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CHAPTER 3

ANGLE DETERMINATION



An FA survey operation consists of many tasks. One task is the measurement of horizontal and vertical angles. Field artillery surveyors use two angle-measuring instruments--the T16 and the T2 theodolites.

Section I

T16 THEODOLITE



The T16 and T16-84 theodolites (Figure 3-1) are compact, lightweight dustproof, optical-reading, directional-type instruments equipped with a fixed reticle (Figure 3-2) in the telescope and a horizontal circle (repeater) clamp. Surveyors use these theodolites to measure horizontal and vertical angles for artillery fifth-order survey. The horizontal and vertical scales of the theodolite are enclosed and are read through a built-in optical system. The scales, graduated in mils, are read directly to 0.2 mil and by estimation to the nearest 0.1 mil. Illumination of the scales is provided by either sunlight or artificial light.

3-1. ACCESSORIES

Accessories issued with the T16 include the following:

  • Canvas accessory kit, which contains the following:
    • - Eyepiece prisms.

      - Sun filters.

      - Two jeweler's screwdrivers.

      - Two adjusting pins.

      - Camel's-hair brush.

      - Lubricant.

      - Plastic instrument head cover.

      - Circular compass.

      - Two operation and service manuals.

  • Battery case with lighting devices and three spare bulbs.
  • Universal tripod with plumb bob, plug-in sleeve, and tripod key in a leather pouch attached to the tripod.

In addition to the accessories issued with the older model T16, the T16-84 accessories include a GST20 tripod and an eye prism. The fixing screws on all Wild tripods have the same threads. This allows both theodolites to mount on any Wild tripod.

Note. For detailed information on the T16 theodolite, see TM 5-6675-200-14 or TM 5-6675-270-15.

3-2. PREPARING THE T16 THEODOLITE FOR USE

a. Set up the tripod as discussed below.

(1) Upend the tripod, place the tripod head on the toe of your boot, and unbuckle the restraining strap.

(2) Loosen the leg clamp thumbscrews, and extend the tripod legs to the desired length. Tighten the leg clamp thumbscrews. Do not force the thumbscrews.

(3) Spread the legs, and place the tripod over the occupied station with one leg bisecting the angle(s) to be measured. Set up the tripod head so that the telescope will be at a convenient height for the operator.

(4) Insert the plug-in sleeve of the plumb bob into the instrument fixing screw. The plumb bob should hang about 1 inch above the station. Center the tripod approximately over the station.

(5) Firmly embed the tripod legs in the ground. Be sure the plumb bob is within 0.5 inch (laterally) of center of the station. The tripod head should be approximately level when the legs are embedded.

(6) Remove the tripod head cover.

b. Remove the theodolite from its case as discussed below.

(1) Grasp the carrying strap with both hands just above the two clamping levers. Pull outward to release the clamping levers from the base assembly.
(2) Lift the dome-shaped cover directly off the instrument, and lay it to one side.
(3) Pull upward on the two base clamping levers that secure the theodolite to the base assembly. Grasp the theodolite by the standard on which the trademark is inscribed, and lift the theodolite off the base assembly. Handle the theodolite by this standard only.

Note. The carrying handle is used to lift the T16-84 theodolite off the base assembly.

(4) Attach the instrument to the tripod head by screwing the fixing screw snugly into the base of the tribrach.
(5) Replace the cover on the carrying case to keep dust and moisture from entering the case. Move the case away from the tripod to provide a working area for the instrument operator.

c. Plumb and level the T16 theodolite as discussed below.

(1) Loosen the fixing screw slightly, and carefully move the instrument around on the tripod head to center the point of the plumb bob exactly over the station.

(2) Tighten the fixing screw. Be sure the point of the plumb bob remains centered over the station. Remove the plumb bob, and return it to its case.

CAUTION

Excessive tightening of the fixing screw will bend the slotted arm and damage the tripod head.

(3) Loosen the horizontal clamping screw, and rotate the instrument until the axis of the plate level is parallel to any two of the three leveling screw knobs. This is the first position. Center the bubble by using these two leveling screw knobs. Grasp the leveling screw knobs between the thumb and forefinger of each hand. Turn the knobs simultaneously so the thumbs of both hands move either toward each other or away from each other. (See Figure 3-3.) This movement tightens one screw as it loosens the other. The bubble always moves in the same direction as the left thumb.

(4) Rotate the instrument 1,600 mils. This second position places the axis of the tubular level at a right angle to the first position. Using the third leveling screw knob only, center the bubble. (See Figure 3-4.)

(5) Return the instrument to the first position (Figure 3-3), and again center the bubble.

(6) Return the instrument to the second position (Figure 3-4), and again center the bubble.

(7) Repeat (5) and (6) above until the bubble remains centered in both positions.

(8) Rotate the instrument 3,200 mils from the first position (Figure 3-3) so the axis of the tubular level vial is on a line parallel with the first position. If the bubble remains centered in this position, rotate the instrument 3,200 mils from the second position. (See Figure 3-4.) If the bubble remains centered in this position, rotate the instrument throughout 6,400 mils. The bubble should remain centered. If the instrument remains centered, it is level.

(9) Rotate the instrument 3,200 mils from the first position ((8) above). The level vial is out of adjustment if it does not remain centered. If time permits, perform a plate level adjustment. If the situation does not allow time for a plate level adustment, perform the following. To compensate, with the instrument in this position (Figure 3-3), move the bubble halfway back to the center of the level vial by using the same two leveling screw knobs used for the first position. Rotate the instrument 3,200 mils from the second position, and move the bubble halfway back to the center of the level vial by using the one remaining leveling screw knob. Rotate the instrument through 6,400 mils. If the bubble does not move more than one graduation, the instrument is considered level. If the bubble moves more than one graduation, repeat the leveling procedure. If the bubble continues to move more than one graduation, turn the theodolite in to the supporting maintenance unit for repair.

(10) After the instrument is level, cheek the optical plumb to ensure that the instrument is centered exactly over the station. If it is not, center the instrument over the station by loosening and shifting it on the tripod head. Check the level of the instrument. If necessary, repeat the leveling process, and again check the optical plumb. Repeat this process until the instrument is level and centered over the station.

Note. A valid check with the optical plumb can be made only if the optical plumb is in proper adjustment.

3-3. TAKING DOWN THE T16 THEODOLITE

After completing observations at a station, march-order the theodolite and tripod as discussed below.

a. Remove the dome cover from the carrying case.

b. Place the telescope in a vertical position with the objective lens down, and lightly clamp the vertical clamping screw.

c. Turn the leveling screws halfway down and to the same height.

d. Position the horizontal clamping screw directly over one of the leveling screws, and lightly clamp it.

Note. On the T16-84, place the optical plumb over any one of the leveling screws.

e. Close the illumination mirror, and turn the hinge to the top.

f. Hold the instrument by its right standard, and unscrew the instrument-fixing screw. Lift the theodolite from the tripod, and secure it in the carrying case. Replace the dome-shaped cover.

g. Replace the tripod head cover, collapse the tripod, and strap the tripod legs together.

3-4. READING AND SETTING HORIZONTAL AND VERTICAL CIRCLES WITH THE T16

a. With the T16 theodolite prepared for observing as described in paragraph 3-2, open the illumination mirror. Adjust the light so both the horizontal and vertical circles are uniformly illuminated when viewed through the circle-reading microscope. When operating at night with unfavorable lighting, attach lighting devices and connect them to the battery pack. Adjust the focus of the circle-reading microscope until the numbers on the horizontal and vertical scales are sharp and clear.

b. When viewed through the circle-reading microscope (Figure 3-5), the vertical circle (marked "V") appears above the horizontal circle (marked "AZ"). Both circles are graduated from 0 to 6,400 mils with a major graduation each 10 mils. Unit mils and tenths are viewed on an auxiliary scale graduated in 0.2-mil increments from 0 to 10 mils. Circle readings are estimated to the nearest 0.1 mil. The scale reading is taken at the point where the major (10-mil) graduation (gauge line) is superimposed on the auxiliary scale. When the telescope is not in a horizontal position, the scales will appear tilted. The amount of tilt depends on the inclination of the telescope.

Note. The horizontal circle of the T16-84 theodolite (Figure 3-6) is yellow and marked "HZ."

c. All horizontal angle measurements with the T16 theodolite begin with an initial circle setting of 0001.0 mil on the horizontal circle. This practice prevents working with a negative mean of direct (D) and reverse (R) readings on the rear station. To set this value on the horizontal circle, release the horizontal clamping screw and rotate the instrument until the major graduation 0 appears on the horizontal circle. Tighten the horizontal clamping screw, and use the horizontal tangent screw to set the 0 gauge line directly over the 1.0-mil graduation on the auxiliary scale. Engage the horizontal circle clamp by setting it to the locked position (down). The horizontal circle is now attached to the alidade of the instrument. The reading of 0001.0 mil will remain on the horizontal circle as long as the horizontal circle clamp is engaged.

3-5. FOCUSING THE TELESCOPE TO ELIMINATE PARALLAX

Before using the theodolite to measure angles, focus the telescope to eliminate parallax. Parallax is eliminated by bringing the focus of the eyepiece and the focus of the objective lens to the plane of the reticle (crosslines). To do this, first point the telescope toward the sky or a neutral background. Rotate the knurled ring on the telescope eyepiece until the reticle cross lines are distinct lines. When doing this, be very careful to focus your eyes on the crosslines, not on the sky. Next point the telescope toward a well-defined distant point. While focusing on the crosslines, bring the distant point into a clear, sharp image by rotating the knurled focusing ring on the telescope. Use the horizontal tangent screw to center the vertical crossline on the point. To check for parallax, move your eye horizontally back and forth across the eyepiece. If parallax is eliminated, the crossline will remain fixed on the object as you move your eye. If all parallax is not eliminated, the crossline will appear to move back and forth across the object. To eliminate any remaining parallax, change the focus of the eyepiece slightly to bring the crosslines into sharper focus and refocus the telescope accordingly until there is no apparent motion. Each time an angle is measured, the telescope should be focused to eliminate parallax. Accurate pointings with the instrument are not possible if parallax exists.

3-6. MEASURING HORIZONTAL ANGLES WITH THE T16

a. In artillery survey, the T16 theodolite is used as a directional-type instrument. The horizontal circle clamp is used only to set the initial circle setting (0001.0 mil) on the horizontal circle before making a pointing on the initial station. Measuring horizontal angles consists of determining, at the occupied station, the horizontal circle readings to each observed station, beginning with an initial (rear) station. The angle between two observed stations is the difference between the mean (mn) horizontal circle readings determined for each of the observed stations. The mean horizontal circle readings used to determine the angles are determined from two pointings (circle readings) on each observed station. (See Figure 3-7.)

b. The direct and reverse pointings on each station should differ by 3,200 mils, plus or minus the amount of horizontal spread (twice the actual error in horizontal collimation) in the instrument. No value can be specified as the maximum allowable spread for an instrument however, it should be small (1.0 mil or less) for convenient in meaning of the pointings. The amount of the spread should be constant; otherwise, there are inconsistencies in operating the instrument. If the mean spread of an instrument exceeds 1.0 mil, the instrument should be adjusted at the first opportunity.

c. With the telescope in the direct position, the initial circle setting of 0001.0 mil on the horizontal circle, and the horizontal circle clamp engaged (down), make a pointing on the initial, or rear, station at the lowest visible point. This establishes the direction of the station at 0001.0 mil with respect to the horizontal circle. Record in the field notes (Figure 3-7) the value of the direct reading on Station A. Release (lift) the horizontal circle clamp. This causes the horizontal circle to detach itself from the alidade and remain in its fixed position. Then make a pointing on each station around the horizon in a clockwise direction. After making a pointing to the last station, plunge the telescope to the reverse position (by rotating the telescope to an upside down position). Make a pointing on each station in a counterclockwise direction, beginning with the last station and ending with the initial station. This sequence of operations is defined as a one-position angle.

d. In making pointings, the horizontal and vertical clamping screws are used to place the crossline approximately on the object marking the station. After placing the crosslines approximately on the target, tighten the horizontal and vertical clamping screws. Then use the horizontal and vertical tangent screws to plain the crosslines exactly on the lowest visible point of the station. (See Figure 3-8.) The final rotation of the tangent screws must be in a clockwise direction.

Note. The view through the T16-84 is the same as the T16 except that the view is not inverted.

e. After making the reverse pointing on the initial station and recording the horizontal circle reading, plunge the telescope to the direct position. Then make a direct pointing on the initial station. Although zeroing the instrument is not a part of the angle measurement, it will save time in setting the initial circle setting for the angle measurement at the next station.

3-7. DETERMINING VERTICAL ANGLES WITH THE T16

a. Normally, each time a horizontal angle is measured, a vertical angle to the forward station is determined. If possible, measure vertical angles to the height of the instrument.

b. Vertical angles cannot be measured directly with the theodolite. The vertical circles of the theodolite reflect readings of 0 mils at the zenith (straight up), 1,600 mils horizontal direct, 3,200 mils at the nadir (straight down), and 4,800 mils horizontal reverse. Hence, values read from the vertical circle are not vertical angles but are zenith distances that are converted to vertical angles. When the collimation level bubble is centered, vertical circle readings are taken from what is, in effect, an upward extension of the plumb line of the theodolite. (See Figure 3-8.) One value of the vertical angle is computed from the vertical circle reading made with the telescope in the direct position arid pointed at the station. With the telescope in the direct position, a vertical circle reading of less than 1,600 mils indicates that the station observed is above the horizontal plane of the theodolite and the vertical angle is positive. A vertical circle reading greater than 1,600 mils indicates that the station observed is below the horizontal plane of the theodolite and the vertical angle is negative. To determine the value of a positive vertical angle, subtract the vertical circle reading from 1,600 mils. (See Figure 3-9, (A).) Determine the value of a negative vertical angle by subtracting 1,600 mils from the vertical circle reading. (See Figure 3-9, (B).) A second value of the vertical angle is computed from the vertical circle reading made with the telescope in the reverse position and pointed at the station. With the telescope reversed, a vertical circle reading greater than 4,800 mils indicates a positive vertical angle, and a vertical circle reading less than 4,800 mils indicates a negative vertical angle. Determine the value of a positive vertical angle by subtracting 4,800 mils from the vertical circle reading, (See Figure 3-9, (C).) Determine the value of a negative vertical angle by subtracting the vertical circle reading from 4,800 mils. (See Figure 3-9, (D).) Mean the two values by adding them together and dividing the result by two. The result is the vertical angle to the observed station.

c. After placing the crosslines on the station, elevate or depress the telescope until the horizontal crossline is exactly on the point to which the vertical angle is desired. After positioning the telescope on the station, center the bubble of the collimation level (split bubble) on its vial by rotating the collimation level tangent screw until the images of the ends of the bubble coincide. (See Figure 3-10.) Then read the vertical circle reading in the circle-reading microscope. The T16-84 has an automatic vertical index.

Note. If the T16-84 instrument is so off level that the automatic index is no longer within its working range, a red warning screen appears on the vertical circle scale. If a red screen appears, relevel the instrument.

3-8. COMPUTING HORIZONTAL AND VERTICAL ANGLES

After making the direct and reverse pointings and recording the horizontal and vertical circle readings in the field notes (Figure 3-7), determine the size of each angle.

a. To determine the horizontal angle between Stations A and B (Figure 3-7), first determine the mean of the pointings on each station. Do this by mentally adding 3,200 mils to, or subtracting 3,200 mils from, the reverse reading and taking the mean of the direct and reverse readings. Enter the results in the field notes in the proper column. Then determine the horizontal angle between the stations by finding the difference in the mean circle readings. In Figure 3-7, the mean pointing on Station A is 0001.0 mil; on Station B, 0229.4 mils. Therefore, the horizontal angle from Station A to Station B is 0228.4 mils (0229.4 - 0001.0 = 0228.4).

b. In the field notes in Figure 3-7, the direct pointing on Station B resulted in a vertical circle reading of 1,598.5 mile, or a vertical angle of +1.5 mils. With the telescope revered, the vertical circle reading on Station B was 4,801.4 mils, or a vertical angle of +1.4 mils. The mean vertical angle from the instrument to Station B is +1.4 mils (1.5 + 1.4 + 2 = 1.45 rounded to the nearest even 0.1 mil =1.4.

3-9. CARE OF THE T16 THEODOLITE

The T16 theodolite is a delicate instrument. Take care not to drop it or bump it against any object. If the instrument gets wet, dry it before storing it in the carrying case. As soon as possible, place the instrument in a dry room or tent. Remove it from the carrying case so it can dry completely. If left in the closed carrying case, it will absorb the humidity in the air if there is an increase in temperature. Should the temperature drop later, the moisture will condense on the inside of the instrument and may render the instrument inoperable. A man on foot may carry the instrument mounted on the tripod from station to station. Always carry the theodolite in an upright position. Clamp all motions with the telescope in the vertical position. For transport over rough terrain, the instrument should be in its carrying case. When transported in a vehicle, the theodolite should be in the carrying case and the case should be in the padded box. For short distances in vehicles, the instrument operator may hold the carrying case in an upright position on his lap.

3-10. CLEANING THE T16 THEODOLITE

Keep the theodolite clean and dry. During use, as necessary, and after use, clean the instrument as discussed below.

a. Wipe painted surfaces with a clean cloth.

b. Clean the lenses only with a camel's-hair brush and lens tissue. Clean the lenses first with the brush to remove any dust or other abrasive material and then with the lens tissue. Remove any smudge spots remaining after using lens tissue by slightly moistening the spot and again cleaning with lens tissue. Be careful not to scratch the lenses or remove their coating. The coating reduces glare for the observer.

c. Clean all metal parts of the tripod with a cloth moistened with an approved cleaning solvent. Then wipe them dry. Clean the wooden parts with a soft cloth moistened with water, and dry the parts thoroughly. Clean the leather strap with a suitable leather cleaner.

3-11. REPAIR OF THE T16 THEODOLITE

Theodolites in need of major adjustment or repair should be turned in to the engineer unit responsible for providing maintenance service.

3-12. TESTS AND ADJUSTMENTS OF THE T16 THEODOLITE

a. Keep the T16 theodolite in correct adjustment to obtain accurate results. There are four tests and adjustments of the T16 theodolite that the instrument operator must make. He makes these tests and adjustments in the following sequence:

  • Plate level test and adjustment.
  • Optical plumb teat and adjustment.
  • Horizontal collimation teat and adjustment.
  • Vertical collimation test and adjustment.

When a test indicates that an adjustment is necessary, the operator makes the adjustment and retests the instrument for accuracy before making the next test in sequence.

b. The four tests and adjustments of the theodolite are made with the instrument mounted on its tripod. For these tests and adjustments, set the instrument up in the shade on firm ground with the head of the tripod as nearly level as possible. Also protect the theodolite from the wind.

c. If handled properly, an instrument will remain in adjustment indefinitely. Needless and excessive movement of the adjusting screws will cause them to become worn and the instrument will not hold an adjustment.

3-13. T16 PLATE LEVEL TEST AND ADJUSTMENT

a. Purpose. The purpose of the plate level adjustment is to make the vertical axis of the theodolite truly vertical when the bubble of the plate level is centered in the vial. (See Figure 3-11.)

b. Test.

Step 1. Test the adjustment of the plate level by placing the axis of the plate level bubble parallel to any two of the three leveling screws. (See Figure 3-12.)

Step 2. With these two leveling screws, center the bubble in its vial. Rotate the instrument 1,600 mils, and align the plate level with the third leveling screw. (See Figure 3-13.)

Step 3. Center the bubble in this position by using the third leveling screw. Rotate the instrument back to the first position, and see if the bubble remains centered. Repeat leveling in these two positions until the bubble remains centered in both positions.

Step 4. Carefully rotate the instrument 3,200 mils from the first position. If the bubble does not remain centered within one graduation, an adjustment is required. The discrepancy noted between the position of the bubble and the center position is the apparent error, or twice the actual error of the plate level. The level vial has graduated lines used to determine the apparent error. (See Figure 3-14.)

c. Adjustment. To adjust the plate level, use the adjusting pin from the accessory case. With the instrument 3,200 mils from the first position, insert the adjusting pin into the hole of the capstan screw and remove one-half of the apparent error (actual error) by turning the capstan adjusting screw. On the T16 theodolite, the plate level adjusting screw is about 1.5 inches above the horizontal clamping screw on the right standard. (See Figure 3-15.) After adjusting, repeat the plate level test and adjust as necessary to correct any error remaining in the plate level bubble. The plate level is in proper adjustment when the bubble remains centered throughout 6,400 mils rotation.

Note. On the T16-84, the plate level adjustment screw is on the top right of the plate level vial. (See Figure 3-16.)

3-14. T16 OPTICAL PLUMB TEST AND ADJUSTMENT

a. Purpose. The purpose of the optical plumb adjustment is to make the vertical axis of the theodolite pass through the station mark when the theodolite is properly leveled and the station mark is centered in the reticle of the optical plumb. (See Figure 3-17.)

b. Test.

Step 1. To test the optical plumb, set up the theodolite over a station that is clearly marked by a cross or other well-defined point. Accurately plumb and level the instrument. The image of the point should be centered exactly in the center of the optical plumb. This is accomplished by recentering and releveling as necessary.

Step 2. When the instrument is leveled and the image of the point is centered, rotate the alidade 6,400 mils about its vertical axis. If the image of the point does not remain centered in the reticle, an adjustment is required. The amount of displacement is the apparent error, or twice the actual error, of the optical plumb. Position the instrument at the point where the displacement is farthest from the point.

c. Adjustment.

Step 1. To adjust the optical plumb, correct one half of the displacement (actual error) by turning the two optical plumb adjusting screws. The adjusting screws are 1.25 inches to the right and left of the optical plumb eyepiece. To gain access to the adjusting screws, remove the cover screws. (See Figure 3-18.)

Step 2. Use the screwdriver from the accessory case to turn the adjusting screws. Loosen the adjusting screw on the side opposite the side to which the reticle must be moved. Tighten the other screw, and the reticle moves as the screw is tightened. Move the reticle image one-half the distance to the station mark by moving first one screw and then the other in small increments. The last movement of both adjusting screws must be clockwise. This compresses a counterspring positioned under each screw and holds the optical system stationary.

Step 3. Check the adjustment again by centering the instrument over the station mark and leveling it. Rotate the instrument through 6,400 mils. If the image of the reticle remains centered on the station mark throughout the full circle, the optical plumb is in adjustment. If the image of the reticle does not remain centered on the station mark throughout the full circle, repeat the adjustment until the image of the reticle remains centered. After the adjustment is complete, replace the cover screws.

3-15. T16 HORIZONTAL COLLIMATION TEST AND ADJUSTMENT

a. Purpose. The purpose of the horizontal collimation adjustment is to make the line of sight perpendicular to the horizontal axis of the telescope. (See Figure 3-19.)

b. Test.

Step 1. To test the horizontal collimation, select a well-defined point at least 100 meters (m) from the instrument and at about the same height as the instrument. (see Figure 3-20.)

Step 2. With the telescope in the direct position, center the vertical crossline on the selected point. Read the horizontal circle reading to the recorder. For illustration purposes, assume your direct reading is 0001.0 mil.

Step 3. Plunge the telescope to the reverse position, and give the recorder a second reading to the same point. Assume the reverse reading is 3,202.6 mils.

Note. Only one set of direct and reverse readings is required. Be very careful, therefore, to prevent pointing or reading errors.

Step 4. The difference between the direct and reverse readings should be exactly 3,200 mils. Assuming you did not make any error in the pointings or readings, the discrepancy between the actual difference in the two readings and 3,200 mils is the apparent horizontal collimation error, or twice the actual horizontal collimation error. If the apparent horizontal collimation error exceeds the initial circle setting (0001.0 mil for the T16), you should perform the horizontal collimation adjustment.

EXAMPLE

Direct reading to                                   0001.0
selected point

Reverse reading to the                          3202.6 - 3200 = 0002.6
same point

Spread (0002.6 - 0001.0)                          1.6

Apparent horizontal                                    1.6 mils
collimation error

Actual error (apparent error                        0.8 mil
divided by 2) (1.6 ÷ 2)

c. Adjustment.

Step 1. With the telescope sighted on the point in the reverse position and using the horizontal tangent screw, set the circle to the mean value of the direct and reverse pointings. (Mean reverse reading: 3202.6 - 0.8 = 3201.8) This will move the vertical crossline of the telescope reticle off the point.

Step 2. Move the vertical crossline back to the point by turning the two pull-action capstan adjusting screws that are arranged horizontally and on opposite sides of the telescope near the eyepiece. To move the crosslines to the left, loosen the right screw first and tighten the left screw. If the adjusting screw is tightened too much, it will cause the reticle to get out of adjustment later. Do not try to make the entire adjustment in one step. Loosen and tighten the opposite screws in small amounts. Do this until the crossline is centered exactly on the sighted point.

Note. On the T16-84, the cover on the telescope eyepiece must be removed to expose the capstan adjusting screws.

Step 3. Perform the test again, and make additional adjustments until the difference is less than 0.2 mil.

Note. It is preferred that the adjustment be made with the theodolite in the reverse position. This adjustment also can be made with the telescope in the direct position and by using the mean value for the direct pointing (0001.0 + 0.8 = 0001.8).

3-16. T16 VERTICAL COLLIMATION TEST AND ADJUSTMENT

a. Purpose. The purpose of the vertical collimation adjustment is to make the line of sight horizontal when the vertical circle reading is 1,600 mils with the telescope in the direct position (4,800 mils with the telescope in the reverse position) and the ends of the collimation level bubble in alignment. (See Figure 3-21.)

b. Test.

Step 1. To test the vertical collimation, select a well-defined point at least 100 meters from the instrument and about on a horizontal plane with the theodolite. (See Figure 3-22.)

Step 2. With the telescope in the direct position, give the recorder a direct vertical circle reading to the point. Be sure to precisely align the collimation level bubble.

Step 3. Plunge the telescope to the reverse position. Give the recorder a reverse circle reading to the same point. Be sure the collimation level bubble is precisely aligned before you read the vertical circle. Check the alignment after you take your reading.

Note. Only one set of direct and reverse readings is required. Be very careful, therefore, to prevent pointing or reading errors.

c. Adjustment. The sum of the direct and reverse readings should be 6,400 mils. Any difference between the sum of the two readings and 6,400 mils is the apparent vertical collimation error, or twice the actual error. If the difference exceeds 0001.0 mil for the T16 theodolite, the vertical collimation level should be adjusted. To compensate for the error, you must compute the correct vertical pointing by using the two vertical readings. If the sum of the two readings is greater than 6,400 mils, determine the difference between the sum and 6,400 mils. Then subtract one-half the difference from your reverse reading to obtain a corrected reading. If the sum of the two readings is less than 6,400 mils, find the difference between the sum and 6,400 mils. Add one-half of the difference to the reverse vertical reading.

EXAMPLE

Direct reading                                            1063.4
Reverse reading                                       +4797.8
Sum                                                           6401.2
                                                                -6400.0
Apparent error                                                 1.2 mils

Actual Error (1.2 ÷ 2)                                       0.6 mils

Corrected reverse vertical                          4,797.2 mils
reading (4797.8 - 0.6)

EXAMPLE

Direct reading                                             1594.6
Reverse reading                                        +4804.2
Sum                                                            6398.8
Apparent error (6400 - 6398.8)                        1.2 mils

Actual error (1.2 ÷ 2)                                        0.6 mils

Corrected reverse vertical                           4,804.8 mils
reading (4804.2 + 0.6)

Step 1. With the instrument in the reverse position and the telescope sighted on the selected point, use the collimation level tangent screw to place the corrected vertical reading on the vertical circle scale. The collimation level bubble will not be centered.

Step 2. Remove the cover from the collimation level bubble, and use the capstan adjusting screw to center the collimation bubble. Rotate the single adjusting screw to align the images of the collimation bubble. On the T16-84, slacken the screw located 1/2 inch above the illumination mirror and open the cover. The adjustment screw for the vertical index is now seen. Turn the adjustment screw carefully until the correct vertical reading is set, then close the cover.

Step 3. Continue the test and adjustments until the difference between the direct and reverse readings is less than 0.2 mil.

Note. It is preferred that the adjustment be made with the theodolite in the reverse position. This adjustment also can be made with the telescope in the direct position by using the corrected direct vertical reading to adjust the instrument.

3-17. TESTS AND ADJUSTMENTS OF THE T16-64 THEODOLITE

a. There are five tests and adjustments on the T16-84 theodolite that are made by artillery survey personnel. The tests and adjustments are performed in the following order:

  • Plate level adjustment.
  • Circular bubble test and adjustment.
  • Optical plumb test and adjustment.
  • Horizontal collimation test and adjustment.
  • Vertical collimation test and adjustment.

The procedure for tests and adjustments on the T16-84 are very similar to those of the T16. Perform the plate level, horizontal collimation, and vertical collimation tests and adjustments by following the procedures outlined for the T16 theodolite in paragraphs 3-13, 3-15, and 3-16 respectively.

b. Always perform the tests and adjustments in the sequence in which they are listed. After you make an adjustment, perform the test again to check for accuracy before you go to the next test and adjustment. When making adjustments, do not force the movement of the adjusting screws or exert too much pressure. The screws could be damaged and render your instrument unserviceable. A correctly adjusted theodolite is essential in obtaining accurate survey results.

3-18. T16-84 CIRCULAR BUBBLE TEST AND ADJUSTMENT

a. Purpose. The purpose of the circular bubble adjustment is to make the circular bubble center of its setting circle when the plate level is in adjustment.

b. Test. Level the instrument by using the plate level. The circular level should be center of its setting circle. If not, it must be adjusted.

c. Adjustment. To adjust the circular bubble, tighten and/or loosen the two small adjustment screws in the side of the level holder (Figure 3-23) until the level is centered in its setting circle.

3-19. T16-84 OPTICAL PLUMB TEST AND ADJUSTMENT

a. Purpose. The purpose of the optical plumb adjustment is to make the vertical axis of the theodolite pass through the station mark when the theodolite is properly leveled and the station mark is centered in the reticle of the optical plumb.

b. Test.

Step 1. Place a piece of paper on the ground under the instrument. Accurately plumb the instrument by using the plumb bob, and mark the point on the paper. Label the mark "Point 1."

Step 2. Remove the plumb bob, and bring the point into focus. Turn the leveling screws to set the cross hairs of the optical plumb exactly on Point 1. Move the eye slightly to ensure there is no parallax between the cross hairs and Point 1. If there is parallax, adjust the focus.

Step 3. Rotate the alidade 3,200 mils, and carefully mark the position of the cross hairs on the paper as Point 2. If Points 1 and 2 coincide, the optical plumb is in adjustment. If not, it must be adjusted.

c. Adjustment.

Step 1. Mark a point (Point 3) halfway between points 1 and 2. Using the leveling screws, set the cross hairs of the optical plumb on Point 3.

Step 2. Loosen the four screws in the plate around the optical plumb eyepiece until the optical plumb and plate can just move. (See Figure 3-24.)

Step 3. Move the plummet and plate carefully until the cross hairs are exactly on Point 1. Tighten the four screws. Repeat the test, and if necessary, repeat the adjustment.

Note. In the procedure described above the plate level has no influence and must be ignored.

CAUTION

It may be difficult for you to remember where the various adjusting screws are located when you need to make adjustments on the theodolite. You should always refer to the technical manual published for each type of theodolite.

3-20. CARE AND USE OF THE TRIPOD

The tripod should be used and cared for as discussed below.

a. Turn the tripod to its upright position. Test the adjustment of the tripod legs by elevating each leg, in turn, to a horizontal position and then releasing it. If properly adjusted, the leg should fall to about 800 mils and stop. If it does not, adjust the tripod leg by tightening or loosening the tripod clamping nut. Repeat the test until it is successful.

b. Clean the wooden parts with a soft cloth moistened with water, and dry them thoroughly. Clean the leather strap with a suitable leather cleaner.


Section II

T2 THEODOLITE



The T2 theodolite (Figure 3-25) is the authorized angle-measuring instrument for artillery fourth-order survey. The theodolite is a directional-type instrument and is used to measure horizontal and vertical angles. It has interior scales, which are read by means of a built-in optical system. The scales, graduated in mils, can be read directly to 0.002 mil and by interpolation to the nearest 0.001 mil. The scales may be illuminated by sunlight or by means of a built-in wiring system using artificial light. All parts of the instrument that can be seriously damaged by dust or moisture are enclosed. The T2 theodolite is issued with a canvas accessory case containing the following:

  • An instructional pamphlet.
  • Diagonal eyepieces for the telescope and reading microscope.
  • A sun filter.
  • A jeweler's screwdriver.
  • Two adjusting pins.
  • A camel's-hair brush.
  • A plastic instrument cover.
  • Two lamp fittings for artificial illumination.

Also issued with the T2 is a battery case containing lighting devices and spare bulbs and a universal tripod with a plumb bob, plug-in sleeve, and tripod key in a leather pouch attached to the tripod. The accessories of some models of the theodolite are stored in the base of the carrying case.

Note. For detailed information on the T2 theodolite, see TM 5-6675-205-20P, TM 5-6675-233-20P, and TM 5-6675-296-14.

3-21. PREPARING AND TAKING DOWN THE T2 THEODOLITE

a. Setting Up the Tripod. The tripod used with the T2 theodolite is similar to that used with the T16 theodolite. The procedure for setting up this tripod is the same as that for setting up the T16 theodolite tripod (paragraph 3-2).

b. Removing the Theodolite From Its Case. The T2 theodolite is removed from its case in the same manner as the T16 theodolite (paragraph 3-2) except that the T2 theodolite is fastened to the base by three supports with locking devices.

c. Plumbing and Leveling the Theodolite. The procedure for plumbing and leveling the T2 theodolite is the same as that for the T16 theodolite. (See Figures 3-3 and 3-4.)

d. Focusing the Telescope to Eliminate Parallax. The telescope of the T2 theodolite is the same as the telescope of the T16 theodolite. It is focused to eliminate parallax in the same manner (paragraph 3-5).

e. Taking Down the T2 Theodolite. The procedure for taking down the T2 theodolite is discussed below.

(1) Remove the dome cover from the carrying case, and prepare the case to receive the theodolite by opening the locking devices.

(2) Place the telescope in a vertical position with the objective lens down, and lightly clamp the vertical clamping screw.

(3) Turn the leveling screws about halfway down and to the same height.

(4) Lightly clamp the horizontal clamping screw.

(5) Close the illumination mirrors, and turn the hinges to the top.

(6) Hold the instrument by its right standard, and unscrew the instrument-fixing screws. Lift the theodolite from the tripod, and secure it in the carrying case. Replace the dome-shaped cover.

(7) Replace the tripod head cover, collapse the tripod, and strap the legs together.

3-22. CIRCLE READINGS WITH THE T2

a. A system of lenses and prisms permits the observer to see small sections of either the horizontal circle or the vertical circle. The circles are viewed through the circle-reading microscope eyepiece located alongside the telescope. The observer selects the circle to be viewed by turning the circle selector knob on the right standard. The field of view of the circle-reading microscope contains two small windows. (See Figure 3-26.) The upper window shows images of two diametrically opposite parts of the circle (horizontal or vertical). One image of the circle is inverted and appears above the other image. The lower window shows an image of a portion of the micrometer scale.

b. The micrometer coincidence knob on the side of the right standard is used in conjunction with the micrometer scale to obtain readings for either of the circles. Optical coincidence is obtained between diametrically opposite graduations of the circle by turning the micrometer coincidence knob. When this knob is turned, the images of the opposite sides of the circle appear to move in opposite directions across the upper window in the circle-reading microscope. The image of the micrometer scale in the lower window also moves. The graduations of the circle (upper window) are brought into coincidence so they appear to form continuous lines across the dividing line. The center of the field of view in the upper window is marked by a fixed vertical index line. The final coincidence adjustment should be made between circle graduations near this index line.

3-23. HORIZONTAL CIRCLE READINGS WITH THE T2

A reading on the horizontal circle is determined as discussed below.

a. Rotate the circle selector knob until the black line on the face of the knob is horizontal.

b. Adjust the illuminating mirror so both windows in the circle-reading microscope are uniformly lit.

c. Focus the microscope eyepiece so the graduations of the circle and the micrometer scale are sharply defined.

d. Observe the images in the microscope eyepiece. Bring the circle graduations into coincidence at the center of the upper window by turning the coincidence knob.

e. Read the horizontal circle and the micrometer scale.

3-24. STEPS IN CIRCLE READING

On the T2 theodolite, the main scale (upper window) is graduated in 2-mil increments (Figure 3-26). Each fifth graduation is numbered with the unit digits omitted. For example, 10 mils appears as 1; 250 mils, as 25; and 3,510 mils, as 351. The micrometer scale (lower window) is graduated from 0.000 mil to 1.000 mil. Each 0.002 mil is marked with a graduation and each fifth graduation is numbered (hundredth of a mil). The scale may be read to 0.001 by interpolation. The steps in reading the circles are discussed below.

a. With the circle graduations in coincidence (Figure 3-26), determine the first erect numbered graduation to the left of the index line that marks the center of the upper window. This numbered graduation indicates the value of the circle reading in tens of mils. In Figure 3-26, this graduation is 121.

b. Locate on the inverted scale the graduation for the number opposite 121 (the number +320). In Figure 3-26, this number is 441 (viewed as ). The inverted number normally is to the right of the index line that marks the center of the field of view. Both values always end in the same number--in this case, the number 1. When the unit mil of the circle reading is zero, coincidence is obtained when the circle reading and its diametrically opposite number are in coincidence with each other.

c. Count the number of spaces between graduations from 121 to the inverted 441. Each of these spaces represents 1 mil. There are five spaces, representing 5 mils.

d. Convert 121, which is tens of mils, to 1,210 mils. To this value, add the unit mils determined in paragraph c above (1,210 + 5 = 1,215 mils, the angular value obtained from the main scale).

e. On the micrometer scale (lower window), the index line that marks the center of the field of view also indicates the value to be read from the micrometer scale. In Figure 3-26, this value is 0.475 mil.

f. Add the values determined in paragraphs d and e above (1215 + 0.475= 1,215.475 mils, the angular value displayed in Figure 3-26).

3-25. VERTICAL CIRCLE READINGS WITH THE T2 THEODOLITE

To view the vertical circle, turn the circle selector knob to the vertical position (the black line on the face of the knob is vertical.) Adjust the vertical circle illuminating mirror so both windows in the circle reading microscope are uniformly lit. The vertical circle is read in the same manner as the horizontal circle. Before reading the vertical circle, center the vertical collimation level (split bubble) by using the procedures described in paragraph 3-7, and bring the images of the vertical circle into coincidence by using the procedures described in paragraph 3-22.

3-26. SETTING THE HORIZONTAL CIRCLE

There are two situations in which it is necessary to set the horizontal circle.

a. The first instance is when the initial circle setting of 0000.150 ( 0.100 mil) is used.

(1) Point the instrument at the rear station.

(2) Using the coincidence knob, place a reading of 0.150 on the micrometer scale.

(3) Using the circle-setting knob, zero the main scale as accurately as possible, ensuring that the numbered lines, which are 3,200 mils apart (the erect 0 graduation and the inverted 320 graduation), are touching each other. Ensure that the circle-setting knob cover is closed when this step is finished.

(4) With the coincidence knob, bring the main scale graduation into precise coincidence.

(5) Read the horizontal circle. The reading should be 0000.150 ( 0.100 mil).

b. The second instance is when it is desired to orient the instrument on a line of known direction from a reference direction (or to measure a predetermined angle).

(1) Sight the instrument on the station for which the reference direction is provided, and read the circle.

(2) Add the angular difference between the reference direction and the desired direction (or the predetermined angle) to the circle reading. The result is the circle reading for the instrument when it is pointed in the desired direction.

(3) Using the coincidence knob, set the micrometer scale to read the fractional portion of the desired circle reading to the nearest thousandth of a mil.

(4) Using the horizontal clamping screw and the horizontal tangent screw, rotate the alidade to obtain coincidence on the main state at the mils value corresponding to the reading obtained in (2) above. When coincidence is obtained, the instrument is pointing in the desired direction.

3-27. MEASURING HORIZONTAL ANGLES WITH THE T2

a. Since the T2 theodolite is a directional-type instrument, the values of horizontal angles are determined by differences in circle readings. The procedures for measuring and determining horizontal angles (Figure 3-27) are discussed below.

(1) With the telescope in the direct position, point to the rear station (Station A). Set and record the initial circle setting (0000.166 mil).

(2) With the telescope in the direct position, point to the forward station (Station B). Record the horizontal circle reading (1,215.475 mils).

(3) Plunge the telescope to the reverse position, and point to Station B. Record the circle reading (4,415.503 mils).

(4) With the telescope in the reverse position, point to Station A. Record the circle reading (3,200.200 mils).

(5) Subtract 3,200 mils from the reverse pointing on Station A. Mean the remainder with the direct pointing on Station A (0000.183 mil).

(6) Subtract 3,200 mils from the reverse pointing on Station B. Mean the remainder with the direct pointing on Station B (1,215.489 mils).

(7) Subtract the mean pointing on Station A from the mean pointing on Station B to determine the horizontal angle from Station A to Station B (1215.489 - 0000.183 = 1,215.306 mils).

Note. Steps (1) through (7) above constitute one direct and one reverse pointing on each station which is referred to as one position.

b. When it is necessary to measure the angle to more than one station, make a pointing on the initial station with the telescope in the direct position and then on each station around the horizon in a clockwise direction. After getting a direct reading on the last station, reverse the telescope and make a pointing at each station in a counterclockwise direction, ending with the initial station. One set of direct and reverse pointings on all of the observed stations constitutes one position.

c. The direct and reverse pointings on each station should differ by 3,200 mils, plus or minus the amount of horizontal spread (twice the actual error in horizontal collimation) in the instrument. No value can be specified as the maximum allowable spread for an instrument; however, it should be small (0.150 mil or less) for convenience in meaning the pointings. The amount of the spread should be constant; otherwise, there are inconsistencies in operating the instrument. If the mean spread of an instrument exceeds 0.150 mil, the instrument should be adjusted at the first opportunity.

d. In FA survey, one position is normally observed for traverse.

e. It may be necessary, as in fourth-order triangulation, to measure two positions. The second position is measured in the same manner as the first position, except that the second position is measured with the telescope in the reverse position for the initial pointing on each station. The initial circle settings should be as follows: first position, 0000.150 (+ 0.100) mil; second position (reverse), 4,800.150 mils (+ 0.100) mil.

f. Determine the angle between two observed stations by measuring and meaning the horizontal circle reading to each station and computing the difference between the mean circle readings. When two positions are taken, determine the value of the angle by taking the mean of the values of the angle as determined from each of the two positions.

g. When two positions are observed, if the two observed values for any angle differ by more than 0.050 mil, these observed values should be rejected. If the observed values are rejected, the angle(s) must be remeasured.

3-28. DETERMINING VERTICAL ANGLES WITH THE T2

a. The procedure for determining vertical angles with the T2 theodolite is the same as that for the T16 theodolite (paragraph 3-7).

b. After sighting on the observed station and with the circle selector knob in the vertical position, make the vertical circle reading in the same manner as the horizontal circle reading.

3-29. TESTS AND ADJUSTMENTS OF THE T2 THEODOLITE

a. The T2 theodolite must be kept in correct adjustment if accurate results are to be obtained. There are five tests and adjustments of the T2 theodolite that the instrument operator must make in the following sequence:

  • Plate level test and adjustment.
  • Optical plumb test and adjustment.
  • Verticality test and adjustment.
  • Horizontal collimation test and adjustment.
  • Vertical collimation teat and adjustment.

When a test indicates that an adjustment is necessary, the operator makes the adjustment and retests the instrument for accuracy before making the next test in sequence.

b. The five tests and adjustments of the theodolite are made with the instrument mounted on its tripod and accurately leveled. For these tests and adjustments, the instrument is set up in the shade on firm ground with the head of the tripod as nearly level as possible. The theodolite should also be protected from the wind.

c. If handled properly, an instrument will remain in adjustment indefinitely. Needless and excessive movement of the adjusting screws should be avoided, as it will cause the screws to become worn, and the instrument will not hold an adjustment.

3-30. T2 PLATE LEVEL TEST AND ADJUSTMENT

a. Purpose. The purpose of the plate level adjustment is to make the vertical axis of the theodolite truly vertical when the bubble of the plate level is centered in its vial.

b. Test. The plate level adjustment test for the T2 theodolite is the same as that for the T16 theodolite. See paragraph 3-13 for the steps in this test.

c. Adjustment. To adjust the plate level, use the adjusting pin from the accessory case. Insert the adjusting pin into the hole of the capstan screw, and remove one-half of the apparent error (actual error) by turning the capstan adjusting screw. The capstan adjusting screw is located on the lower portion of the large standard of the T2 theodolite, directly below the collimation level bubble reflector. (See Figure 3-28.) After adjusting, repeat the plate level test to detect and adjust for any error remaining in the plate level bubble. The plate level is in proper adjustment when the bubble remains centered throughout 6,400 mils rotation.

3-31. T2 OPTICAL PLUMB TEST AND ADJUSTMENT

a. Purpose. The purpose of the optical plumb adjustment is to make the vertical axis of the theodolite pass through the station mark when the theodolite is properly leveled and plumbed.

b. Test.

Step 1. To test the optical plumb, set up the theodolite over a station that is clearly marked by a cross or other well-defined point.

Step 2. Suspend the plumb bob from the instrument. Move the instrument until the plumb bob is suspended directly over the well-defined point. Accurately level the instrument.

Step 3. Remove the plumb bob from the instrument, and check to ensure that the instrument is accurately leveled (that the vertical axis is truly vertical). Look into the eyepiece of the optical plumb. If it is in correct adjustment, the mark on the ground will be centered in the reticle.

c. Adjustment. If the point on the ground is not centered in the optical plumb reticle, center the point by using the three adjusting screws located near the optical plumb eyepiece. Two of these adjusting screws are on opposite sides of the eyepiece, and the third adjusting screw is below the eyepiece. The bottom adjusting screw is locked in place by a check nut, which is located immediately above the head of the adjusting screw. (See Figure 3-29.)

Step 1. With an adjusting pin, loosen the check nut. Raise or lower the reticle by turning the bottom adjusting screw to move the reticle image along the axis of the optical plumb in the same direction that the screw travels.

Step 2. Use two adjusting screws on each side of the eyepiece to move the image of the reticle in the opposite direction that the screws travel. If it is necessary to use these screws, they should be rotated an equal amount in opposite directions. It is usually necessary to loosen the screw below the eyepiece slightly to adjust the screws on the side.

Step 3. When the adjusting is complete, the two opposed adjusting screws must be fairly tight. Lock the bottom adjusting screw in place by tightening the check nut.

3-32. T2 VERTICALITY TEST AND ADJUSTMENT

a. Purpose. The purpose of the verticality adjustment is to make the vertical crossline of the reticle lie in a plane that is perpendicular to the horizontal axis of the telescope.

Note. The newer model T2 theodolites (64 series or newer) require no verticality adjustment at the operator or organizational maintenance level.

b. Test.

Step 1. To teat the verticality of the vertical crossline, select a well-defined distant point as near the horizontal plane of the instrument as possible. Center the vertical crossline on the selected point.

Step 2. Elevate and depress the telescope by using the vertical tangent screw. If the vertical crossline does not continuously track on the point as the telescope is elevated and depressed, an adjustment is necessary.

c. Adjustment. To adjust the vertical crossline and make it truly vertical, use the three adjusting screws on the enlarged portion of the telescope housing between the eyepiece and the knurled focus ring. The two screws on the right side are slant screws (Figure 3-30), which change the vertical position of the crossline. Loosen one slant screw and tighten the other one an equal amount to adjust the verticality of the crossline. The adjusting screw on the left side moves the crossline horizontally.

3-33. T2 HORIZONTAL COLLIMATION TEST AND ADJUSTMENT

a. Purpose. The purpose of the horizontal collimation adjustment is to make the line of sight perpendicular to the horizontal axis of the telescope.

b. Test.

Step 1. To test the horizontal collimation, select a well-defined point at least 100 meters from the instrument and at about the same height as the instrument.

Step 2. With the telescope in the direct position, center the vertical crossline on the selected point and read the horizontal circle reading to the recorder. For illustration purposes, assume your direct reading is 0000.202 mil.

Step 3. Plunge the telescope to the reverse position, and take a reverse reading to the same point. Assume the reverse reading is 3,200.802 mils.

Note. Only one set of direct and reverse readings is required for horizontal collimation. Be very careful, therefore, to prevent pointing and reading errors.

Step 4. The difference between the direct and reverse readings should be exactly 3,200 mils. Assuming you do not make any error in the pointings or readings, the discrepancy between the actual difference in the two readings and 3,200 mils is the apparent horizontal collimation error, or twice the actual horizontal collimation error. If the apparent horizontal collimation error exceeds the initial circle setting (0000.150 mil for the T2), you should perform the horizontal collimation adjustment.

EXAMPLE

Direct reading to                       0000.202        
selected point

Reverse reading to the               0000.802                
same point (3200.802-3200)

Spread                                            0.600 mil
(0000.802 - 0000.202)

Apparent horizontal                         03.00 mil
collimation error (0.600 mil ÷ 2)

c. Adjustment.

Step 1. With the telescope on the point in the reverse position, set the mean value of the direct and reverse pointings (reverse = 3200.502) on the micrometer scale by using the coincidence knob.

Step 2. Bring the main scale into coincidence by using the horizontal tangent screw. This moves the vertical crossline of the telescope off the point by the amount of the actual horizontal collimation error.

Step 3. You must align the vertical crossline on the selected point by lateral movement of the reticle within the telescope. Newer models of the T2 have two pull-action capstan adjusting screws arranged horizontally and on opposite sides of the telescope. To align the vertical crossline, loosen one screw and tighten the opposite one an equal amount. The crossline will move laterally toward the screw that you tighten. Do not try to make the entire adjustment in one step. Loosen and tighten the opposite screws in small amounts. Proceed in this manner until the crossline is aligned on the station.

Note. On the older-model T2 (63 series or older), the adjusting screws are those used for verticality adjustment. To move the vertical crossline, loosen (tighten) the two adjusting slant screws on the right side of the telescope equally, and tighten (loosen) the single adjusting screw on the left side of the telescope. The adjusting screw or screws on one side of the telescope must be loosened before you tighten the screw or screws on the opposite side. Continue the lateral movement of the vertical crossline until it is aligned precisely on the selected point.

Step 4. Perform the test again, and make additional adjustments until the difference between the direct and reverse pointings is less than 0.050 mil.

Note. It is preferred that the adjustment be made with the theodolite in the reverse position. This adjustment also can be made with the telescope in the direct position by using the mean value for the direct pointing (0000.202 + 0.300 = 0000.502).

3-34. T2 VERTICAL COLLIMATION TEST AND ADJUSTMENT

a. Purpose. The purpose of the vertical collimation adjustment is to make the line of sight horizontal when the vertical circle reads 1,600 mils with the telescope in the direct position (4,800 mils with the telescope in the reverse position) and the ends of the collimation level bubble aligned.

b. Test.

Step 1. To test the vertical collimation, select a well-defined point at least 100 meters from the instrument and approximately on a horizontal plane with the theodolite.

Step 2. With the telescope in the direct position, give the recorder a direct vertical circle reading to the point. Be sure that the collimation level bubble is precisely aligned.

Step 3. Plunge the telescope to the reverse position, and give the recorder a reverse vertical circle reading to the same point. Ensure the collimation level bubble is precisely aligned before you read the vertical circle, and check it after you take your reading.

Note. The T2 adjustment for vertical collimation requires only one set of direct and reverse readings. Be very careful, therefore, to prevent pointing and reading errors.

c. Adjustment. The sum of the direct and reverse readings should be 6,400 mils. Any difference between the sum of the two readings and 6,400 mils is the apparent vertical collimation error, or twice the actual error. If the difference exceeds 0.150 mil for the T2 theodolite, the vertical collimation level should be adjusted.

Step 1. With the telescope in the reverse position and accurately sighted on the selected point, use the coincidence knob to set the fractional part of the corrected reading on the micrometer scale. The corrected reading is the reverse reading with one-half of the apparent error (actual error) applied. Then apply the actual error to determine the corrected vertical reading. In the example below, the actual error must be added. So, add 0.100 mil to 4,804.607 mils to determine the corrected reading of 4,808.707 mils. Set the fractional part of the corrected reading (0.707 mil) on the micrometer scale by using the coincidence knob.

EXAMPLE

Direct reading                                             1595.193
Reverse reading                                           4804.607
Sum                                                            6399.800
Apparent error                                                  0.200 mil
(6400 - 6398.800)

Actual error                                                       0.100 mil
(0.200 ÷ 2)

Note. If the sum of the two readings was greater than 6,400 mils, then the actual error would be subtracted from the reverse reading to determine the corrected vertical reading.

Step 2. Obtain coincidence on the main scale at the correct vertical reading by using the collimation level tangent screw. With the telescope sighted at the selected point and the corrected reading on the vertical scale, the collimation level bubble will not be aligned.

Step 3. Align the images of the ends of the collimation level bubble by rotating the two capstan adjusting screws located directly below and to the right and left of the collimation level mirror (prism). Adjust the bubble images by loosening one adjusting screw and tightening the other by an equal amount. Figure 3-31 shows the location of the adjusting screws. Continue the test and adjustment procedures until the vertical collimation error is less than 0.050 mil.

Note. It is preferred that the adjustment be made with the theodolite in the reverse position. This adjustment also can be made with the telescope in the direct position by using the corrected direct reading to adjust the instrument; that is, 1595.293 (1595.193 + 0.100).

CAUTION

It may be difficult for you to remember where the various adjusting screws are located when you need to make adjustments on the theodolite. You should always refer to the technical manual published for each type of theodolite. Always perform the tests and adjustments in the sequence in which they are listed. After you make an adjustment, perform the test again to check for accuracy before you go to the next test and adjustment. When making adjustments, do not force the movement of the adjusting screws or exert too much pressure. The screws could be damaged and make your instrument unserviceable. A correctly adjusted theodolite is essential in obtaining accurate survey results.





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