Surveying by using Digital Theodolite

Surveying by using Digital Theodolite

South ET-05 Electronic

A theodolite is a telescope mounted to very sensitive horizontal and vertical protractors. It is capable of measuring angles and, when used in conjunction with graduated reference objects, distances with a high degree of accuracy.

1 Setup

1.1 Tripod setup

In this section we learn how to set up the tripod and roughly center and level it over a small positioning mark, such as a divot in a penny glued to the _oor, a nail head, or any other small _xed object.

a. Place the tripod over the positioning mark, setting the legs at a convenient height, and roughly center and level the tripod head by eye.

b. Suspend the plumb bob included in the theodolite box from the hanger beneath the tripod head.

c. Readjust the tripod to center the plumb bob over the positioning mark by moving all three tripod feet by the same amount in the same direction.

d. Firmly _x the tripod feet in position. If necessary, adjust the heights of the tripod legs to re-center the tripod within 1 cm of the reference mark.

e. Tighten the leg clamps on the tripod.

Figure 1: Theodolite-in-a-box.

1.2 Theodolite setup

In this section we learn how to unpack, set up, center, and level the theodolite.

1.2.1 Seating and centering the theodolite

a) Examine how the tripod is seated in its box. See Fig. 1.

b) Lift the theodolite out of its box by the handle never by the telescope.

c) Place the theodolite on the tripod head and screw in the centering screw while holding onto the handle. Leave the centering screw just loose enough that the theodolite can still slide around the tripod head.

d) Looking through the optical plummet, focus the centering index mark. Slide the theodolite on the tripod head until the reference mark is centered in the optical plummet.

e) Fully tighten the centering screw. Look through the optical plummet again and adjust the theodolite foot screws for _ne alignment with the reference mark.

1.2.2 Leveling the theodolite

A. Referring to Fig. 2, roughly level the instrument using the circular level:

1. Turn the leveling screws A and B in opposite directions to center the bubble along the AB axis.

2. Turn leveling screw C to bring the bubble to the center of the circular level.

B. Referring to Fig. 3, precisely level the instrument using the plate level.

1. Free the horizontal motion clamp and rotate the instrument horizontally until the plate level is parallel with line AB.

2. Bring the bubble to the center of the plate level by turning screws A and B in opposite directions.

3. Rotate the instrument by 90◦ around its vertical axis and turn screw C to center the bubble once more.

4. Repeat procedures i. and ii. for each 90◦ rotation of the instrument and check that the bubble is correctly centered for all four points. If after 180◦ of rotation, the bubble is of center, remove half the error in the bubble centering. Check that when you have swung another 180◦ back to the initial point, the bubble offset is the same as the offset you allowed to remain in the 180◦ rotated position.

Once the theodolite is leveled, double check the optical plummet to make sure that it is still centered. If not, repeat the procedure in section 1.2.1 (d) through all of section 1.2.2.

                                                                          

Figure 2: Diagram for rough leveling with the circular level.

          

Figure 3: Diagram for _ne leveling with the plate level.

2 Measurements

In this section we learn how to take accurate readings of horizontal and vertical angles using a theodolite.

a. Determine how many reference points you will need to measure, and make sure that their horizontal and vertical positions are clearly marked. In order of measurement, these reference points will be referred to as X, Y and Z in these instructions.

b. Turn on the theodolite by pressing the green power key.

2.1 Horizontal measurement

To avoid confusion, all horizontal angle measurements should be completed before taking vertical angle measurements.

(a) Take face-left angle measurements.

1. Rotate the telescope about the turning axis until the instrument is in face-left position.

2. Press the OSET key and rotate the telescope horizontally to give a reading between 0◦ and 1◦.

3. Press HOLD to lock this value.

4. After making sure no one is in the telescope line of sight, press the laser on button on top of the telescope. Free the horizontal and vertical clamps and swing the laser spot to reference point X.3

5. Roughly align the laser spot and focus it using the focusing ring on the telescope. Tighten the horizontal and vertical clamps. Turn o_ the laser.

6. Place a blank sheet of paper in front of the telescope, and, looking through the telescope, focus the crosshairs using the eyepiece.

7. Remove the paper and focus the telescope on the reference object using the focusing ring.

8. Check for parallax by moving your head up and down and left to right. If the crosshairs move, refocus the crosshairs, and refocus the reference object as described above.

9. Looking through the telescope, home in on the reference object using the slow-motion screws. For horizontal angle measurements, the vertical crosshair must be perfectly aligned with the reference point, but the horizontal crosshair need not be, as long as approximately the same part of the vertical crosshair is used for each horizontal measurement.

10. Check the telescope pointing, book the horizontal angle reading, and check the booking.

11. Release the HOLD key (only applies to the very first reading).

12. Repeat steps 4 to 10 for reference points Y, Z, etc.

(b) Take face-right angle measurements to complete a round of angles.

1. Rotate the telescope about the turning axis until the instrument is in face-right position.

2. Repeat steps (a) 4 - 10 in the face-right position. Now you've booked a series of angles between 0◦ and 360◦ and a series between 360◦ and 720◦. This completes a round of angles.

(c) Take at least one more round of angles using an initial horizontal reading between 90◦ and 91◦ for the second round, 180◦ and 181◦ for the third round, etc. If you take only two rounds of angles, make sure that the readings agree within twice the precision of the theodolite.

(d) Once the desired precision is achieved, take the mean value of the measured angles.

2.2 Vertical measurement

The vertical angle scale is referenced absolutely to the ground, so it is impossible to use different initial readings for the multiple rounds of angles.

(a) Take face-left angle measurements.

1. Rotate the telescope about the turning axis until the instrument is in face-left position.

2. After making sure no one is in the telescope line of sight, press the laser on button on top of the telescope. Free the horizontal and vertical clamps and swing the laser spot to reference point X.

3. Roughly align the laser spot and focus it using the focusing ring on the telescope. Tighten the horizontal and vertical clamps. Turn o_ the laser.

4. Place a blank sheet of paper in front of the telescope, and, looking through the telescope, focus the crosshairs using the eyepiece.

5. Remove the paper and focus the telescope on the reference object using the focusing ring.

6. Check for parallax by moving your head up and down and left to right. If the crosshairs move, refocus the crosshairs, and refocus the reference object as described above.

7. Looking through the telescope, home in on the reference object using the slow-motion screws. For vertical angle measurements, the horizontal crosshair must be perfectly aligned with the reference point, but the vertical crosshair need not be, as long as approximately the same part of the horizontal crosshair is used for each vertical measurement.

8. Check the telescope pointing, book the vertical angle reading, and check the booking.

9. Repeat steps ii. to viii. for reference points Y, Z, etc.

(b) Take face-right angle measurements to complete a round of angles.

1. Rotate the telescope about the turning axis until the instrument is in face-right position.

2. Repeat steps (a) ii. - ix. in the face-right position. This completes a round of angles.

(c) Take at least one more round of angles. If you take only two rounds of angles, make sure that the readings agree within twice the precision of the theodolite.

(d) Once desired precision is achieved, take the mean value of the angles measured.