THEODOLITE SURVEYING

                                             THEODOLITE SURVEYING

Theodolites are survey instruments designed to measure horizontal and vertical angles precisely. In addition to measuring horizontal and vertical angles, theodolites can also be used to mark out straight and curved lines in the field.

Theodolites (also called transits) have gone through three distinct evolutionary stages during the twentieth century:

1. The open-face, vernier-equipped engineers’ transit (American transit; Figure G.8).

2. The enclosed, optical readout theodolites with direct digital readouts, or micrometer equipped readouts (for more precise readings; Figure 4.4).

3. The enclosed electronic theodolite with direct readouts (Figure 1.7).

Most recent manufactured theodolites are electronic, but many of the earlier optical instruments and even a few vernier instruments still survive in the field (and in the classroom)— no doubt a tribute to the excellent craftsmanship of the instrument makers. In past editions

Of this text, the instruments were introduced chronologically, but in this edition, the vernier transits and optical theodolites are introduced last (in Appendix G) in recognition of their

fading importance.

The electronic theodolite will probably be the last in the line of transits/theodolites. Because of the versatility and lower costs of electronic components, future field instruments will be more like the total station which combines all the features of a theodolite with the additional capabilities of measuring horizontal and vertical distances (electronically),

and storing all measurements, with relevant attribute data, for future transfer to the computer.

By 2005, some total stations even included a global-positioning system (GPS) receiver

Electronic Theodolites

   Electronic theodolites operate similarly to optical theodolites (Section G.4); one major difference is that these instruments usually have only one motion (upper) and accordingly have only one horizontal clamp and slow-motion screw.

   Angle readouts can be to 1_, with precision ranging from 0.5_ to 20_. The surveyor should check the specifications of new instruments to determine their precision, rather than simply accept the lowest readout as relevant (some instruments with 1_ readouts may be capable of only 5_ precision).

  Digital readouts eliminate the uncertainty associated with the reading and interpolation of scale and micrometer settings. Electronic theodolites have zero-set buttons for quick instrument orientation after the backsight has been taken (any angular value can be set for the backsight).

  Horizontal angles can be turned left or right, and repeat-angle averaging

is available on some models. Figures 1.7 and 4.5 are typical of the more recently introduced theodolites.

  The display windows for horizontal and vertical angles are located at both the front and rear of many instruments for easy access. Figure 1.7 shows the operation keys and display area typical of many of these Instruments.

After turning on some instruments, the operator must activate the vertical circle 

by turning the telescope slowly through the horizon; newer instruments don’t require this

referencing action. The vertical circle can be set with zero at the zenith or at the horizon; the

factory setting of zenith can be changed by setting the appropriate dip switch as described in

the instrument’s manual. The status of the battery charge can be monitored on the display

panel, giving the operator ample warning of the need to replace and/or recharge the battery.

 Angle Measurement in Theodolite

  Most surveying measurements are performed at least twice; this repetition improves precision and helps eliminate mistakes. Angles are usually measured at least twice:

 Once with the telescope in its normal position and once with the telescope inverted. After the theodolite has been set over a station, the angle measurement begins by sighting the left hand (usually) target and clamping the instrument’s horizontal motion. The target is then sighted precisely by using the fine adjustment, or slow-motion screw.

   The horizontal angle is set to zero by pressing the zeroset button. Then the horizontal clamp is loosened and the right-hand target sighted.

   The clamp is tightened and the telescope fine-adjusted onto the target. The hold button is pressed and the angle is read and booked (pressing the hold button ensures that the original angle stays on the instrument display until the surveyor is ready to measure the angle a second time).

  To prepare to double the angle (i.e., measure the same angle a second time), loosen

the clamp and transit the telescope. The left-hand point is now resighted, as described

above. To turn the double angle, simply press (release) the hold button, release the clamp,

and resight the right-hand target a second time. After fine-adjusting on the target, the double angle is read and booked.

   To obtain the mean angle, divide the double angle by 2. See Figure 4.6 for typical field notes.

Typical Specifications for Electronic Theodolites

The typical specifications for electronic theodolites are listed below:

•        Magnification: 26_ to 30_
•       Field of view: 1.5°
•          Shortest viewing distance: 1.0 m Angle readout, direct: 1_ to 20_; accuracy: 1_ to 20_*
•  Angle measurement, electronic and incremental: see Figure 45(b
• ·         Level sensitivity:
•    Plate bubble vial—40_/2 mm
•  Circular bubble vial—10_/2 mm

  *Accuracies are now specified by most surveying instrument manufacturers by reference to DIN 18723. DIN (Deutsches Institut fur Normung) is known in English-speaking countries as the German Institute for Standards.

  These accuracies are tied directly to surveying practice. For example, to achieve a claimed accuracy (stated in

terms of 1 standard deviation) of _5 seconds, the surveyor must turn the angle four times (two on face 1 and two

with the telescope inverted on face 2). This practice assumes that collimation errors, centering errors, and the like have been eliminated before measuring the angles.

Electronic theodolites are quickly replacing optical theodolites (which replaced the verniertransit). They are simpler to use and less expensive to purchase and repair, and their use of electronic components seems to indicate a continuing drop in both purchase and repair costs.

   Some of these instruments have various built-in functions that enable the operator to

perform other theodolite operations, such as determining remote object elevation and distance between remote points .

   The instrumentation technology is evolving so rapidly that most new instruments now on the market have been in production for only a year or two, and this statement has been valid since the early 1990s.

Theodolite station setup

1. Place the instrument over the point, with the tripod plate as level as possible and with

two tripod legs on the downhill side, if applicable.

2. Stand back a pace or two and see if the instrument appears to be over the station. If it

does not appear centered, adjust the location, and check again from a pace or two away.

3. Move to a position 90° opposed to the original inspection location and repeat step 2.

(Note: This simple act of “eyeing-in” the instrument from two directions, 90°

opposed, takes only seconds but could save a great deal of time in the long run.)

4. Check to see that the station point can now be seen through the optical plummet (or

that the laser plummet spot is reasonably close to the setup mark) and then firmly

push in the tripod legs by pressing down on the tripod shoe spurs. If the point is now

not visible in the optical plumb sight, leave one leg in the ground, lift the other two

legs, and rotate the instrument, all the while looking through the optical plumb sight.

When the point is sighted, carefully lower the two legs to the ground, keeping the

station point in view.

5. While looking through the optical plummet (or at the laser spot), manipulate the leveling

screws until the cross hair (bull’s-eye) of the optical plummet or the laser spot

is directly on the station mark.

6. Level the theodolite circular bubble by adjusting the tripod legs up or down. This

step is accomplished by noting which leg, when slid up or down, would move the circular

bubble into the bull’s-eye. Upon adjusting that leg, either the bubble will move

into the circle (the instrument is nearly level) or it will slide around until it is exactly

opposite another tripod leg. That leg is then adjusted up or down until the bubble

moves into the circle. If the bubble does not move into the circle, adjust the leg until

the bubble is directly opposite another leg and repeat the process. If this manipulation

has been done correctly, the bubble will be centered after the second leg has

been adjusted; it is seldom necessary to adjust the legs more than three times.

(Comfort can be taken from the fact that these manipulations take less time to perform

than they do to read about!)

7. Perform a check through the optical plummet or note the location of the laser spot to

confirm that it is still quite close to being over the station mark.

8. Turn one (or more) leveling screw(s) to ensure that the circular bubble is now centered

exactly (if necessary).

9. Loosen the tripod clamp bolt a bit and slide the instrument on the flat tripod top

(if necessary) until the optical plummet or laser spot is centered exactly on the station mark.

Retighten the tripod clamp bolt and reset the circular bubble, if necessary. When sliding the instrument on the tripod top, do not twist the instrument, but move it in a rectangular fashion. This precaution ensures that the instrument will not go seriously off level if the tripod top itself is not close to being level.

10. The instrument can now be leveled precisely by centering the tubular bubble. Set the

tubular bubble so that it is aligned in the same direction as two of the foot screws.

Turn these two screws (together or independently) until the bubble is centered. Then

turn the instrument 90°; at this point, the tubular bubble will be aligned with the third

leveling screw. Next, turn that third screw to center the bubble. The instrument now

Should be level, although it is always checked by turning the instrument through

180° and noting the new bubble location. See Section 4.13.2 for adjustment procedures.

On instruments with dual-axis compensation, final leveling can be achieved

by viewing the electronic display [Figure 5.24(b)] and then turning the appropriate

leveling screws. This latter technique is faster because the instrument does not have

to be rotated repeatedly.