Theodolite Reading

7/29/2019 Theodolite Reading

1/26

CMgt2340 Surveying

Chapter 5 Open-faced

transit, Vernier readingsand Theodolites


2/26

Angular Measurement Terms

Optical Plummet Theodolite

Plumb Bob Transit

Theodolite Precision instrument

Transit Repeating instrument

Vernier Shorter scale on top of alonger scale used to measure angles


3/26

Uses of Transits and Theodolites

Accurate measurement or layout of

horizontal and vertical angles

Low order differential leveling

Prolonging straight lines


4/26

Major Parts of a Transit

Alidade

Horizontal Circle AssemblyLeveling Head Assembly


5/26

Transits Versus Theodolites

Transits Repeating instruments, 4 screw, plumb bob and string, vial level

on telescope, metal hor./vert. circles with verniers inscribed, and

rotation of instrument occurs on a brass spindle rather than ball

bearings

Theodolites Precision instruments, 3 leveling screws, optical plummet, fish-

eye level on leveling head, Shorter telescope, smallergraduations on verniers if not electronic


6/26

Major Types of Verniers

Direct or single

Double or double direct

Folded Vernier


7/26


8/26

Reading Double Vernier Transits

Clockwise (Angle Right) Reading- Find the zero mark on the vernier.

Find the set of numbers on the bottom scale that increase going

right to left.


9/26


Clockwise (Angle Right) Reading- Determine the last known mark going right to left on the bottom

scale (57 degrees and just past 30 minutes).


10/26

Reading Double Vernier Transits Clockwise (Angle Right) Reading- Moving left on the vernier from zero, find the vernier line that

vertically aligns with a scale mark. These two marks should beframed by two vernier marks right inside two scale marks.


11/26


Clockwise (Angle Right) Reading- Add these minutes to the original reading

(57d 30m + 7m = 57 d 37m).


12/26


Counter-Clockwise (Angle Left) Reading-\ Find the zero mark on the vernier.

Find the set of numbers on the bottom scale that increase going left to

right.


13/26


Counter-Clockwise (Angle Left) Reading- Determine the last known mark going left to right on the bottom

scale (302 degrees).


14/26


Counter-Clockwise (Angle Left) Reading- Moving right on the vernier from zero, find the vernier line that

vertically aligns with a scale mark. These two marks should be

framed by two vernier marks right inside two scale marks.


15/26


Counter-Clockwise (Angle Left) Reading- Add these minutes to the original reading (302 degrees 23

minutes).


16/26

Typical Mistakes in Reading

Verniers Not using a magnifying glass.

Reading the wrong direction from zero.

Failing to read directly on the line.


17/26

Setting up a Transit

1.Remove transit from its case. Hold it by its standards, neverby its telescope.

2. Place on tripod. Screw leveling head on tripod snuggly while

holding instrument with one hand.

3. Place instrument roughly over the desired point and adjust

the legs of the tripod so that 1) the instrument is at a convenient

height, and 2) the tripod plate is nearly level.

4. Tighten the wing nuts on the tripod legs when in position.

5. Secure a plumb bob to the plumb bob chain and hook slightly

over desired point.


18/26


6. Slowly secure the tripod legs into the ground without jarring

the instrument.

7. Loosen two adjacent leveling screws slightly to release

tension so that the transit can be shifted laterally until it isprecisely over the point. Then retighten the same two screws.

8. Center the leveling tubes or plate levels directly over two

opposite leveling screws by rotating the alidade.

9. Begin adjusting the level bubble by turning the two (opposite)

leveling screws that are directly beneath the level vial. Propertension is important at this stage. Leave the screws firmly

secure but not bound.


19/26


10. Duplicate this procedure using the two remaining leveling

screws and the opposite level vial.

11. Repeat the leveling steps a second or third time to eliminate

any minor adjustments to the level bubbles. 12. Once the instrument is accurately leveled, the screws may

be loosened slightly and the leveling head shifted to position the

plumb bob point directly over the desired point of setup. Care

must be taken if this is done. It can be shifted right or left but

not turned on the leveling head. Any leveling screw must beretightened back to its original point.


20/26

Measuring Angles in the Field

1. After setting the instrument over the first point, set

the scales to zero.

A. Loosen both the upper and lower motion clamps.

B. Hold the alidade stationary, and revolve the circle by

pushing on the circle underside with the fingertips.

C. When zero is close to the index point of the vernier,

tighten the upper clamp.

D. With a magnifying glass, slowly turn the upper tangent

screw until the zeros are precisely in line.


21/26


2. Sight the initial point (backsight).

A. With the upper clamp tightened and the lower clamp

loose, turn and point the telescope towards the initial point.

B. Once relatively close to the desired position, tighten thelower clamp.

C. While observing the point through the telescope, slowly

turn the lower tangent screw until precisely in line with the

initial point.


22/26


3. Turn the horizontal angle.

A. Loosen the upper clamp and turn the telescope clockwise

or counter-clockwise toward the final point (foresight).

B. When point is close to the vertical cross hair, tighten theupper clamp.

C. Slowly turn the upper tangent screw until the vertical

cross hair is precisely in position.


23/26


4. Read the angle.

A. Observe the vernier and determine the angle using a

magnifying glass.

B. Record the value in the field book in the appropriatecolumn.


24/26


5. Repeat the angle.

A. After the initial angle has been recorded, plunge or

invert the telescope.

B. Loosen the lower motion and sight the initial point ororiginal backsight point.

C. Tighten the lower clamp.

D. Repeat steps 5 and 6 except that the telescope is now

inverted and the initial horizontal angle setting is that of the

initial angle.

E. Record the doubled angle in the appropriate column.


25/26

Recording Angles in the Field Book


26/26

References Cited

Examples and step by step tutorials

were copied directly from the following:

Mid-America Vocational CurriculumConsortium, Inc., Basic Surveying

Technology, Stillwater, OK: Oklahoma

State Department of VocationalTechnical Education, 1987

Documents

Theodolite Reading