How to Measure Angles Using a Theodolite - Part 2 Taking a measurement

How to Measure Angles Using a Theodolite

A theodolite is an instrument used commonly by builders and engineers to measure precise angles, which is necessary for large scale construction projects. A basic modern optical theodolite typically consists of a small telescope which is connected to two angle measuring mechanisms, one for measuring horizontal angles and one for measuring vertical angles. It sits atop a rotatable base with a leveling mechanism on a tripod. Once the theodolite is set up, the telescope is turned to spot the desired point and then the angle from the point that the theodolite is placed to the point spotted in its telescope can be read through the eyepiece of the scope.

Taking a measurement

Step 1

Unlock the upper horizontal clamp, and rotate the theodolite until the arrow in the rough sights is lined up with the point you wish to measure, then lock the clamp. Use the upper horizontal adjuster (not the clamp) to align the object between the two vertical lights in the sight.

Step 2

Look through the small eyepiece, and using the fine adjustment knob to get a precise horizontal line up with your object. The degrees from your reference are measured on the horizontal degree scale, the minutes and seconds on the fine adjustment scale (ex. 30 degrees 10'30").

Step 3

Unlock the vertical clamp and look through the sight while moving the theodolite up and down to find the precise spot vertically on your object that you'd like to measure. Lock the clamp and use the fine vertical adjustment knob to get a precise fix on the point you've chosen. Then look through the small eyepiece and read off the degrees, minutes and seconds from the vertical scale and the fine adjustment scale as you did for the horizontal scale. If your object is up high you'll need to do a rough horizontal adjustment first, then do the vertical measurement, then readjust for the final horizontal measurement. These two coordinates give the exact angle between your reference and your point of interest, but you can also measure the angle between two points by comparing their two measurements, or by setting the first point as the reference.

HOW TO USE A SURVEYOR'S THEODOLITE

A surveyor's theodolite is used to measure horizontal and vertical angles. These measurements are used to plot boundary lines, building foundations and utility routing. A theodolite measures distance manually by chains of standardized length or metal measuring tapes along the length of the desired angle. Land-surveying students use a theodolite as a way to learn the principles of angle measurement. Surveying on the job requires the use of more-advanced surveying instruments.

Drive a surveyor's nail into the ground at the point where you want to set up the theodolite. Angles will be measured from this point, as will distances.

Set up the tripod legs, taking care to set the height where the instrument sight will be at a comfortable eye level. Be certain to check that the hole in the center of the mounting plate is located over the nail. Press each leg into the ground by stepping on the bracket at the bottom of each leg.

Fine-tune the position of the legs so that the mounting plate on the top of the tripod is as eye-level as possible.

Remove the theodolite from the case. Most theodolites have a sturdy handle on top. This is the best place to lift the instrument. Gently place the instrument on the mounting plate and screw in the mounting knob beneath the instrument.

Adjust the theodolite to level by adjusting the tripod legs, using the bull's-eye level. Fine-tune the adjustment with the leveling knobs on the instrument.

Adjust the small sight, called the vertical plummet, at the bottom of the theodolite. This sight allows you to make certain the instrument is centered directly over the nail. Fine-tune the vertical plummet by adjusting the knobs on the bottom of the theodolite.

Look through the main scope and aim the crosshairs at the point to be measured. Twist the locking knobs to hold the theodolite in position on the exact point. View the horizontal and vertical angles in the viewing scope on the side of the instrument.

Tip

To fully understand the use of surveying instruments, take a course from a licensed surveyor. The hands-on experience is invaluable in understanding the more abstract concepts associated with mathematics and instrument operation.

Warning

Use care when transporting and operating the theodolite. These precise measuring devices are very sensitive, and they are expensive to repair if damaged.

THEODOLITES SET-UP

THEODOLITES SET-UP

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, 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.
4. Check 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). Then push the tripod legs firmly by pressing down on the tripod shoe spurs.
5. While looking through the optical plumb (or at the laser spot), manipulate the leveling screws (one, two or all the three at a time) 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.
7. Perform a check through the optical plummet or note the location of the laser spot to confirm that it is still quite being over the station mark.
8. Turn one (or more) leveling screws to be sure that the circular bubble is now exactly centered (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 exactly centered on the station mark. Retighten the tripod clamp bolt and reset the circular bubble, if necessary.

10. The instrument can now be precisely leveled 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 which 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°.
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