Slope Profiles Part 2: Measuring Slope Angles

The basic principle of slope angle measurements, which applies to the use of protractors, clinometers and total stations.


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Basic principle 

There are several ways to calculate slope angle. Common measuring equipment such as clinometers and total stations both use a similar method of raising the geometry of the slope below.

angle

By placing poles of equal heights at Points A and B, the line from the top of Pole A to Pole B is parallel to the actual slope. The line has the same angle as the slope below.

angle

The “easiest” way (which is not the same as the most accurate way) to measure this angle would be to place a protractor on the top of Pole A, drag a string from the top of Pole A to Pole B and read the angle. Note: This method would only be accurate if your protractor is perfectly flat at 180°.

prot

Clinometers 

Clinometers are essentially protractors that are a little bit more sophisticated. Clinometers provide more accurate measurements because their design helps the surveyor correct for holding error (i.e. ensuring that the base of the “protractor” component is perfectly horizontal).

clinometer

A clinometer

clinometer2

Some compasses have clinometers built into them.

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… you can also make one of your own with a protractor, straw, and a weighted string (if you make one as shown in the picture, you’d have to subtract the angle from 90 degrees. Can you figure out why?).

Total stations 

Total stations also use the same principle to calculate slope angle. The total station would be positioned at Point A, and the total pole would be positioned at Point B.

A total station and a person holding a total pole.

A total station and a person holding a total pole.

Slope angle is measured by aiming the total station at the top of the total pole. An eyepiece allows the surveyor to see where the total station is pointing towards.

total

At the same time, total stations can also measure the distance between the total station and the total pole. Once the total station has been aimed at the total pole, a laser is shot towards a mirror at the top of the total pole. The amount of time it takes for the laser to be reflected back to the total station is a function of distance (the farther it is, the longer it takes for the laser to return).

This is done through a press of a button, and the machine will record both the slope angle and the slope distance for you. Because total stations can measure long distances, the total station can remain at the same spot (i.e. Point A) while the total pole is moved to farther distances (Point C, D etc.) for more measurements.

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