In previous classes we used the Brunton Pocket Transit for measuring horiztontal angles, and as a hand level. It has, however, one more very important usage--as a clinometer, or device for measuring vertical angles. For example, if a field worker needed to calculate the height of a tree or a cliff, or the depth of a canyon (a vertical distance), she or he could do so without having to climb the object or feature, thereby risking life and limb [example].
1. Open the cover of the Brunton so that the sighting arm is fully extended parallel with the face of the instrument.
2. Turn the peep sight or sighting tip at a right angle to the sighting arm.
3. Position the mirror at approximately a 45 degree angle to the instrument's face.
4. Hold the compass in a vertical plane, with the sighting arm pointed toward your eye, but approximately one foot away so that the point being sighted and the axial line in the sighting window can be focused clearly.
5. Look through the window of the lid and find the point to be sighted, then tilt the compass until the point of the sighting arm, the axial line of the window, and the point sighted coincide.
6. Move, actually rotate, the clinometer by the lever on the back of the compass body until the tube bubble, as observed in the mirror, is centered.
7. Check to make sure the sights are still aligned, then bring the compass down and read and record the angle.
8. Repeat the entire procedure as a check.
This is easy if a few simple steps are followed. 1. Measure the distance from the object to some point where the top of the object is visible. 2. Standing at this point, measure the vertical angle to the top of the object. 3. Given that the object is at a 90 degree angle to the earth's surface, the field worker now knows the length of one side, and two adjacent angles, of a triangle. Using a little trigonomety, the height of the object equals the horizontal distance times the tangent of the vertical angle plus the observer's eye height [example].
A bit more difficult, but not much. There are two ways to do it. The first involves measuring the horizontal distance from the top of the landform to the bottom, and a second (rod) person. It also requires the use of a range pole with the eye height of the observer (instrument person) marked with black tape. Standing at the edge of the landform, the instrument person sights on the eye height marker on range pole held by the rod person at the bottom of the landform [example]. Noting the vertical angle and applying simple trig, the depth of the landform is obtained using the same equation as that for measuring the height of an object on flat ground, except the eye height should not be factored in.
Now, here is where it starts getting a bit more complicated and fun at the same time. In some cases, such as with the sides of a gently sloping valley, it is easier to make a direct slope measurement than a horizontal distance measurement. When this is done, the equation changes slightly as the tangent of the vertical angle is used rather than the sine [example]. Furthermore, it is possible to make this calculation without the aid of a second person holding a range pole. In such situations the observer must envison a range pole at the bottom of the landform and estimate a point approximating her or his eye height.
It shouldn't take a genius to figure out that what works going one direction works going the opposite direction [example].
If one can use a clinometer and a bit of trigonometry to measure the heights and depths of objects and landforms, they can make multiple calculations and determine the height of an object on top of another (e.g. the height of a cross on top of a church) [example], the height of an object or feature on top of a landform [example], and even the height of something in a valley when looked at from above.
In some cases, such as when one needs to know the slope of a surface, he or she can determine the gradient by simply placing the clinometer directly on the ground. Once in place, the clinometer lever is rotated until the tube bubble is centered. The compass is then picked-up and the angle read and recorded.
Obviously, the accuracy of this method is specific to only one point on the surface. Several readings, in various places up and down the slope should, therefore, be taken and averaged. For soft surfaces, such as sandy slopes, fieldworkers can improve the accuracy of direct clinometer readings by placing the Brunton on the field notebook after it is placed on the surface. On rocky surfaces better readings can be taken by placing a range pole or stadia rod on the slope--oriented up and downslope--and placing the Brunton on the pole, thereby compensating for the surface irregularities.
Simple trigonometry is almost as basic to field work as is arithmatic. It is not only used in taking vertical measurements but can be used in horizontal measurements as well. Take for instance the case in which someone would like to know the width of a river without a boat or getting wet [example]. Remember, all one needs to know is a distance and two adjacent angles, or one angle and two adjacent sides. If one of those angles is 90 degrees, the job is super easy.
Created by William E. Doolittle. Last revised 26 June 2013, wed