The trigonometrical correlation is
H = h x cos α
Where H is the true or corrected height, h is the observed height and α (alpha) is the ground slope angle. With the aid of the above equation the correction can also be made in the distance. In this case h means the distance measured along the ground and H is the horizontal distance sought. If the corrected distance is used no correction in the height observed is needed. When calculating the horizontal distance by using the ground distance and the slope, it must be pointed out that an error is introduced if the slope is measured from eye level to the trunk base. Measuring the slope along the ground would be cumbersome and inconvenient. No error is introduced, however, when the slope angle is measured from eye level to sighting mark made or placed on the trunk at eye level whereby the two lines of measurement become parallel. The true angle of slope is 9 degrees.
The example shown in the follow- ing figure illustrates both meth- ods of calculation.
Method 1. Measure the ground distance. This is found to be 25 m / 82 ft. Then measure the slope angle. This is 9 degrees. Read percentages of top and ground points. These are 29 and 23 per cent.
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