SUUNTO PM5 - Manual (Page 14)

13
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.