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When calculating the hori-
zontal distance by using
the ground distance and
the slope angle, it must be
pointed out that an error is
introduced if the slope is
measured from eye level to
the pillar base. Measuring
the slope along the ground
would be cumbersome and
inconvenient. No error is
introduced, however, when
the slope angle is mea-
sured from eye level to a
sighting mark made or
placed on the pillar at eye level (Fig. 11) whereby the two lines of measurement
become parallel. The true angle of slope is 9 degrees. The example shown in Fig. 11
illustrates both methods 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.
Calculate:
1.6 m
[5½ ft]
29%
1.6 m
[5½ ft]
9%
25 m
[82 ft]
23%
24.7 m [81 ft]
Fig. 11. Calculating horizontal distance by using
ground distance and slope angle
23
100
-------- -
29
100
-------- -
52
100
-------- -
=+