Compass and Map : Page 235

Draw a dotted line between the observation station and the peak whose visibility is to be tested and determine their difference in elevation. Draw a line from the higher point at right angles to the dotted line, and, if the difference in elevation is, for example, 500 feet, mark five equal divisions on the right-angle line. Checking along the dotted line, draw a second right-angle line to represent the ridge that might obstruct the view. Using the same divisions as before, mark on this second line the difference in elevation between the station and the ridge. A line drawn from the "summits" of the right-angle lines representing the station and obstruction is then extended as far as the peak. If the peak line is high enough to rise above the extended line, the peak can then be seen from the observation station.