2-54. Height of Obstructions. It is often necessary to locate stations so that lines of sight will miss
obstructions requiring signals to impractical heights. Blocks of higher timber will often be seen in
heavily wooded areas. Plot the blocks on the map and ensure that all lines miss them. Intersections
from water tanks, section corners, and other established stations give the positions of the obstructions.
The heights of trees must be measured in wooded areas. The simplest and most accurate measurement is
to lower a tape from the top of a representative tree. There are also various instrumental and improvised
methods using an Abney hand level, a celluloid triangle, or a theodolite.
2-55. Clearance of Lines. The distance by which lines of sight must clear obstructions in order to
avoid excessive refraction and dispersion of light varies with the type of vegetation cover and the other
physical conditions of the line. The determining factor seems to be the amount of heat transferred to the
atmosphere by the ground over which the line passes. Regions combining a bare ground surface with a
large daily temperature range require the greatest amount of clearance, and areas heavily timbered and
with a humid climate require the least.
a. Determining the amount of clearance required for a line is largely a matter of experience. The
following information shows the average minimum values:
Over water surfaces - 3 meters.
Over open plains where the sun is hot during the day and the atmosphere is dry - 9 to 12 meters.
Over cultivated land interspersed with wooded areas - 4.5 to 6 meters.
Over treetops - 3 meters.
b. If the obstruction is a narrow ridge with relatively small capacity for heat radiation, the
clearances may be safely reduced. If the line is parallel to the ground nearly the entire distance between
stations, the clearance may have to be increased. Extreme heat and drought present special problems.
The usual solution under unfavorable conditions is to specify the highest signal that the triangulation
party is equipped to build economically.
2-56. Final Selection of Station Sites. The final selection of a station site requires a compromise of
several requirements--the intervisibility of stations, the permanence of marks, the strength of figures, the
wishes of property owners, and accessibility.
a. Intervisibility of stations is a very important requirement, along with the topics of curvature,
refraction, and clearance. The possibility of securing visibility with a signal building at optional
locations should also be investigated. The essential lines must be clear--this condition must govern the
selected site.
EN0593
2-36
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