surveying iii (topographic and geodetic surveys) - Modern Prepper

2-54. Height of Obstructions. It is often necessary to locate stations so that lines of sight will missobstructions requiring signals to impractical heights. Blocks of higher timber will often be seen inheavily wooded areas. Plot the blocks on the map and ensure that all lines miss them. Intersectionsfrom 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 isto lower a tape from the top of a representative tree. There are also various instrumental and improvisedmethods 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 toavoid excessive refraction and dispersion of light varies with the type of vegetation cover and the otherphysical conditions of the line. The determining factor seems to be the amount of heat transferred to theatmosphere by the ground over which the line passes. Regions combining a bare ground surface with alarge daily temperature range require the greatest amount of clearance, and areas heavily timbered andwith a humid climate require the least.a. Determining the amount of clearance required for a line is largely a matter of experience. Thefollowing 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, theclearances may be safely reduced. If the line is parallel to the ground nearly the entire distance betweenstations, 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 triangulationparty is equipped to build economically.2-56. Final Selection of Station Sites. The final selection of a station site requires a compromise ofseveral requirements--the intervisibility of stations, the permanence of marks, the strength of figures, thewishes 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 optionallocations should also be investigated. The essential lines must be clear--this condition must govern theselected site.EN0593 2-36