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Compass and Map - part 5
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If a compass is not available, one's position can be determined by sighting three known landmarks.
To identify unknown landmarks reverse the procedure for identifying position. With only one line of sight, the exact position of the landmark can only be determined if one has enough knowledge of the country and of maps to know how distant the feature is, its shape, and its position with reference to intervening ridges. Determining intervisibility.—Experience in reading topographic maps can usually show approximately whether any two points should be intervisible. This may be checked by a procedure such as follows: 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. An easy variation is to substitute for the dotted line a rubber band upon which the equal divisions of the difference in altitude have been marked. By stretching the rubber band between the station and the peak, one can easily determine whether the elevation of any intervening obstruction is higher or lower than the elevation division marked on the rubber band at that point. If a distance of more than a few miles exists between the station and the peak, allowance must be made for curvature of the earth and refraction of light by the atmosphere. Contour topography.—The most accurate method of representing land forms upon a map is provided by contour topography, the method used throughout the United States in maps of the Geological Survey. On the back of most of these maps is an excellent diagram showing a landscape in relief drawing and in contour topography. A contour is a line every point of which is at the same elevation. The shore line at mean low tide is the zero contour for all United States surveys and maps. If the coast has a ten-foot tide, the water will follow up each inlet and cross each beach to establish, in effect, a new contour line, ten feet higher. Ten feet would then be the contour interval. The interval between contours usually varies according to the scale of the map and roughness of the country, but is uniform throughout a particular map.
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