the SEABEEs will generally concern topographicmaps for use in construction.HYDROGRAPHIC MAPS.— A hydrographicmap shows the shorelines, the location and depth ofsoundings, and often the topographic and otherfeatures of lands adjacent to the shorelines. It alsoshows the locations of both horizontal and verticalcontrol in the area.SPECIAL-PURPOSE MAPS.— These are mapsdeveloped for specific purposes. A PRELIMINARYMAP developed from a preliminary survey of ahighway, a LOCATION MAP showing the alignmentof the located line, and a RIGHT-OF-WAY MAPshowing the boundaries of the right-of-way and theadjacent lands all come under the heading of special-purpose maps.MOSAIC AND OVERLAYS.— The aerialphotographic mosaic is constructed from two or moreoverlapping prints joined so that they form a singlepicture. Usually, vertical photographs are used and amaplike result is obtained; however, obliquephotographs may be used, in which case the result isa panorama. The mosaic has become increasinglyuseful in cartography and related fields since WorldWar I. Large geographic areas may be represented inthis manner with each feature of terrain assuming itsnatural appearance and approximating itsproportionate size. The U.S. Army TopographicCommand has a vari-colored map of the entire UnitedStates and other countries that was developed frommosaics. The Army calls it a PICTOMAP; this is thetype of map that is generally used in a war zone.Aerial photographs may be converted into linemaps by the use of overlays. Usually, these are madeby tracing the details from the photograph ontransparent paper or vellum and adding suchmarginal data as desired. This line map may then bereproduced quickly by blueprinting or by lithography.Figure 11-2 shows a vertical aerial45.735Figure 11-2.-Example of an aerial photograph.11-11
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