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Show charts which it produces, also does topographic mapping in such areas if the Geological Survey has not mapped the required areas or is unable to map them in time for the Coast Survey's use. The Forest Service of the Department of Agriculture is another producer of topographic maps, its operations being confined to the national forests. The Tennessee Valley Authority also has a staff producing such maps in the area under its control, in cooperation with the Geo- logical Survey. By interdepartmental agreements, the Geological Survey is recognized as the principal producer of topographic maps, in the sense that it does the great majority of the work, and that the maps made by other agencies are submitted to it for final editing before publi- cation and general distribution. Publication Scales The topographic maps published by Federal mapping agencies vary in scale from 1 : 24,000 to 1 : 250,000. In general, maps distributed to the public are now pro- duced at scales of 1 : 24,000, 1 : 31,680, and 1 : 62,500. The choice of scales is based upon the type of terrain and the particular use to be made of the maps. In recent years, there has been an increasing demand for maps at the larger scales and at present mapping at these scales is considerably in excess of that at the smaller scales. Topographic maps are produced for purely military purposes at scales of 1 : 25,000, 1 : 50,000, 1 : 100,000, and 1 : 250,000. In general, these maps are prepared by photographic reduction or enlargement of the civil editions. In the process, the military grind is added and the border information changed to provide data which the civil editions do not require. Contour Intervals The contour intervals for topographic maps at scales of 1 : 24,000, 1 : 25,000 and 1 : 31,680 are generally 5, 10, or 20 feet, depending upon the terrain. Maps at scale 1 : 50,000 and 1 : 62,500 use intervals of 10, 20, or 40 feet and sometimes 100 and 200 feet where the terrain is particularly rugged. Maps at scales smaller than 1 : 62,500 usually have larger contour intervals in proportion to their scales. National Map Accuracy Standards Since 1941, nearly all topographic maps, produced by or for Federal mapping agencies, have been prepared on the basis of standard specifications for accuracy. By this procedure, each user of a topographic map is assured that it has been properly made and that he can rely upon the information shown. To bring this fact to the attention of the map user there is imprinted at the bottom of each sheet the statement, "This map complies with National Map Accuracy Standards." Status of Topographic Mapping The status of topographic mapping is indicated in figure 32. There are available to the public two status maps in greater detail which are published annually by the Geologi- cal Survey. One map shows the status of mapping at scales of 1 inch equals 1 mile or larger, and gives indica- tions of the relative quality of the maps produced. The other shows coverage by maps of reconnaissance quality at scales smaller than 1 inch equals 1 mile. In general terms, approximately 25 percent of the area of the United States is covered by acceptable topographic maps on scales of 1 : 62,500, 1 : 31,680, and 1 : 24,000. Maps at these scales are available for a very small percentage of Alaska, but an attempt is being made to obtain interim map coverage with maps at scale of 1 : 250,000. The demand for topographic maps is evidenced in the annual map priority requirements submitted by Federal agencies to the Bureau of the Budget, letters from the public, contacts with State or municipal governments, and the resolutions of technical and civic societies. There are few areas within the United States and its territories and possessions for which there is not a request for topographic maps from some source. The critical pro- gram deficiency is revealed in a startling manner in that the present annual mapping program constitutes less than 2 percent of the expressed demand. Recommendations It is recommended that topographic mapping of the continental United States, at scales of approximately 1 : 62,500 or 1 : 24,000, be completed during the next 20 years. It is recommended that topographic mapping of the critical areas in Alaska (approximately one-third of the total area) be mapped at the scale of 1 inch equals 1 mile or larger during the next 20 years. Provision should be made for periodic revision of topographic maps to insure that they show up-to-date information. C. Geology Geologic maps, with related reports and records, por- tray the make-up of the earth's crust. They show the kind, character and distribution of all the solid rocks, and of the unconsolidated rocks as well, since loose bodies of sand, gravel, and glacial clay are just as much geologic formations as are the hard rocks. They also show the structural relations of one rock body to another, their folds and faults or dislocations, and the distribution and character of mineral deposits. The Geological Survey is the Federal agency primarily responsible for making and publishing basic geologic maps and reports. In addition, nearly all of the States main- tain staffs of geologists and produce some geologic maps, of quadrangles, counties or other areas, either independ- ent of, or in cooperation with, the Geological Survey. Basic geologic maps are published at various scales, depending upon the particular purpose for which they are prepared. In general, the scale range is between approxi- mately 1 inch equals 1 mile and approximately 1 inch equals one-half mile. These are general purpose geologic maps. Larger scale maps are prepared for specific cover- age of small areas, where the geologic information required must be shown in rather minute detail. Standard topo- graphic maps are utilized as a base for field studies and for plotting the geologic information. 355 |