graphic maps for general distribution, but supplied them only to its own members and to other government services. Authority to sell the maps to the general public was finally granted by Congress in 1897 (act of February 18, 1897, 29 Stat. L., 701).

During the first few years of the Survey's map work it continued to use the same scale that had been employed by the earlier independent surveys-about 4 miles to the inch, or 1:250,000, with relief shown by 200-foot contours; but when the coöperative surveys of the states were begun it was seen to be desirable to increase the scale and decrease the contour interval, not only for the work in those states, but for that in other states in which coöperation was not being carried on.

The earlier topographic mapping of the Survey was subjected to some criticism on account of lack of refinement in detail. It was a question of judgment whether the appropriation should be expended in accurately mapping a limited area or in making reconnaissance surveys over a more extended area. The less accurate maps were less expensive, and served a useful purpose in establishing the value of topographic surveys. The results have proved the wisdom of a policy whereby the people were gradually educated to the use of refined and detailed maps, while Congress learned to appreciate the cost and the comparative slowness with which accurate surveys would be prosecuted over so vast a territory as that of the United States. Had fewer square miles been mapped annually in the early years, but with the accuracy attained at present, it is doubtful if Congress would have continued to maintain the organization. As it was, valuable results were rapidly put forth, and in time it has been possible to supersede the older reconnaissance maps with more detailed maps.1

1

As the topographic work steadily advanced, not only in the area covered, but in accuracy, it was found desirable to obtain from Congress authority to run careful spirit-level lines and to establish bench marks for permanent record of eleva1U. S. Geological Survey, Bulletin No. 227, 1904, p. 57.

tions upon the ground, and also to carry on control by triangulation or traverse with greater accuracy and to mark the positions more permanently than had been possible without statutory authority. Accordingly, on June 11, 1896, an act was approved which provided that "elevations above a base level located in each area under survey shall be determined and marked on the ground by iron or stone posts or bench marks," etc. Thereafter it was possible to provide for the running of careful spirit levels over all the areas under survey and to extend the amount of spirit leveling, thereby securing more accurate location of contours. Since that act was passed the topographic mapping has reached as high a degree of detail and quality as seems desirable for the scales adopted.

The selection of areas for survey is determined by a variety of considerations. It is of course desired to extend the benefits of the work as widely and equally over the country as a whole as is economically practicable. Other things being equal, however, it is more economical to extend surveys outward continuously from areas already surveyed than to undertake new work in areas entirely unconnected with them by triangulation or otherwise. A special economic need or project in an isolated area may nevertheless occasionally warrant the initiation of a survey there. As between areas equally economical of survey, preference is determined by a number of factors, among which are the apparent local demand for surveys, and the probable economic value of the resultant maps. The methods of field work followed by the Survey in making topographic surveys are unique and have been developed by the topographers of the Survey through nearly forty years of field experience. Wherever possible the results of triangulation by other organizations are utilized. The degree of refinement in the detailed surveying depends upon the scale upon which the resulting map is to be drawn.

The results of the topographical surveys are published in the form of Topographic Atlas Sheets, measuring 17 by 20 inches. These sheets are published on different scales, the

scale selected varying with the character of the area mapped and the degree of detail required. These scales are 1:62,500, 1:125,000, and 1:250,000; that is, approximately 1 mile, 2 miles, and 4 miles to the inch, respectively. A sheet thus represents an area of approximately 230 square miles, 920 square miles, or 3,700 square miles. The largest scale, showing the most detail and representing the smallest area, is used for densely populated or otherwise important areas; the next largest for sparsely inhabited mountain regions; the smallest for areas covered by reconnaissance or exploratory surveys, especially in the western states and Alaska.

The boundaries of each quadrangle, as the area mapped on a single sheet is known, are meridians of longitude and parallels of latitude, the differences in longitude or latitude between the boundaries being in the largest scale maps 15', and in the smaller scales, 30' and 1°, respectively.

The maps show the topographic features of the land, such as mountains, hills, valleys, and gulches; all bodies of water, such as lakes, marshes, streams, and springs; the routes of travel, such as railroads, wagon roads, and trails; political boundaries; cities, towns, and permanent buildings; and the names of natural and other features. They also indicate exactly the location of permanent survey monuments and bench marks whose positions and whose elevations above sea level have been determined by precise methods. The maps of areas covered by public land surveys show all township and section lines as well as the boundaries of all land grants.

Each of these maps is printed in three colors-black for the cultural features, such as boundary lines, roads, railroads, houses, towns, and cities, and for the names of all things represented; brown for the relief, or the element of elevation, indicated by contour lines showing the heights of all parts of the area above sea level; and blue for the water-the rivers, lakes, and the sea and its bays and inlets.

Each sheet is named from some prominent natural or other feature within the quadrangle it represents, such as a lake,