Page:Popular Science Monthly Volume 46.djvu/498

482 whole domains, and that outside of Europe the United States claim first attention by their grand topographic works.

Similar surveys are also in progress in the principal colonies of the European powers, such as India, Canada, Algeria, Tunis, etc. The scale of the maps varies from 1:2,000 to 1:420,000.

According to Lowinsin, seventy per cent of the area of Europe has had a fairly satisfactory topographic survey; and of the land area of the world, about twenty-seven per cent has been surveyed more or less accurately. Bartholomew estimates that only one seventh of the whole land surface of the globe has been exactly surveyed. He publishes an instructive map exhibiting the area of topographic surveys, both exact and general, and of geographic surveys, both fairly reliable and approximate or hypothetical.

Most European topographic maps were made, primarily, for military purposes, under the supervision of military officers, and secondarily for the scientist and statesman only. In the United States the necessities of the geologist developed the first interior surveys, and they are now being carried forward under the direction of the Geological Survey, and, along the ocean borders, by the Coast and Goedetic Survey.

The methods employed are the same in all topographic surveys, in respect to the two essential divisions of work, viz., location of points of control and sketching in of contours, streams, culture, etc. The minor methods of procedure differ in details within these two divisions; but geometrically located points of control are in all cases obtained, and the contours, roads, streams, and all features shown on the map are sketched in, whether the located points of control are ten inches or a thousand feet apart. Usually it is only the features sketched that appear on the map, as the geometrically located points that control the sketch are mathematical points. If desired they can be represented by conventional signs.

The first contoured topographic maps of the United States for geologic purposes were on the scale of 1:250,000 (four miles to the inch), with contour intervals of two hundred to two hundred and fifty feet; but the necessities of science and the demands of the public called for a more detailed map, and the scale of 1:125,000 (two miles to the inch) was adopted. This was again enlarged in certain regions to 1:62,500 (one mile to the inch). The topographic maps of the Geological Survey are now being made, in the rougher mountain region and thinly populated areas, on the 1:125,000 scale, with contour intervals of from ten to one hundred feet, and in the more valuable, economic, and thickly populated areas, on the 1:62,500 scale, with contour intervals of from five to one hundred