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Show I 5OTES IN RELATION TO THE ACCOMPANYING CONTOUR- MAP OF THE UNITED STATES. Bv HENRY GANXETT, M. E. I I have been engaged for several years in collecting elevations in the United States, particularly in that portion west of the Mississippi River. ID connection with this work, I have made a quite extensive study of the geography, both horizontal and vertical, of the country, having had access to nearly all sources of such information extant. By means of the numerous reconnaissances and surveys of different parts of the West, there is scarcely any part of this vast area of which the general features at least are not known. The necessity for further reconnaissance, as such, no longer exists, while the need of actual surveys, based on exact methods, increases year by year, as the work of settlement goes on. In the accompanying map, I have sought to embody some of the results of my studies. It represents approximately and as nearly as the information available at present will enable me to do it, the locations of contour- curves over the country. The vertical distance between the curves is 1,000 feet. This map is published merely as a beginning in this direction, and as much with a desire of gaining information by criticism as of imparting knowledge on the subject The map on which the curves were drawn, and of which this is a photo- lithographic copy, is that used by the Census Bureau for their atlas. It is not all that could be wished for the purpose, as it contains many errors in drainage, geographical positions, & c, due to the fact that the latest geographical work is not represented upon it. In general, I have made the contours conform to the drainage; but in a few cases, where this would necessitate marked errors in the curves, the coarse of the drainage has been neglected, and the contours have been drawn as I know them to exist, as in the case of Preuss Lake, Utah, which has lately been proven to have no existence, and the mouth and lower coarse of the Bio Dolores in Eastern Utah, which have been sadly misplaced by map- makers since the time of Captain Gunnison. The scale of the map is smaller than I should wish, and makes it impossible to represent properly many features which could be brought out on a larger map. Exact measurements of elevation above sea- level are in this country entirely unknown, except near the coast. No leveling for scientific purposes, on any considerable scale, has been done; and the levels of railroad and canal lines, though at first they might seem to be all that is required, on examination are found to be as conflicting as possible. No two lines give the same height for the terminus. The profiles of the railroads, when compared, present a mass of contradictions almost beyond belief. The discrepancies, however, are due, not to the instrumental work, but to the office- work, the computations, the connection of different sections, & c. The matter is greatly complicated by uncertainty concerning the identity of datum- planes, especially when such datura- planes are given as mean, high, or low water in lakes or rivers. |
