94 J. W. Powell—Organization and Plan 
city of points, a system of checks is afforded, so that the railroad 
surface of the country can be determined therefrom with a 
the accuracy necessary for the most refined and elaborate topo- 
graphic maps. From such a hypsometric basis the reliefs for 
the whole country are determined, by running lines of levels, 
by trigonometric construction, and in mountainous regions by 
barometric observation. 
The primary triangulation having been made, the topography 
is executed by a variety of methods, adapted to the peculiar 
conditions found in various portions of the country. To a 
large extent the plane-table is used. In the hands of the topo- 
graphers of the Geological Survey the plane-table is not 
simply a portable drafting table for the field; it is practically 
an instrument of triangulation, and all minor positions of the 
details of topography are determined through its use by trigo- 
nometric construction. 
he scale on which the map is made is variable. In some por- 
tions of the prairie region, and in the region of the great plains, 
the topography and the geology alike are simple, and maps on 
a comparatively small scale are sufficient for practical purposes. 
For these districts it is proposed to construct the sheets of the 
map on a scale of 1-250000, or about four miles to the inch. 
In the mountain regions of the West the geology is more com- 
plex, and the topography more intricate; but to a large extent 
these regions are uninhabited, and to a more limited extent 
‘uninhabitable. It would therefore not be wise to make a topo- 
States, there will be several hundred special maps on large 
scales, as above described. 
