TOPOGRAPHY. 61 
at the bottom of each sheet the scale is expressed in three ways — by a 
graduated line representing miles and parts of miles in English 
inches; by a similar line indicating distance in the metric system, 
and by a fraction. Special larger scales, from 2,000 feet to the inch 
up to 100 feet to the inch, are used for detailed surveys of important 
mining regions, for the planning and construction of irrigation 
projects, etc. 
The maps are printed on sheets approximately 13 by 17 inches. The 
area represented on a sheet on the 1-mile scale is 15 minutes of lati- 
tude and of longitude, or, on the average, about 225 square miles; on 
the 2-mile scale, 30 minutes of latitude and of longitude, or about 1,000 
square miles; and on the reconnaissance or 1-mile scale, 1 degree of 
latitude and of longitude, or approximately 1,000 square miles. An 
atlas sheet is known by the name of the most important town, city, or 
natural feature within the quadrangle represented. 
The methods of field work followed in making these topographic sur- 
veys are as follows: At rather distant intervals throughout the terri- 
tory under survey, say 10 to 25 miles, monuments are established, the 
positions of which are accurately determined by trigonometric methods 
and recorded in degrees of latitude and longitude. For this purpose 
base lines from 1 to 6 miles in length are measured with great accu- 
racy, the location of one end of each base and of its azimuth being 
determined astronomically. From this base a network of triangulation 
is expanded to hilltops, mountain summits, and other prominent 
objects, the positions of which are carefully determined by computa- 
tion. This is the special work of the section of triangulation and com- 
puting (see p. 61). All existing triangulation by other organizations 
is utilized. Where the nature of the country renders the extension 
of triangulation impracticable, primary control is extended by means 
of careful lines of traverse. The details of the methods employed are 
elaborated in another portion of this pamphlet (pp. 65-69). 
The resultant positions are plotted accurately, by latitude and lon- 
gitude, on large sheets of drawing paper, approximately 18 by 21 
inches in size, called plane-table sheets. To each topographer is 
assigned, as his season's work, the mapping of one or more quadran- 
gles, and lie enters the field equipped with the plane-table sheet, which 
has on it only the trigonometric positions. A field party usually con- 
sists, in addition to the topographer in charge, of one skilled assistant, 
of the grade of assistant topographer or topographic aid, and several 
surveyors temporarily employed as traversemen or levelmen, with the 
necessary rodmen, teamsters, cooks, and other camp hands where 
subsistence is by camping. 
The levelmen proceed to run lines of spirit levels over the various 
roads and paths, to determine elevations. The higher order of pri- 
mary leveling, elsewhere described, is run at such distances apart that 
