TOPOGRAPHY. 59 
and so on. In the space between any two contours are found eleva- 
tions above the lower and below the higher contour. Thus the contour 
at 150 feet falls just below the edge of the terrace, while that at 200 feet 
lies above the terrace; therefore all points on the terrace are shown to 
be more than 150 but less than 200 feet above sea. The summit of the 
higher hill is stated to be 670 feet above sea; accordingly the contour at 
650 feet surrounds it. In this illustration all the contours are num- 
bered, and those for 250 and 500 feet are accentuated by being made 
heavier. Usually it is not desirable to number all the contours, and 
then the accentuating and numbering of certain of them — say, every 
fifth one — suffice, for the heights of others may be ascertained by 
counting up or down from a numbered contour. 
• 2. Contours define the horizontal forms of slopes. Since contours 
are continuous horizontal lines, they wind smoothly about smooth 
surfaces, recede into all reentrant angles of ravines, and project in 
passing about prominences. These relations of contour curves and 
angles to forms of the landscape can be traced in the map and view. 
3. Contours show the approximate grade of any slope. The alti- 
tudinal space between two contours is the same, whether the} 7 lie 
along a cliff or on a gentle slope; but to rise a given height on a gentle 
slope, one must go farther than on a steep slope, and therefore con- 
tours are far apart on -gentle slopes and near together on steep ones. 
The topographic maps of the United States Geological Survey 
represent, besides the natural features mentioned, such artificial 
features as are of a public or more enduring nature, as State, county, 
township, and city boundary lines; reservation, land-grant, and public- 
land township lines; railways, streets, roads, and paths; bridges, 
ferries, dams, locks, and wharves; the location of permanent bench 
marks and triangulation stations, and the positions of light- ships and 
light-houses, mines, shafts, and tunnels. The only private features 
shown are houses, factories, stores, etc., no barns or outbuildings 
being represented. 
The uses of such topographic maps are many. For the purposes of 
the National Government and the State they are invaluable, as they 
furnish data from which may be determined the value of projects for 
highway improvement, for railways, for city water supply and sewer- 
age, and for the subdivision into counties, townships, etc. They serve 
the military departments of the Government, National and State^ in 
locating encampment grounds, in planning practice or actual operations 
in the field, and, during war, in indicating the precise situations of 
ravines, ditches, buildings, etc. The Post-Office Department utilizes 
them in considering all problems connected with the changing of 
mail routes, star routes, and especially in connection with contracts 
and assignments of rural free-delivery routes. As the outlines of 
wooded areas are to be indicated on these maps, National and State 
