TOPOGRAPHIC MAPS 223 



244. Uses of Topographic Maps. Topographic maps are 

 much used by tourists traveling for pleasure, by carriage or 

 automobile. The most direct roads between towns, as well 

 as the more circuitous routes, are easily traced. Contour lines 

 show the ascent or descent of the grade. If the lines cross- 

 ing the road are far apart, the grade is gentle; if near to- 

 gether, a steep grade is indicated. By consulting a map, the 

 traveler can choose at a fork of a road whether he will take 

 a short, steep road or a longer one with a more gradual rise; 

 whether he will drive along a river course or away from it. 



It is by careful study of elevations as shown by contour 

 lines that sites are selected for reservoirs for irrigation, 

 water power, or city water supply. The first condition is, 

 of course, a permanent and sufficient supply of water. This 

 is generally determined from the number of square miles 

 included in the basin of which the stream is the outlet, and 

 the average rainfall for the region. 



People planning for a summer camp can select the site 

 from a topographic map nearly as well as from the place 

 itself. The positions of brooks and ponds, hills and water- 

 falls are accurately shown. Distances from town centers 

 and roads are correctly indicated. 



In planning for new trolley routes, the company interested 

 not only ascertains the best grades between large towns, 

 but sees which roads would go through regions likely to 

 give patronage, and where the line could leave a highway 

 to make a short cut through the woods or across open 

 country with the least loss of business. All these things can 

 be learned from a study of the maps. 



Maps of any locality whose survey is completed can be 

 obtained at Washington for a small sum. The survey of 

 many of the eastern states is already finished; but in the 

 great plain and prairie regions, only the populous areas or 

 those most important geologically are completed. 



