CHAPTER XVIII 

 TOPOGRAPHIC MAPS 



237. Map Making. By the topography of a continent 

 or region is meant the description of its physical features - 

 mountains, plains, rivers. There are several ways of repre- 

 senting differences of elevation of a tract of country. One is 

 the method of relief maps by which the region is pictured 

 as if carved from a solid block. Another is the method of 

 hachures or shadings, by which the steeper slopes are rep- 

 resented by heavier shadings. 



A third method is that of contour lines. This method 

 is now used in all topographic maps published by the United 

 States government. It is superior to the other methods 

 because it shows not only the location of all physical fea- 

 tures, but also the exact elevation of any point on the map. 



238. Contour Lines and Intervals. A contour line is a 

 line on which every point represents the same level. Sup- 

 pose we stand at the shore of the ocean; we are at sea level. 

 Other people at various places on the shore are at the same 

 level. A line representing the shore is the contour line of 

 zero feet above sea level. One person might climb a cliff 

 a few feet away from the water and at the top stand twenty 

 feet above the sea level, while another might walk twenty 

 rods from the shore before he rises to the same level as 

 the top of the cliff. The same contour line would pass 

 through the points where these two people stand, but it 

 would not be parallel to the shore line. 



The space between the two contour lines would represent 

 the horizontal distance from the shore to the twenty-foot 

 elevation. The contour interval, or vertical distance between 



