Page:Graphic methods for presenting facts (1914).djvu/241

 limited by the fact that there is no key scale shown from which an exact numerical interpretation may be obtained.

Courtesy of Graham Romeyn Taylor

Fig. 181. Methods of Marking Maps When an Increased Density of Population May be Expected in Following Years

Each shading can be made by adding with a pen to the shading used for the next lower density. Pins of different colors placed in a map have an advantage over this scheme in that pins can be removed if population density should happen to decrease

The scheme for indicating population density shown in Fig. 181 is necessary only when very large and valuable maps are used. For ordinary purposes it would be better to have a new map for each Census, and then to use colored crayons on the different areas of the map, rather than to attempt the complicated scheme of Fig. 181. If, however, a photograph must be taken periodically to produce line cuts showing the map in printed form on a reduced scale, colored crayons cannot be used and the scheme of Fig. 181 may be of great assistance. The result obtained by the method of Fig. 181 may be obtained by using map pins with spherical heads and pushing the pins in until the heads touch the map. The number of pins in any city block or district would indicate the population according to some simple scale. If it is not necessary to photograph the map, pins with different colors of heads may be used to show density of population. There is one very great advantage in using pins instead of crayons or the pen-and-ink system of Fig. 181. It sometimes happens that an error is made which may spoil a very valuable map because of the impossibility of erasure. When short pins are used instead of crayon or ink, an error can be instantly