Even the mention of railroad tracks in the title does not make up for not showing them on the chart.

In preparing Fig. 198 a scale was very carefully selected to use one dot to represent a definite number of people so as to avoid having dots crowd each other too closely on the map. A map of this kind could be made in a very large size, and then be reduced photographically to a

MAPS AND PINS

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size which could be used in a report or magazine article. The reduction must ordinarily be so great for such a map that considerable forethought and care must be used or the dots will not show up distinctly enough in the final illustration. Anyone wishing to see many maps of this type shown in very excellent manner should consult Volume II of the Report of the Transit Commissioner, of the City of Philadelphia, published in July, 1913.

In placing dots for outlying districts on maps made by the method of Fig. 198, judgment must be used to have each dot placed at exactly the right point to locate accurately the people represented. In Fig. 198 each dot represents two hundred people. A dot in the suburbs may therefore represent all the people in one square mile of territory. If a map were first made with two hundred dots for the two hundred people, the one dot actually used on the final map would have to be placed not at the geographical center of the area represented, but at the center of gravity of the two hundred dots which it replaces.