The vertical dimension of the group of bars for each district shows the relative total number of houses. The horizontal dimension shows the relative number of houses of each height by stories

up about half of the whole United States. A uniform scale varying by $25 per acre would give an erroneous impression regarding those important areas which contain land valued at less than $25 per acre but in which there is a large amount of farming.

Map diagrams of the type shown in Fig. 180 are sometimes useful. There is danger, however, of making a chart of this kind so popular in character that it loses in accuracy. The utility of Fig. 180 is at once

MAP PRESENTATIONS

Population Density per Acre.

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0-50

50-75

75-100

100-1 50

150-200

200-E50

250-300 300-350 350^400

400-45O Over 450

Courtesy of GTaham Romeyn Taylor

Fig. i8i. 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

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