Then, you can format basic properties of the chosen shape, such as the color and outline. In the case of line geometry, you choose whether the data represents Width or Distance from center. When you choose Area, the attribute value is multiplied by the square of the Unit value. Height can be specified only when you are working in a scene. You must also specify whether this value should correspond to the Area, Radius (one-half of the width in the case of a square), or Height of the symbol in the Data represents property. The attribute value is multiplied by this unit to determine the size of each symbol. Set the Unit property to the unit of measure that your data represents.
For example, you may have a point layer of individual trees that includes an attribute for tree crown radius, measured in feet, and want to represent each tree with a circle using its real-world crown size. If the attribute being symbolized is directly related to an actual value with a known unit of measure, you can use proportional symbols to draw symbols that are sized with these exact sizes. Because Flannery's research applies specifically to the perception of circles, appearance compensation should be used with circular symbols only. You can use appearance compensation only when you have relatively sized proportional symbols (proportional symbols with no unit specified) and no Maximum size specified. Appearance compensation uses an algorithm defined by James Flannery, where larger symbols are scaled up to compensate for this perception.
To counteract this perception, you can check Appearance compensation (Flannery). There is a tendency for map readers to underestimate the size of proportional circles on maps, especially at the larger end of the size range. Leaving the range open-ended by not defining a maximum symbol size allows the range of symbols to be truly relatively sized, but it may result in some excessively large symbols if there is a large value spread. In this case, the Minimum size corresponds to the smallest value in the data, and all other symbols are sized proportionally relative to this minimum size. Values that fall below or beyond this range are drawn with the smallest and largest symbols, respectively. All symbols draw between these two sizes, and you can use the histogram to interactively set the data values that correspond to the minimum and maximum sizes. When the attribute values represent a count (such as population or number of bushels harvested) rather than a measurable amount, or if the unit is unknown, you can specify a minimum and maximum symbol size. When the symbology is based on an expression, this sorting does not occur, and some smaller symbols may be obscured by larger ones. When proportional symbology is based on a single field, the symbols are drawn in a sorted order, where the larger features draw first and the smaller features draw above. Proportional symbols can be based on an attribute field in the dataset, or you can write an Arcade expression on which to generate numeric values to symbolize. This animated map uses proportional symbols to show the fluctuation of precipitation in Mexico. For reference, you can specify a uniform background symbol for the polygons that draw below the points. When applied to polygon features, a proportionally sized point symbol draws at the center of the polygon. When applied to point or line symbols, the feature's size is modified directly. Proportional symbols can be defined for point, line, or polygon feature layers.
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But where graduated symbols distribute features into distinct classes, proportional symbols represent quantitative values as a series of unclassed symbols, sized according to each specific value. Proportional symbology is similar to graduated symbols symbology in that both draw symbols sized relative to the magnitude of a feature attribute.
Proportional symbology is used to show relative differences in quantities among features.
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