Proportional+Symbol


 * Proportional Symbol Mapping**

Proportional symbol mapping, also called //graduated symbol mapping//, is used to show relative magnitudes of phenomena at specific locations. A symbol form, such as a circle, is chosen and the symbol size is varied in proportion to the quantity it represents.

There are three methods for setting symbol size: absolute scaling, apparent-magnitude scaling, and range-grading.
 * __Absolute scaling__, or square root scaling when using two-dimensional symbols, is the direct proportional scaling of symbol //area//. For example, the symbol for a point with a data value of 100 should be twice the area of the symbol for a point with the value of 50.
 * __Apparent-magnitude scaling__ incorporates correction factors to compensate for map reader underestimation of symbol area.
 * With __range-grading__, a symbol represents a range of data values. Data are classed, as in choropleth mapping, and then each class is assigned a symbol of a distinctly different size.

Historically, proportional symbols have been used to map economic data, but physical and cultural data may also be mapped using this technique. Data may occur at points, such as rainfall data collected from weather stations, or data may be aggregated at points within areas, such as population data collected at the county level and mapped at points located within the centers, or centroids, of each county.


 * General rules for proportional symbol mapping are as follows:**
 * Use either raw data or data that have been processed in some way, including data proportions or ratios. Densities (i.e. variable/area) are normally mapped using choropleth techniques.
 * Avoid proportional symbol mapping, particularly when using square root scaling, if the total range of data values is small. With small data variation, a dull, homogeneous map results.
 * Circles, squares, and triangles are the most common symbol forms. When using square root scaling, circles and squares are the easiest to construct.
 * Limit the number of variables symbolized by proportional symbols to one or two. When two variables are used, keep one variable at the interval/ratio level and the other at the nominal or ordinal level to avoid //symbol overload//.
 * Use outlined, opaque symbols.
 * Use a legend that shows representative symbols for low, medium, and high values. If using the range-grading technique, the recommended number of ranges is four or five. The maximum number of ranges is five or six. With range-grading, all symbols must be displayed in the legend.

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