proportional symbol maps

proportional symbol maps

= represents the magnitude of a data with a proportionally sized symbol

• Good at representing count data or absolute data or raw totals

• a solution to the issue of choropleth maps being unrepresentative due to population size

types of data it represents data used

True point data = represents data measured at a given point - ex: number of people per city

Conceptual point data = data collected all over areas but symbolized as point location, centered in the area

what symbols can be used

literally any

• circles used the most → visually stable, preferred by users, easy to tell where the center is, easy to compare/construct

• squares

• pictographic symbols → hard to compare size and hard to see when overlapping

three types of scaling methods

1. mathematical/absolute

2. perceptual scaling

3. range grading

mathematical/absolute scaling

= size the area of point symbol increases in direct proportion of the data value → as value increases, area increases

• starts by setting the value of the largest data value

perceptual scaling (flannery scaling)

a study showing people tend to under estimate the size of circles on maps, to correct this underestimation they use 0.57 as the exponent to create the effect of having the area of circles increasing faster

range grading (graduated symbol maps)

= class based symbols

• Similar to choropleth maps because you have to choose: number of classes, classification method, symbol size for each class

• Useful for pictographic symbols where it is hard to create precise sizes and compare sizes

• Useful for when data varies too much or too little -> when it is hard to see pattern

• Have to be careful to not create a pattern that isn't really there

labelling symbols

nested legend (left picture) = conserves space

linear legend (middle and right) = easiest to visualize

• use round numbers to represent thee range of data