Proportional Symbol Maps Choropleth Maps Cartograms Isarithm Maps
Choropleth Maps
Conceptual basis for Chloropleth Mapping: The cartographer selects a distinctive color or shading, and applies it to artificial collection units (chorograms). Chorograms are statistical or administrative areas. It is these chorograms that are "colored or shaded",i.i. the symbology is applied so that the map reader can see within which symbol class the chorogram belongs. .
When to select this mapping method:
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when discrete data occue, or can be attributed to definite enumeration units-either statistical (as in census tracts, voting districts, school districts, etc.) or administrative political subdivisions (as in townships, counties, states, etc.)
- Note: geographical phenomena that are continuous and smooth should not be mapped via this method, as their distributions are not controlled by political or administrative boundaries. For example, aveage annual rainfall, or temperature are better mapped with isolines.
- The difference here is that the boundary of the chorograms have no value, but the lines of an isoline do have a value.
- The cartographer should not map values (raw data) with this method, e.g., population distribution, but instead should map derived values, such as population densities. In other words, if the data cannot be dealt with as a ratio or proportion, then they shouldn't be portrayed with the choropleth technique.
Map Scale
Map scale will influence the "resolution" at which you can map the data. E.g., it is difficult to map data at the county level if you are trying to show the entire U.S. within an 8.5 x 11 page.
Data Classification Techniques
Exogenous data classification: Values not related to the way the data are arrayed are chosen to subdivide the data into classes. E.g., an income level to define poverty.
Arbitrary data classification: Use regular, rounded numbers having no particular relevance to the distribution as class divisions, such as 10, 20,30,40.
Idiographic data classification:
- Typically used by cartographers
- Class intervals are based on the array of the data. E.g., natural breaks, quantiles, means and standard deviations, equal intervals.
