Map Types and their Application

 

What map should I use?

Learning outcomes:

  • Five types of thematic map and when to use them
  • Enumeration units (EUs)
  • Data classification

 

Many different maps can be made:

  • Location maps
  • Road maps
  • Weather maps
  • Cancer maps
  • Topographic maps
  • Cadastral maps
  • Aviation or oceanographic maps


London Underground map, first designed in the 1930s

Question is, which one to use!?

Two broad categories of all maps:

  1. Locational or general reference maps
  2. Thematic maps

Thematic maps show a certain theme, usually by showing data in different categories

Thematic maps are often quantitative

  • Review nominal, ordinal, interval, ratio levels of measurement


Typical thematic map: the attribute is population with four categories

How to choose the right map!
Thematic Maps

There are four main map types of thematic map as shown on the right.

Proportional Symbol

Dot Density

Choropleth

Isarithmic (surface)

Although they look very different three of them are based on data collected in areas or enumeration units (EUs). One (isarithmic) is based on point data across a surface.

To choose the right map we think about underlying data:

  1. are the data continuous or discrete?
  2. does the data vary smoothly or abruptly?

If the data are discrete = choropleth

If the data are continuous = surface map

If the data vary smoothly = dot density

If the data vary abruptly = proportional symbol


The four main thematic map types

 

 

1. Proportional symbol map

A map in which point symbols are scaled proportional to their data value

——Use when underlying data are abrubtly distributed
——Use when need to show absolute or derived data

2. Dot density distribution map

——Use when underlying data are smoothly distributed.
——Use when need to show absolute data.

Gives total number of something, with each dot representing a given number (here, of people)

Notice legend tells you how many things each dot represents

Problems:

  • People might think dots are exactly where data are, in fact dots are placed randomly within the EU

3. Choropleth map

A map in which areal units are shaded according to the rate of intensity or density of the data

In this example, African countries are shaded according to their population density

Choropleth maps never show absolute numbers. They only show "derived" data or ratios, densities, percentages.

Has severe deficiencies long recognized in the literature (since at least 1940s by J.K. Wright): data are areally aggregated and do not follow actual boundaries of the data


Choropleth map. In this example, animals have been overlaid on the choropleth.

4. Surface or Isarithmic map

A map made from points sampled across an area. Interpolation is used to predict values at unsampled locations

Surface or sometimes called isoline map

5. Dasymetric map

A map that is like a cross between a choropleth map and an isarithmic map

Has many advantages: boundaries of the map follow boundaries of the data

 

How to classify data

Most often in GIS we do not represent each data value exactly. Instead data are classified or put into categories. This is sometimes known as "binning".

There are several statistical methods, including:

  • "natural breaks" or Jenks method (named after George Jenks)
  • equal interval (10-20, 21-30, 31-40, etc.)
  • quantile (equal number of observations in each category)
  • standard deviation to emphasise how much deviates from the mean (best used on normal distributions)

Terms to know:

Frequency distribution or histogram
Normal distribution or bell curve
Skewness and positive or negative skew

 

 

Summary

There are at least five major types of thematic map

  • Proportional symbol
  • Dot density
  • Choropleth
  • Surface or isarithmic map (also called isoline map)
  • Dasymetric

To decide which one to use we must think carefully about the underlying data. Are they discrete or continuous? Are they smooth or abrupt?

Surfaces are a special case. We can interpolate (fill in) missing or hidden values by converting a set of points to a continuous surface.