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:
 Locational or general reference maps
 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:
 are the data continuous or
discrete?
 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 (1020,
2130, 3140, 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.

