Geographic boundary thematic maps
A popular mapping technique for representing spatial distributions are geographic
boundary thematic maps. The geographic boundaries (also often referred to as regions
or polygons) are usually defined administrative or political areas such as beats,
enumeration districts, polling districts, wards, or district boundaries. Crime events
mapped as points can be aggregated to these geographic region areas. These counts
of events by their geographic areas can then be thematically mapped to display the
spatial pattern of crime across the area of interest.
When creating thematic maps of this type a variety of range methods are offered
to represent the distribution of crime (e.g. equal ranges, natural break, standard
deviation, or custom range). This freedom to choose a range of any type creates variability
in thematic maps produced for identifying hotspots of crime or disorder. Different
range types structure the groupings of geographic boundary incident counts into differing
threshold categories.
Geographic boundary thematic mapping methods have an important application for
providing summarised management information across areas of accountability, but may
mislead focused crime and disorder reduction targeting because of failing to reveal
patterns within and across the geographic division of boundary areas.
Guidance points for creating a thematic map.
Consider the theory and application behind the map that is being produced and
the audience to whom the map will are address. If the application is to identify
the hotspots of crime, the range thresholds need to be structured to focus on revealing
the locations of these high volume crime and disorder areas. At the same time, the
thematic map should show the variability in crime and disorder across the area, including
those areas where there has been no crime or disorder reported.
The thematic ranges need to be structured in a manner that is easy for the
audience to understand. The map should be the central message, and the thematic ranges
should follow in a logical sequence. If the thematic ranges require explanation or
confuse the audience, the opportunity to present the central message – identifying
the hotspots of crime – has been lost. Several range methods are suitable for identifying
those areas that have the highest crime concentrations (e.g. a standard deviation
range). However, it is often more useful to choose a custom range setting where the
ranges that are chosen follow in a logical sequence, rather than confusing the map
reader who can be easily distracted by a legend that has few logical breaks and which
they question.
A suitable number of ranges to choose for the thematic map is 5 or 6. Too
few ranges will often over-generalise the crime and disorder patterns, whilst too
many may confuse map interpretation.
Natural colours need to be used to represent the different ranges. Hotspots
are usually represented in red or other ‘warm’ colours. Areas where crime is less
are represented in cooler colours such as pale green and pale blue.
Thematic mapping is an iterative process. The first map that is produced is
unlikely be the one that is printed or included in a report. Decide on the message
that is to be presented and follow through with this message by testing different
settings available for the thematic thresholds.
Problems with geographic boundary thematic mapping for accurately identifying
hotspots.
The geographic boundary thematic mapping method does however, have problems for
accurately identifying hotspots.
The varying size and shape of the geographic boundaries in the thematic map
can mislead the identification of hotspots. Large geographic areas may often have
crime concentrated into a small part or spread evenly across the geographic boundary.
As the thematic map displays counts of crime by these areas, the whole of the large
area is shaded, diverting visual attention away from the smaller geographic areas
where there is a higher concentration of events.
Geographic boundaries suffer from the Modifiable Area Unit Problem (Bailey
and Gatrell, 1995; Openshaw, 1994). This is where changes in the geographic boundary
areas used to thematically represent the distribution of crime can change the way
the map looks and alter the interpretation from the map.
Suitable applications for geographic boundary thematic maps.
The geographic boundary thematic mapping method does have several important applications.
Boundary thematic maps are important map outputs as the areas they represent
are often geographic regions used for political and administrative purposes. For
example, a community safety officer may have management responsibility for reducing
crime and disorder in a group of geographic areas. This officer will be keen to be
informed with general information such as a thematic map based on crime and disorder
counts for their area. The officer will also most likely be interested in summarised
information of patterns that exist in neighbouring geographic areas.
The map below shows robbery data thematically
shaded by enumeration districts across the London Borough of Croydon.
A custom range has been used to present the distribution of this
crime type. The custom range follows in a logical and easy to
understand sequence. The map serves the purpose of identifying those
enumeration districts that are grouped in the highest crime count
categories. The map also makes use of natural colours that
complement the central message of the map (i.e. where are the
hotspots?), and a sensible number of ranges to represent the pattern
of this crime across Croydon.
When identifying hotspots from this map early
attention is drawn to the large enumeration districts that are
coloured in the darker orange shading, rather than the EDs of the
same colouring but less than a tenth the size. This type of mapping
can often mislead identifying and prioritising crime reduction
resources to areas that appear as problems, but where the problem is
exaggerated from a thematic map of areas of varying size and shapes
Click here for an enlarged version of
the map
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