Grid thematic maps
Grid thematic maps use a uniform grid of a specified user width, where each cell
(a quadrat) is of the same size and shape. Each grid cell can have a crime count
associated to it, which can then be thematically mapped. The unit to thematically
map could also be a density value calculated from the count and cell area. If the
grids are small enough, hotspots of crime within large geographic boundary areas can
be revealed, whilst retaining and displaying high crime volumes in the smaller geographic
boundary areas. A suitable size of grid to use initially is where each cell represents
250m*250m. Some police forces georeference their crime and disorder by a grid network.
Where this is the case, use the standard grid network that is already applied.
This method is an accurate approach for identifying and interpreting the location
of hotspots. However, problems with this technique include,
the range method range used, and the threshold set can mislead the interpretation
of hotspots (see also Geographic boundary thematic maps)
the size and orientation of the grid cells can hide spatial detail, causing
problems for accurate interpretation of hotspots (see also Geographic boundary
thematic maps)
The ‘blockiness’ in the appearance of grid thematic maps can be distracting.
Increasing the resolution often only has the effect of making the map look ‘specklely’.
The map below shows robbery patterns across the
London Borough of Croydon thematically shaded by using grid cells of
250m in size. 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 more accurately
identifies areas of high robbery concentration than the example for
geographic boundary thematic maps. The map also makes use of natural
colours that help to distinguish areas that are hot and not so hot.
Problems with this technique relate to,
- defining when ‘hot’ is ‘hot’
- spatial detail can be lost as hotspots are not naturally
formed as grid blocks
- the size and orientation of the grids can lead to different
interpretations of where the hotspots are.
Click here for an enlarged version
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