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Geo-Refer logo Geographical Referencing Learning Resources

Spatial Data Linkage - Joining Point Data to a Set of Polygons

Researchers frequently need to link together sets of data spatially. This is different to matching data using tabular data linkage. Spatial data linkage usually involves some form of intersection or overlay of more than one set of spatial units. For example, a set of points may be intersected with a set of polygons in order to identify within which polygon each point lies. The number of points within each polygon or a statistical summary of the attributes of the points within each polygon can then be calculated. This type of operation is often necessary where the researcher is using non-standard geographies and where pre-existing look-up tables do not exist.

Car Crimes Example

For example, if seeking to address the question "What is the relationship between car crime and policing policies?", the researcher may have the location of car crimes as a set of geo-referenced points and may wish to link these to police beats which are available as a set of polygons. The researcher could then calculate the total number of car crimes per police beat and compare this to differences in policies between beats. Since neither police beats nor the location of car crimes are standard geographical units there are no pre-existing look-up tables available to help the researcher. Instead, they will probably need to perform a purpose-specific spatial data linkage operation.

Location of car crimes (available as a points shapefile in ArcGIS format):

Location of crimes shown as points in a GIS map produced by ArcGIS

Police beat areas (available as a polygons shapefile in ArcGIS format):

Map showing the boundary of the police beat areas that is produced by ArcGIS

The researcher's objective is therefore to link the points and polygons spatially and to then calculate how many car crimes took place in each beat. This type of spatial data linkage is called a point-in-polygon overlay, or an intersection of points and polygons. It is available within most vector Geographical Information Systems (GIS).

Copyright Attribution

ArcGIS® and ArcMap® are registered trademarks of Environmental Systems Research Institute, Inc. (ESRI). The software screen shots shown in this page are reprinted with permission from ESRI. The maps shown above are based on data provided through EDINA UKBORDERS with the support of the ESRC and JISC and uses boundary material which is copyright of the Crown.