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Geo-Refer logo Geographical Referencing Learning Resources

Spatial Data Linkage - Joining Polygon Data to a Set of Points

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 attributes of the relevant polygon can then be joined to the point data either to carry out mapping or further statistical or spatial analysis. 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, in seeking to explore the relationships between car crime and policing policies implemented at beat level, the researcher may have the location of car crimes available as a set of geo-referenced points but may not know within which beat each car crime occurred. 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. If they have the spatial boundaries of the police beats available as a set of polygons, they can use GIS operations to link each point to the polygon within which it falls and then transfer the polygon’s attribute data (including, for example, the number of officers assigned to each beat) to the relevant point.

Location of car crimes (available as a points shapefile in the ArcGIS shapefile format, click [here] to download ):

Attribute data for car crimes shapefile:

Police beat areas with attributes (available as an ArcGIS polygons shapefile, click [here] to download):

Attributes of police beats with attributes (police_beats_atts) shapefile, click [here] to download:

The researcher's objective is therefore to link the points and polygons spatially (a point-in-polygon overlay) and to then transfer the polygon attribute data to the set of points. This type of spatial operation 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.