GIS Files 4: Putting it all in a system

4.3: What happens where? - The power of the spatial query (1)

Basic spatial querying

GIS can not only tell you what information exists about particular features in the map data but it can also analyse where things are in relation to each other. What can this tell us?

GIS can go beyond visual analysis of thematic mapping as described in the previous section. The software can identify trends across a given area as well as performing specific queries. Such queries can select attribute data depending on its geographical location and then interrogate the attributes by performing calculations and statistical analysis. Selecting data based on the geometry of objects is known as performing a spatial query.

The simplest spatial query can be performed on screen using the selection tools that are provided with the GIS software. For instance, you can draw a circle on screen and select all objects falling inside it. This example shows addresses that have been selected="selected" because they fall within the circle.

This technique could be used by the emergency services to quickly identify all houses within 500 m of a spillage of a dangerous chemical.

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