I'm getting started with a QGIS map showing the quality of roads in my town. I have various data which create an index of 1-5.

How do I link this index data to road data in a map, showing the bad roads (5) in red, and good roads (1) in green?

  • How is your index data related to the roads? ie use your common identifier field to execute a join.
    – mikeLdub
    Nov 19, 2013 at 14:12

2 Answers 2


What you're trying to create is either a categorized or graduated symbology. This website walks you through it: http://qgis.spatialthoughts.com/2012/02/tutorial-styling-vector-data-in-qgis.html


It seems like your trying to create a spatial matrix on your road layer from other intersecting layers. To populate the road layer you will need to perform either a combination of Join Attribute by Location, Spatial Query, or Union.

Spatial Query - will select one table features based on spatial intersection parameter. You may find the tool in the Vector menu>Spatial Query>Spatial Query.

Join Attribute by Location - will join the two tables together based on spatial intersection. You may find the tool in the Vector menu>Data Management>Join Attribute by Location.

Union - You should be mindful of that the joining of spatial data between polyline and polygon data is not always a one to one relationship, since sometimes polyline features overlap polygon features. Union tool is located in the Vector menu>Geoprocessing Tools>Union.

With each method you will most likely have to clean up your final database eliminating un-wanted fields and devising a matrix schema for each criteria (or field) that may add up to the total classified field (summation of the other criteria fields).

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