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? What is the correct term for this process? Any pointers on this would be greatly appreciated. Regards, Case.
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).