How to combine information from raster and vector layers?

Question

in QGIS I have 1 raster-layer (elevation) and 1 vector-layer (administrative units).

Now I would like to be able to query one of the administrative units and see the mean and min/max of the elevation within its area.

Which QGIS tool can do that?

THX, stn

Answer 1

Yes, this is called Zonal Statistics. You can run it with Zonal Statistics plugin, this is the core plugin, so it doesn't need to be installed separately, simply turn it on. Note that there is not that many statistics available.

Answer 2

although this is not exact answer for you as it does not use QGIS, I still think it could be valueable for you.

Some time ago I had to compare outputs from different modeling softwares so I had plenty of grids (=rasters, mostly in ascii or SAGA format) and add their min/max/averages to the shapefile polygons.

I used SAGA GIS (opensource like QGIS) tool Grid Statistics for Polygons:

Which added all the statistical data calculated from the raster file to the shapefile. It should be also able to add data from multiple rasters, but then there is a problem with the DBF limitation as it cuts the names of the columns containing the raster name and the statistical parameter. So you should use short names for the rasters or later edit the column names in QGIS to better represent which data is it.

Answer 3

This is a great question. I can think of a semi-automatic way to do this and it's not as elegant. Hope someone else can give a better answer.

Here is goes

• Covert the raster to vector using Raster - Conversion - Polygonize. Check the 'Field Name', so you have a field with value of elevation. Call this layer raster_polygon
• Intersect the vector layer with raster_polygon using Vector - Geoprocessing - Intersect. Call the new layer intersect_join.
• Assume your vector layer has unique value field called ID. The intersect_join layer will have attributes for both ID and Elevation. Export this as a CSV.
• Open the CSV in a spreadsheet program and find min/max elevation for each ID. Use built in Max() or Min() functions. Goal is to create another CSV file with following fields ID, min_elev, max_elev. For example.

ID, min_elev, max_elev

0, 500, 1000

1, 200, 250

....

100, 100, 1000

• Open this CSV file in QGIS as a vector layer and join with original vector layer using 'Joins' in properties menu. Use the unique field ID in both the vector layer and your CSV layer to perform the table join.

Voila! Now when you query any polygon in the vector layer you will see min_elev and max_elev attributes.