polygonqgisraster
I have two layers: a raster layer and a vector layer (polygon). I can see the values of the raster pixels but I don't know how to combine them with information from the polygon layer. More specifically, I would like to :
Can anybody help and advise on how to do this in QGIS?
This kind of thing is best done with Spatialite and SQL.
First you will need to load you data into a Spatialite database which can be doing using the DBManager plugin that ships with QGIS. Click the Import Layer/File button.
With your data into a database you can then run the following query using the SQL button. You will just have to change the names of the columns and tables to suit your data.
SELECT COALESCE(SUM(a2.pop),0) as pop_neighbours,
a1.pop,
a1.name,
a1.id,
a1.geomm FROM areas a1
LEFT OUTER JOIN areas a2 ON NOT a1.id = a2.id
AND intersects(a2.geomm, a1.geomm)
GROUP BY a1.id
Tell the query tool your unique id column (id) and geometry column (geomm), then just click load.
You should having something like this, once you label it of course
The Query Breakdown
We are joining the layer onto itself using:
LEFT OUTER JOIN areas a2 ON NOT a1.id = a2.id
AND intersects(a2.geomm, a1.geomm)
but only where the geometries intersect and the ids are not the same, otherwise we end up with the same record twice for each polygon. We are also using a LEFT OUTER JOIN so that we include the records that don't join i.e have no neighbours.
In the select part:
SELECT COALESCE(SUM(a2.pop),0) as pop_neighbours,
a1.pop,
a1.name,
a1.id,
a1.geomm
we are using COALESCE in order to convert the NULLS (no neighbours) into a 0 otherwise they just stay NULL.
Then we just GROUP BY a1.id so that we get a single record for each polygon.
You should use the r.series tool from the GRASS commands in the processing toolbox tools.
Here's an example of 3 rasters with 0, 1 data, and their differing extents
The r.series tool lets you choose the algebraic operation, in this case SUM
And the result is 1 raster with a conjoined extent, with values ranging from 0 to 3 (well, in this case only 2 as I didn't have a pixel shared by all 3)
Best Answer
Use to be that you had to use v.rast.stats in the GRASS Plugin, but now you can use the inbuilt Zonal Statistics in the Raster menu.
You have a polygon layer loaded and the raster. You run the menu item.
that will answer part a) of your question.
v.rast.stats and other tools will give you more detailed output, but probably a simple workflow for b) is to extract each polygon (use the Raster|Extraction|Clipper menu) and then run a histogram on each. Depends on the structure of your data, how often you have to do it, etc.