I'm sorry if this question is badly framed, I only started with GIS and QGIS 2 days ago so I don't know any terminology yet - I'll try to explain as best I can.

I have 2 layers in QGIS (1.8, on Win7), one with geological information, and the other with the state boundaries.

I want to get out a report which tells me the composition of each state, i.e. a table of the form:

State name    Rock type 1    Rock type 2
State1            32 %           68 %      
State2             5 %           95 %     

I guess this is a pretty basic thing to want to do, but I'm having trouble finding how on my own because I don't know what to search for.

Sorry if bad question, thanks for any help.

  • 2
    It would be helpful to know what information is available in each layer. Most likely the solution will involve a clip - creating a sub-table that only has geology information for Arizona, and so on. From there, you can calculate the percentages, and then recombine the sub-tables at the end. So look up help on selection, clipping, and area calculation. – L_Holcombe Oct 12 '12 at 20:45

Assuming both layers are vector layers (as opposed to raster, which pixelates when zoomed close), I would do an intersect between the two, found under Vector->Geoprocessing Tools. This creates a new layer that cuts each rock area type out of the state geography and keeps that attribute.

Then you can add an area field per instructions for the field calculator here and then hop into your spreadsheet tool of choice to crosstab area by rock type by state.

  • There is good support for spatial analysis rasters in QGIS, if that is what form the geological information is in. – Willy Oct 14 '12 at 10:13

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