# Raster aggregate values

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Revision as of 08:49, 26 August 2010 by DylanBeaudette (talk | contribs) (added some details regarding r.resamp.stats)

**Q:** How can I calculate the average map values by spatial unit (grid)? For example, I would like to calculate the average slope by square kilometer for a given area.

**A:** Procedure outline:

- Make a slope map from the DEM with r.slope.aspect at the same resolution and bounds as the DEM. (run 'g.region rast=dem' first)
- Make the grid with v.mkgrid, with box=1000,1000 (assuming the location is meters-based)
- Draw a 1 km x 1 km grid (d.grid), and overlay the v.mkgrid vector map (area fill = none, or only display boundary features, colored red or so) just to verify where it will be and what it will look like.
- Run v.rast.stats to calculate stats for each grid box, and upload to that area's attribute table.

**A:** Another approach using r.resamp.stats:

- Generate a region that defines the required grid topology: extent, cell size, origin
- Zoom to that region, and set the resolution accordingly
- Aggregate slope values within each of the cells defined by this new region using r.resamp.stats
- The resulting raster will contain aggregate values of the input raster, using the new grid topology
- Optionally vectorize with r.to.vect