# [GRASS-user] Joining vectors by location

daniel mcinerney daniel.o.mcinerney at gmail.com
Mon Nov 21 09:46:16 EST 2011

```Hi Daniel.

You should use v.rast.stats [1] as it "Calculates univariate statistics
from a raster map based on
vector polygons and uploads statistics to new attribute columns. " Should
be a more straight
forward approach.

Best Regards,
Daniel.

[1] http://grass.osgeo.org/gdp/html_grass64/v.rast.stats.html

On Mon, Nov 21, 2011 at 3:40 PM, Daniel Lee <lee at isi-solutions.org> wrote:

> Hi group,
>
> Does anyone have a good idea of how to join vector areas by location? I'm
> trying to do aggregated statistics on vector maps using extracted values
> from rasters. I want the sum of the raster cells that lie within the
> vectors to be inserted into the vector attribute table. So far I've done
> stuff along these lines, but it's got a problem, and the main problem is
> that when I convert the rasters to polygons the polygons end up with
> stair-like boundaries rather than the smooth boundaries they had before
> they were converted to rasters. A second problem if I overlay the
> stair-like polygons over the original areas, the product of v.overlay is
> two polygons with a single centroid, which is absolutely wrong.
>
> Here's what I've been doing:
> 1. Convert the polygons to rasters with v.to.rast
> 2. Convert the rasters that I want to sum into the polygons to integers
> with r.mapcalc
> 3. Find the sums of the rasters for each polygon (in raster form) with
> r.statistics
> 4. Extract the summed values from the product of r.statistics with
> r.mapcalc, so that each polygon has the sum from the rasters as its value
> for all pixels.
> 5. Convert the rasters to vectors with r.to.vect
> -- This is where the first problem comes up - the boundaries are very
> jagged.
> 6. Connect the attributes with v.overlay
> -- Second problem: I get all the boundaries, not just the smooth ones or
> the jagged ones.
>
> Ideas:
> - Extract the centroids from the polygons and then join the polygon tables
> (extract the centroids with v.select, then use v.overlay? And as a last
> step use v.db.join to join the new values to the old geometries and get rid
> of the jagged boundaries?
> - Convert the rasters to points and then sum them inside the polygons?
> With what tool?
>
> Problems:
> - How can I connect the tables that no longer have thematic connections by
> the position of the geometries they belong to? I can connect the tables to
> the geometries by adding a layer to the map, but then I can't e.g. add the
> values of different fields together. I'm kind of at a loss.
>
> Thanks a lot fo the help!
> Daniel
>
> --
>
> B.Sc. Daniel Lee
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