[GIS] Delineate polygonal catchment areas from a raster using open-source GIS

gdalopen-source-gisqgis

I'm 90% sure this basic question has been asked before but looking through the other posts I simply do not see a useful answer.

I need to calculate polygonal(vector) catchment areas from a raster based on minimum driving distance/cost from a set of points. I've already got the raster, and for that matter, the nodes(intersections with cost attribute) used to interpolate it. What I need now are polygons for each point describing the area nearer it than any other point. This is basically like a voronoi diagram but based on network distance rather than euclidean space.

How exactly can I do this using only open-source tools? I'm comfortable with the command line if I just know what I'm looking for. I feel like GDAL is going to be the answer, but if so, what exactly is the command I'm looking for?

Thanks!

Best Answer

You can use

GRASS GIS (an GRASS GIS from QGIS): Vector aggregate values
SAGA GIS from QGIS: Some hydrological analysis
PostGIS from QGIS: Creating Catchment Areas with pgRouting and QGIS (Underdark)

Related Solutions

Extracting Raster Values at Points Using Open Source GIS

QGIS "Point Sampling Tool" should be the plugin you're looking for.

Here's a detailed description of how to use it: http://pvanb.wordpress.com/2010/02/15/sampling-raster-values-at-point-locations-in-qgis/

Update based on Paolo's comment:

the plugin is not the only solution, and not always the easiest solution anymore. An alternative solution is the Saga function ‘Add raster values to point’ in the processing toolbox. See for details http://pvanb.wordpress.com/2014/07/01/sampling-raster-values-at-point-locations-in-qgis-an-update/

[GIS] Conversion of point grid to a raster with open source tools (Gdal, QGIS …)

A couple of options:

Convert the PostGIS layer to a shapefile in QGIS (save-as), then use the Vector|Conversion|Rasterize tool;
Use the gdal_rasterize command directly.

For the second option:

gdal_rasterize -a VAL -ts [x] [y] PG:'host=localhost dbname=DB user=USER'  
  -sql "SELECT the_geom, VAL FROM table" out.tif

Where:

VAL = the value to assign to the raster (same as the VAL in the SELECT statement);
[x] [y] are the X and Y dimensions of the extent of the point table;
DB and USER are the database and username;
the_geom and VAL are the geometry and value columns you're interested in from 'table', respectively;
out.tif is the output TIF file.

Note: the heatmap plugin doesn't interpolate values - it uses something like a kernel density algorithm to smooth known values for areas around the measured points.

Best Answer

Related Solutions

Extracting Raster Values at Points Using Open Source GIS

[GIS] Conversion of point grid to a raster with open source tools (Gdal, QGIS …)

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