On Tue, Nov 21, 2017 at 9:26 AM, Moritz Lennert < [email protected]> wrote: > > On 20/11/17 21:20, Ken Mankoff wrote: >> >> Dear Helmut, >> >> On 2017-11-02 at 07:17, Helmut Kudrnovsky <[email protected]> wrote: >>> >>> Ken Mankoff wrote >>>> >>>> Does GRASS (or GDAL?, proj4?) have a function (or addon) to provide the >>>> scale error for a given cell? >>> >>> >>> AFAIR there is now an area function in r.mapcalc in GRASS trunk to calculate >>> the pixel area. >> >> >> There is. I've just built GRASS 7.5. But the area() function just returns nsres*ewres for each cell, it does not consider projection errors. > > > You could run area() both in your projected location and in a lat-long location (preferably using a similar datum), although you would have to be careful in the definition of the resolution in each location.
You can avoid problems with raster resolution by creating a vector grid with areas. Calculate the area sizes for the vector grid, then reproject the vector grid and calculate the area sizes again. Maybe the differences in area sizes for the same vector area give you what you are looking for. Markus M > Comparing the output should give you a notion of projection error. > > For visualization you might want to try the Indicatrix mapper Plugin in QGIS... > > Moritz > > > _______________________________________________ > grass-user mailing list > [email protected] > https://lists.osgeo.org/mailman/listinfo/grass-user
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