[GRASS-user] scale error for each pixel

[Markus Metz]

On Tue, Nov 21, 2017 at 9:26 AM, Moritz Lennert <
mlennert at club.worldonline.be> wrote:
>
> On 20/11/17 21:20, Ken Mankoff wrote:
>>
>> Dear Helmut,
>>
>> On 2017-11-02 at 07:17, Helmut Kudrnovsky <hellik at web.de> 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
>
>
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