[GRASS-user] r.surf.area

[Ken Mankoff]

Hi Valter,

On Fri, May 29, 2020 at 3:43 AM Valter Albino <valteralbino at gmail.com>
wrote:

> Suppose you compute the "r.surf.area"(1) to some raster file and find that
> the 3d area is smaller than your 2d area?
> What could be the problem?
> Note:
> The exercise was done with a 5 m cell size raster file, with 37 km2
> watershed with a slope of 0,0341 m/m, within 7.8 version of GRASS GIS 7.8
> console in a 3.12.3 QGIS [with this method the difference is higher (12 ha,
> comparing with ArcGIS)] and outside QGIS, in GRASS GIS GUI
> Thanks in advance
>

Can you provide a small example, perhaps just a few grid cells, that
reproduces the problem? Also, it isn't clear if you see the same problem in
ArcGIS, QGIS, and GRASS, or just GRASS.

How are you calculating the 2D area? Do you note in r.surf.area under
DESCRIPTION that it says " Therefore, area of a flat surface will be
reported as (rows + cols -1) * (area of cell) less than area of flat region
due to a half row and half column missing around the perimeter." and does
this explain what you're seeing?

  -k.
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