r.surf.area

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r.surf.area

Valter Albino
Hi GRASS members
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


Cumprimentos,
Valter Albino -
Geógrafo Físico, M.Sc.
Modelação H&H / Riscos ambientais / OT&U
www.valteralbino.wixsite.com/hydrodynamics

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Re: r.surf.area

Ken Mankoff
Hi Valter,

On Fri, May 29, 2020 at 3:43 AM Valter Albino <[hidden email]> 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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Re: r.surf.area

Valter Albino
Good evening Ken

You should be right, the watershed has a reservoir.
I will test it in another place, with a hillslope, then I will give feedback.
Thank you for your comment!

Cumprimentos,
Valter Albino -
Geógrafo Físico, M.Sc.
Modelação H&H / Riscos ambientais / OT&U
www.valteralbino.wixsite.com/hydrodynamics


Ken Mankoff <[hidden email]> escreveu no dia quinta, 4/06/2020 à(s) 18:03:
Hi Valter,

On Fri, May 29, 2020 at 3:43 AM Valter Albino <[hidden email]> 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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