# bottom area of a lake calculation

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## bottom area of a lake calculation

 Dear all,

I would like to calculate the bottom area of a lake. I have already used marmap to calculate projected area, i.e. surface area.
But i have not found anything to calculate a non projected area. I have the coordinates of points (longitude, latitude and depth) I can convert them into xyz. But then I don't know how to calculate the surface I have found polygonal area calculation but I have a 3D object, a polyheron and I think I can not use polygonal area.
Do you have any idea? do you know an R package that could help?

thanks,
Sophie Leblanc

http://sophieleblanc6.wix.com/photographie         [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list [hidden email] https://stat.ethz.ch/mailman/listinfo/r-sig-geo
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## Re: bottom area of a lake calculation

 Sophie, I think a reasonable approximation might be obtained: (1) if you were to calculate the slope at each grid point using r.slope.aspect (2) use r.mapcalc to calculate A/cos(slope), where A is the pixel projected area (e.g., 100mx100m = 10000 m^2) -- lat-long coordinates will not work. Consequently, if the slope were approximately 0, cos(slope) approaches 1 and A/cos(slope) = A; as slope approaches 90 deg, cos(slope) becomes very small, so A becomes very large. This works where the slope direction is orthogonal to a pixel edge, so a correction to account for other slope directions may be needed. But, with smaller grids, the error may be negligible for you... Tom On Wed, Aug 8, 2018 at 4:55 AM, <[hidden email]> wrote: > Dear all,
>
> I would like to calculate the bottom area of a lake. I have already used > marmap to calculate projected area, i.e. surface area.
> But i have not found anything to calculate a non projected area. I have > the coordinates of points (longitude, latitude and depth) I can convert > them into xyz. But then I don't know how to calculate the surface I have > found polygonal area calculation but I have a 3D object, a polyheron and I > think I can not use polygonal area.
> Do you have any idea? do you know an R package that could help?
>
> thanks,
> Sophie Leblanc
>
> http://sophieleblanc6.wix.com/photographie > >         [[alternative HTML version deleted]] > > _______________________________________________ > R-sig-Geo mailing list > [hidden email] > https://stat.ethz.ch/mailman/listinfo/r-sig-geo>         [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list [hidden email] https://stat.ethz.ch/mailman/listinfo/r-sig-geo
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## Re: bottom area of a lake calculation

 Sophie, My apologies, I was thinking in terms of using GRASS GIS... but the idea is the same Tom On Wed, Aug 8, 2018 at 10:06 AM, Thomas Adams <[hidden email]> wrote: > Sophie, > > I think a reasonable approximation might be obtained: > > (1) if you were to calculate the slope at each grid point using > r.slope.aspect > (2) use r.mapcalc to calculate A/cos(slope), where A is the pixel > projected area (e.g., 100mx100m = 10000 m^2) -- lat-long coordinates will > not work. > > Consequently, if the slope were approximately 0, cos(slope) approaches 1 > and A/cos(slope) = A; as slope approaches 90 deg, cos(slope) becomes very > small, so A becomes very large. This works where the slope direction is > orthogonal to a pixel edge, so a correction to account for other slope > directions may be needed. But, with smaller grids, the error may be > negligible for you... > > Tom > > > On Wed, Aug 8, 2018 at 4:55 AM, <[hidden email]> wrote: > >> Dear all,
>>
>> I would like to calculate the bottom area of a lake. I have already used >> marmap to calculate projected area, i.e. surface area.
>> But i have not found anything to calculate a non projected area. I have >> the coordinates of points (longitude, latitude and depth) I can convert >> them into xyz. But then I don't know how to calculate the surface I have >> found polygonal area calculation but I have a 3D object, a polyheron and I >> think I can not use polygonal area.
>> Do you have any idea? do you know an R package that could help?
>>
>> thanks,
>> Sophie Leblanc
>>
>> http://sophieleblanc6.wix.com/photographie >> >>         [[alternative HTML version deleted]] >> >> _______________________________________________ >> R-sig-Geo mailing list >> [hidden email] >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo>> > > > > > >         [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list [hidden email] https://stat.ethz.ch/mailman/listinfo/r-sig-geo
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