# [R-sig-Geo] bottom area of a lake calculation

Thomas Adams te@3rd @ending from gm@il@com
Wed Aug 8 16:07:45 CEST 2018

```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 <tea3rd using gmail.com> 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, <sophie.leblanc using neuf.fr> wrote:
>
>> <span style="font-family:arial,helvetica,sans-serif;
>> font-size:12px">‌</span>Dear all,<br>
>> <br>
>> I would like to calculate the bottom area of a lake. I have already used
>> marmap to calculate projected area, i.e. surface area.<br>
>> 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.<br>
>> Do you have any idea? do you know an R package that could help?<br>
>> <br>
>> thanks,<br>
>> Sophie Leblanc<br>
>> <br>
>> <!-- +Signature --><font size="2"><em><a href="http://sophieleblanc6.wi
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