<div dir="ltr"><div>Hello Sophie,</div><div><br></div><div>Yes, what you show was my understanding. I'm sure what you want to do can be done in R, but I'm not sure what packages to use. For doing such a thing, I use GRASS GIS. Regardless, I think you should re-project your data from latitude-longitude to Albers Equal Area or Lambert Conic Conformal, etc., that is area preserving. Eventually, you need to do this to get an area anyhow.</div><div><br></div><div>An additional step would be to reproject, then spatially interpolate the point locations you have in your spreadsheet into a grid to Albers Equal Area or Lambert Conic Conformal, etc.</div><div><br></div><div>I could take a stab at this and show you...</div><div><br></div><div>Regards,</div><div>Tom<br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Aug 8, 2018 at 10:39 AM, <span dir="ltr"><<a href="mailto:sophie.leblanc@neuf.fr" target="_blank">sophie.leblanc@neuf.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span style="font-family:arial,helvetica,sans-serif;font-size:12px">Dear Tom,<br>
<br>
thanks for your reply.<br>
Just to be sure I have explain correctly.<br>
With marmap I can get the green area but I also want to calculate the red one.<br>
I have attached the kind of data I have.</span><br>
<br>
<img class="m_4723247046284814012embedded-image" src="cid:pasteImage1533738760024.png@golive.mail" width="undefined" height="undefined"><span style="font-family:arial,helvetica,sans-serif;font-size:12px"></span><br>
<span style="font-family:arial,helvetica,sans-serif;font-size:12px">I do not have any GIS software.<br>
I wanted to convert my spherical coordinate to cartesian ones with</span><br>
<br>
x = radius * cos(latitude) * cos(longitude)<br>
y = radius * cos(latitude) * sin(longitude)<br>
z = radius * sin(latitude)<br>
radius= earth_radius-depth<span style="font-family:arial,helvetica,sans-serif;font-size:12px"> : not fully sure...<br>
Do you think it is correct?<br>
<br>
The slope is not the same everywhere around the lake.<br>
Is it possible to calculate area with your first explanations?<br>
what R package should I use?<br>
<br>
thanks,<br>
Sophie Leblanc</span><br>
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<div class="m_4723247046284814012gl_quote" style="margin-top:20px;padding-top:5px">De : "Thomas Adams"<br>
A : <a href="mailto:sophie.leblanc@neuf.fr" target="_blank">sophie.leblanc@neuf.fr</a>,"<a href="mailto:r-sig-Geo@r-project.org" target="_blank">r-sig-<wbr>Geo@r-project.org</a>"<br>
Envoyé: mercredi 8 août 2018 16:07<br>
Objet : Re: [R-sig-Geo] bottom area of a lake calculation<div><div class="h5"><br>
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<div>Sophie,</div>
<div> </div>
<div>My apologies, I was thinking in terms of using GRASS GIS... but the idea is the same</div>
<div> </div>
<div>Tom</div>
<div class="gmail_extra">
<div class="gmail_quote">On Wed, Aug 8, 2018 at 10:06 AM, Thomas Adams <span dir="ltr"><<a href="mailto:tea3rd@gmail.com" target="_blank">tea3rd@gmail.com</a>></span> wrote:
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<div>Sophie,</div>
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<div>I think a reasonable approximation might be obtained:</div>
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<div>(1) if you were to calculate the slope at each grid point using r.slope.aspect</div>
<div>(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.</div>
<div> </div>
<div>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...</div>
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<div>Tom</div>
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<div class="gmail_quote">On Wed, Aug 8, 2018 at 4:55 AM, <span dir="ltr"><<a href="mailto:sophie.leblanc@neuf.fr" target="_blank">sophie.leblanc@neuf.fr</a>></span> wrote:
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span style="font-family:arial,helve<wbr>tica,sans-serif; font-size:12px"></span>Dear all,<br><br>
<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><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><br>
Do you have any idea? do you know an R package that could help?<br><br>
<br><br>
thanks,<br><br>
Sophie Leblanc<br><br>
<br><br>
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</blockquote></div><br><br clear="all"><br>-- <br><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr">Thomas E Adams, III<div>1724 Sage Lane</div><div>Blacksburg, VA 24060</div><div><a href="mailto:tea3rd@gmail.com" target="_blank">tea3rd@gmail.com</a> (personal)</div><div><a href="mailto:tea@terrapredictions.org" target="_blank">tea@terrapredictions.org</a> (work)<br></div><div><br></div><div>1 (513) 739-9512 (cell)</div><div><br></div></div></div></div></div></div>
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