[GRASS-user] [GRASSLIST:1160] RE: Surface area of a DEM

[Jonathan Greenberg]

I was thinking this, but does the fact that a DEM grid cell is square cause
complications in this formulation?  A 45 degree slope to the west would have
a different surface area than a 45 degree slope to the southwest.  I've come
across some solutions that use a set of 8 triangles created by the lines
between the center cell and each of its 8 neighbors to help in the
calculation, but I was hoping this ability might be buried in a GRASS
command someplace.

 

Thoughts?

 

--j

--

Jonathan A. Greenberg, PhD
NRC Research Associate
NASA Ames Research Center
MS 242-4
Moffett Field, CA 94035-1000
Phone: 415-794-5043
AIM: jgrn3007
MSN: jgrn3007 at hotmail.com 

  _____  

From: Smith, Michael [mailto:Michael.Smith at maine.gov] 
Sent: Wednesday, July 19, 2006 4:21 PM
To: Jonathan Greenberg
Subject: RE: [GRASSLIST:1159] Surface area of a DEM

 

Well wouldn't the true area be the length of one side x (length of other
side / cosine of slope (in degrees))?

 

And since it doesn't really matter which side you choose, it should be:
area of cell / cosine of slope = "real" area on the ground, or in your case
900/cos(slope).

 

Michael Smith
Maine DEP GIS Unit

 

  _____  

From: owner-GRASSLIST at baylor.edu on behalf of Jonathan Greenberg
Sent: Wed 7/19/2006 6:24 PM
To: 'GRASS user list'
Subject: [GRASSLIST:1159] Surface area of a DEM

I'd like to estimate the surface area of each cell in a DEM (r.surf.area
seems to just give you the value for the entire dem, no?) -- by surface area
I am referring to the "true" ground area as opposed to the planar projected
ground area (which is constant for all cells) -- flat areas the area = the
planar projected area (e.g. a 30m cell that is flat has 900m2 of surface
area, but an inclined slope will have > 900m2).  Is there a grass module to
do this, or any recommendation on how to calculate this?  Thanks!

--j

--

Jonathan A. Greenberg, PhD
NRC Research Associate
NASA Ames Research Center
MS 242-4
Moffett Field, CA 94035-1000
Phone: 415-794-5043
AIM: jgrn3007
MSN: jgrn3007 at hotmail.com



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