[GRASSLIST:3311] Re: r.volume help

[Markus Neteler]

On Wed, Mar 06, 2002 at 12:22:16PM +0530, Anil Kumar Bhagat wrote:
> 
> Dear GRASS Users,
>                   I want to calculate the depth of a lake . For that, I
> have generated the DEM map from contour data . Now to calculate the Volume 
> , I am runing the module <r.volume> I am giving the following input for
> <r.volume>
> 
> 1) DEM map 
> 2) output map from r.clump ( The input for  r.clump I am using the
> DEM map and I am getting one output map )
> 
> the module is runing but it is showing all the volume 0.00 
> 
> May I get help in the regard .

as far as I know the r.volume is calculating into positive direction.
For a lake that means that you have to "invert" it to be treated like
a mountain. I assume that you used negative depth. However, r.volume
only likes a positive height for volume calculation.

To change that, generate a temp lake file inverting the depth to a height
('lake' may be the lake DEM):

r.mapcalc lake_mountain="(-1.0) * lake"

If the lake shore is not at 0 elevation, you have to add
this shift to above formula.

Hope you can understand my try for an explanation...

Markus Neteler