[GRASS-user] r.lake units

M S mseibel at gmail.com
Tue Aug 7 11:46:25 EDT 2007

As usual, I continue to be thoroughly impressed with the capabilities and
functionality of GRASS GIS!  A huge thanks out to everyone contributing to
the ever growing success of GRASS..

r.lake is really cool.  I am looking forward to producing some animated
raster series with it.  Currently, I am using it for stage/storage
calculations of natural and man made features.  The blue color ramp for
color depth is a very nice touch for illustrative purposes.  This is another
"out of the box" function in GRASS for which there is no comparative
function in that other commercial GIS software (to my knowledge).  I really
like the calculations r.lake outputs as well such as area and volume.

I have read through the 6.3 man pages, and could not find this answer.  The
output of r.lake gives a warning stating that "Volume is only correct if
lake depth (terrain raster map) is in meters."  To me this was ambiguous if
x,y and z of the terrain map need to be in meters, or just the Z value.

I presumed that at a minimum the Z value needs to be converted to meters. My
data resides in stateplane, units feet, coordinate system.  I did a crude
measurement of the area of the outputted lake area to compare it against the
"lake area" output from r.lake.  Upon converting my measurements (in feet)
against the area reported by r.lake (meters), it seems that the lake area
output converts the area to meters automagically, however, I wanted to
verify this on the list.

After the long preamble, finally the question... For correct lake volumes
outputted by r.lake, can the data be in feet for x,y, and meters for Z to
obtain correct volume?   As a kind suggestion, perhaps input unit
clarification would be a nice addition to the man page for this cool

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