[GRASS-user] r.report units in LatLong location

[Hamish]

Gordon Keith wrote:
> I'm running r.report and specifying units=me in a Lat-Long location but the 
> values it returns appear to be in square degrees not square meters.
> 
> Is there a way to get it to report in square meters?

In general the raster format / libraries / modules did not change between GRASS
5 and 6 except to add the new 3D raster modules. So if it used to work I'd be
surprised if it was anything major.

change history: http://freegis.org/cgi-bin/viewcvs.cgi/grass6/raster/r.report/
(here revs 1.x are GRASS 5, revs 2.x are GRASS 6)

If it never worked it will be a case of calculating the cell area per row for
LL locations [nsres*60nm->m * (ewres*60nm->m * cos(lat))]. I don't really feel
qualified to anticipate what sort of systematic cumulative errors that might
introduce or what a better equal-area projection to use might be on such a
continental scale.

> I remember doing this with r.stats back in grass5, but it doesn't seem to
> work any more.

could you elaborate? exact commands? do you still have GRASS 5 installed to
test against?

etopo2 might be a nice common dataset for testing this; On the same lines as
your query I'm not sure how correctly 'r.buffer units=nautmiles' works in LL
for the simulating the EEZ region (does it create oranges when we want to see
pears?). In general though the old raster code is much more LL aware than the
newer vector code, so I'd be hopeful.

I'm pretty sure d.what.vect in LL uses great circle line length for, well, line
length; I don't know if it's area calculation is smart like that too. If it is,
then it's a simple r.mapcalc, r.to.vect, v.overlay problem.


> The sort of question I am trying to answer is what area of Australia's EEZ is
> the 100-700 m depth range. I have a raster of .0025 degree topography and a 
> shape file of the EEZ. I've converted the shape to a raster and am trying to 
> get the result.

the task seems entirely reasonable.


regards,
Hamish




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