[GRASS-user] r.terraflow on massive DEM

Tue Jan 13 11:03:48 PST 2015

    One more idea is to make sure the DEM is projected.  Flow algorithms, in GRASS and ArcGIS, get messed up by unprojected surfaces.  Be careful choosing a projection for such a large area.        Also projecting should be done after you put the mosaic together, as you may get streaks around the boundaries of the individual DEMs that will affect the flow.  Or just run the individual DEMs and assemble the flow maps after.
-Thayer
      From: Daniel Victoria <daniel.victoria at gmail.com>
 To: Charlie Shobe <chsh5846 at colorado.edu> 
Cc: Thayer Young <thayeray at yahoo.com>; "grass-user at lists.osgeo.org" <grass-user at lists.osgeo.org> 
 Sent: Tuesday, January 13, 2015 10:46 AM
 Subject: Re: [GRASS-user] r.terraflow on massive DEM

Charlie,

I just downloaded some SRTM 1arc sec. from EarthExplorer. The data is supplied in 3 different file types, GeoTIFF, DTED or BIL and they are all in Integer values (Int16). No floating point elevation values.

Cheers
Daniel

On Tue, Jan 13, 2015 at 12:12 PM, Charlie Shobe <chsh5846 at colorado.edu> wrote:

Hi Daniel,
I believe that SRTM data does in fact provide floating point elevation values, so you may want to try working with the DEM as type FCELL from the very beginning. I don't know if this will help solve your problem, but the last time I brought in SRTM (1 arc second) data it was type FCELL and contained several decimal places of precision.
Good luck,
Charlie

On Jan 13, 2015, at 4:00 AM, Daniel Victoria <daniel.victoria at gmail.com> wrote:

Stephan, I'll give r.watershed a try and let it run for a couple of days. Thanks

Thayer, I used r.recode because the person that sent me the data messed up the null values. So in order to fix that I did:
1) use r.external to bring the data to Grass
2) fix null values with r.recode, which was faster than r.null

But I gave up on that path and since imported the data (r.in.gdal) and fixed the null values with r.null.
I'm also using integer values (CELL type) because from what I heard, SRTM does not provide floating point data.

Now I tried to run r.terraflow but I got a type error which asked me to use r.terraflow.short. Since I don't have that command in my grass instalatin, I converted the SRTM data to float. But that was to no avail since it's now giving me a dimensions type overflow error and asking me to change the dimension_type and recompile.

I'll give r.watershed a try. If that does not work, I'll see if I can recompile.

Thanks
Daniel

On Mon, Jan 12, 2015 at 9:22 PM, Thayer Young <thayeray at yahoo.com> wrote:

With regards to the size of DEM, if you can get the command to run it may take several days to finish, this is more than twice the size of the Washington State DEM that took the authors 33 hours in 2003 (Arge, Chase, Halpin, Toma, Urban, Vitter, Wickremesinghe  Efficient Flow Computation on Massive Grid Terrain Datasets Geoinformatica Volume 7 Issue 4, December 2003 Pages 283 - 313 ).  My mid-2014 computer runs at about twice the speed quoted in the paper.
Did you r.recode it to make it smaller?  In flat areas you need the data contained after the decimal point to allow the flow a chance to make it down hill, otherwise you can get streams going no where near where they do in real life.  I have never tried running r.terraflow on an integer raster, I know that it works on decimal rasters.

-Thayer

Date: Mon, 12 Jan 2015 09:59:40 -0200
From: Daniel Victoria <daniel.victoria at gmail.com>
To: grass <grass-user at lists.osgeo.org>
Subject: [GRASS-user] r.terraflow on massive DEM
Message-ID:
    <CA+irsJjf7xhWgU968D+4+xc2YGJ=_n4xJtP0_WeU1ZpoRTt9ng at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

Hi list,

I'm trying to run r.terraflow on a very large DEM but I wander if it's
__too large__.

The region dimensions are:
ncol=141114
nrow=140487
Data type = CELL

The map is actually a reclass map of a raster that I imported using
r.external.

When I run r.terraflow I get:

r.terraflow elevation=srtm_brasil at PERMANENT filled=srtm_fill
direction=flowdir swatershed=sink accumulation=flowacc tci=srtm_tci
directory=E:\terraflow_temp

WARNING: raster srtm_brasil is of type CELL_TYPE --you should use
r.terraflow.short
ERROR: [nrows=140487, ncols=141114] dimension_type overflow -- change
dimension_type and recompile
(Mon Jan 12 09:57:18 2015) Command finished (0
sec)

However, I don't have an r.terraflow.short command
I'm running Grass 7.0.0beta4 from OsGeo4W

Cheers
Daniel
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