[GRASS-user] gdal_translate

Wed Jul 7 03:18:25 EDT 2010

Hi

Which module do I use to change the resolutions?

2010/7/6 <micha at arava.co.il>

> Hello Sandile:
> It seems you are importing two raster with vastly different resolutions.
> (I think we already came across this). See below...
>
> > Hi
> >
> > Below is a step-by-step of what I have done but I'm getting an error when
> > running v.rast.stats vector=catchments raster=rainfall layer=1
> > colprefix=area.
> >
> >
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > r.in.gdal
> > in=/home/tgumede1/grassdata/Cape_Town/TRMMLast1day.tif out=rainfall
> >
> > Projection of input dataset and current location appear to match
> >  100%
> > r.in.gdal complete. Raster map <rainfall> created.
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > g.region rast=rainfall -p
> > projection: 3 (Latitude-Longitude)
> > zone:       0
> > datum:      wgs84
> > ellipsoid:  wgs84
> > north:      33:30S
> > south:      33:45S
> > west:       18:15E
> > east:       19E
> > nsres:      0:15
> > ewres:      0:15
> > rows:       1
> > cols:       3
> > cells:      3
>
> Here, the rainfall data has a resolution of 0:15 = 15 minutes or 1/4
> degree. THat's approximately (at the equator) about 27 km. So *one* raster
> cell is 27 km X 27 km =~ 730 sq.km. Your region is covered by 3 cells, 1
> row by 3 columns. Not very helpful data!
>
> Next...
>
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > r.in.gdal
> > in=/home/tgumede1/grassdata/Cape_Town/Dem_CF.tif out=dem target=SRTMDEM
> >
> >
> > Projection of input dataset and current location appear to match
> >  100%
> > r.in.gdal complete. Raster map <dem> created.
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > g.region rast=dem -p
> > projection: 3 (Latitude-Longitude)
> > zone:       0
> > datum:      wgs84
> > ellipsoid:  wgs84
> > north:      33:40:46.499215S
> > south:      34:00:52.499215S
> > west:       18:17:55.500436E
> > east:       19:10:16.500436E
> > nsres:      0:00:03
> > ewres:      0:00:03
> > rows:       402
> > cols:       1047
> > cells:      420894
>
> Your DEM layer, on the other hand, is of resolution 3 arc seconds, or
> about 90 meters on a side. So each cell is 90 m. X 90 m = 8100 sq.m. =~
> 0.0081 sq.km.
>
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > r.watershed elevation=dem
> > accumulation=acc drainage=direction basin=catch stream=str threshold=200
> >
>
> Here you chose a threshold of 200. That's 200 cells, so about 200 X 0.0081
> sq.km, or 1.6 sq km. As a result (see below, the output of r.to.vect) you
> are getting over 19,000 tiny little catchments. Are you sure that's what
> you want??
>
> Finally, you're trying to get raster values for 19,000 tiny vector areas
> where the raster (rainfall) is only 3 cells! You'll have 1000's of
> catchments with all the same values. And I guess that some of these
> catchments are extending outside of the three rainfall cells, and causing
> the  NULL value error.
>
> In short: I think you'll need to match the resolution of the DEM to that
> of the rainfall data. If the rainfall is only as accurate as 1 data value
> per 730 sq.km. then you will be able to do vector-raster analyses only at
> that resolution = i.e. continent scale maps.
>
> HTH
> --
> Micha
>
> >
> > SECTION 1a (of 5): Initiating Memory.
> > SECTION 1b (of 5): Determining Offmap Flow.
> >  100%
> > SECTION 2: A * Search.
> >  100%
> > SECTION 3: Accumulating Surface Flow.
> >  100%
> > SECTION 4: Watershed determination.
> >  100%
> > SECTION 5: Closing Maps.
> >  100%
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > r.to.vect -s in=catch out=catchments
> > feature=area
> > Extracting areas...
> >  100%
> >  100%
> > Building topology for vector map <catchments>...
> > Registering primitives...
> > 60653 primitives registered
> > 314051 vertices registered
> > Building areas...
> >  100%
> > 19885 areas built
> > 1 isles built
> > Attaching islands...
> >  100%
> > Attaching centroids...
> >  100%
> > Number of nodes: 40769
> > Number of primitives: 60653
> > Number of points: 0
> > Number of lines: 0
> > Number of boundaries: 40768
> > Number of centroids: 19885
> > Number of areas: 19885
> > Number of isles: 1
> > r.to.vect complete.
> >
> > GRASS 6.4.0RC5+39438 (SRTMDEM):~ > v.rast.stats vector=catchments
> > raster=rainfall layer=1 colprefix=precip
> >
> > DBMI-DBF driver error:
> > SQL parser error: syntax error, unexpected NULL_VALUE processing 'NULL'
> > in statement:
> > UPDATE catchments SET precip_min=-NULL WHERE cat=10163
> > Error in db_execute_immediate()
> >
> > ERROR: Error while executing: 'UPDATE catchments SET precip_min=-NULL
> > WHERE
> >        cat=10163'
> >
> >
> >
> > Here is the output of gdalinfo TRMMLast1day.tif
> >
> > Origin = (18.250000000000000,-33.500000000000000)
> > Pixel Size = (0.250000000000000,-0.250000000000000)
> >
> > --------------------
> > coordinates--------------------------------------------
> > Corner Coordinates:
> > Upper Left  (  18.2500000, -33.5000000)
> > Lower Left  (  18.2500000, -33.7500000)
> > Upper Right (  19.0000000, -33.5000000)
> > Lower Right (  19.0000000, -33.7500000)
> > Center      (  18.6250000, -33.6250000)
> >
> >
> > Here is what I did to clip the region of interest.
> >
> > gdal_translate -a_srs EPSG:4326 -projwin 18.2987501 -33.6795831
> 19.1712501
> > -34.0141665 3B42RT.2010032900.1day.tif TRMMLast1day.tif
> >
> >
> > Is there something I have done wrong in these steps or there is something
> > wrong with my coordinates?
> >
> >
> >
> > 2010/7/6 <micha at arava.co.il>
> >
> >> > Hi
> >> >
> >> > Is it wrong to use  -a_ullr option in gdal_translate to clip a small
> >> > portion
> >> > of the region from the big geotiff file region?
> >> >
> >>
> >> The option -a_ullr will change the georeference of the resulting file,
> >> so
> >> you could say it's "wrong" if you want to keep the original referencing.
> >> The way to clip a portion of the original and still maintain
> >> geo-referencing is with the -projwin option.
> >>
> >> --
> >> Micha
> >>
> >> > --
> >> > Kind Regards
> >> > TS Gumede
> >> > CSIR, Meraka Institute
> >> > 072 258 1650
> >> >
> >> > This mail was received via Mail-SeCure System.
> >> >
> >> >
> >> >
> >>
> >>
> >
> >
> > --
> > Kind Regards
> > TS Gumede
> > CSIR, Meraka Institute
> > 072 258 1650
> >
> > This mail was received via Mail-SeCure System.
> >
> >
> >
>
>

-- 
Kind Regards
TS Gumede
CSIR, Meraka Institute
072 258 1650
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