[GRASS-user] Reprojecting and resampling raster map

Nikos Alexandris nikos.alexandris at felis.uni-freiburg.de
Thu Jul 30 14:51:58 EDT 2009

stephen sefick wrote:

> When you reproject a raster map to a new location does it
> automatically re-sample it to the regions resolution.  I have a 10x10
> meter grid in albers equal area projection.  I have reprojected it
> into UTM zone 17 res=50.  This is a base flow index map bfi.  Does
> grass resample the map to 50x50m?

Yes, given that the _target_ location is of res=50. You can, however,
control this by changing the resolution (g.region) or directly at the
reprojection process by setting the "resolution=" parameter of the
"r.proj" module. You can even set the resampling algorithm (default
nearest neighbour or bilinear).

Hope this is what you've been looking for.

> The problem is this:
> I have an average runoff value (arv) in m/s (converted from mm/day
> into m/s by dividing by 86,400s*1000mm).
> flow accumulation grid produced by r.terraflow (which was 30x30m
> resampled to 50x50m).
> flow accumulation * 250 (to get square meters (50x50))
> bfi*arv*flow accumulation (sqm)
> to get discharge in cms
> I converted this to cfs (multiplied by 35.314454)
> the resulting map is discharge in cfs
> The estimate for discharge is ~3x what the long term average is at
> usgs gauging stations along the rivers of interest (point layer to
> identify where they are located).  The only thing that I have been
> able to think of at this point is that there is a resampling step I am
> missing because 10x10=100 and 50x50=250.  which would account for
> almost the 3x discrepancy.
> If you need anymore information please don't hesitate to ask.  I
> appreciate any and all help.

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