[GRASS-user] Reprojecting and resampling raster map

[stephen sefick]

a resolution of 50 in UTM would translate to 2500 sqm for the area of
a grid cell?  Is there any way to preform a weighted flow
accumulation?
thanks,

Stephen Sefick

On Thu, Jul 30, 2009 at 1:51 PM, Nikos
Alexandris<nikos.alexandris at felis.uni-freiburg.de> wrote:
>
> 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.
> Nikos
>
>> 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.
>
>
>



-- 
Stephen Sefick

Let's not spend our time and resources thinking about things that are
so little or so large that all they really do for us is puff us up and
make us feel like gods.  We are mammals, and have not exhausted the
annoying little problems of being mammals.

								-K. Mullis