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?
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
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.
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@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
stephen sefick wrote:
When you reproject a raster map to a new location does it
automatically re-sample it to the regions resolution.
The new map is created using the destination region. The original map
is read at its native resolution, regardless of the source region.
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?
For each 50x50m destination cell, r.proj reverse-projects the cell's
centre coordinate to the source projection, and samples the input map
at the projected coordinate. Exactly how it samples the map depends
upon the method= option (nearest, bilinear, cubic), but the sampling
or interpolation is based upon the raw source map, without any further
resampling.
--
Glynn Clements <glynn@gclements.plus.com>
Thank you all for your helpful comments
On Fri, Jul 31, 2009 at 3:04 PM, Glynn Clements<glynn@gclements.plus.com> wrote:
stephen sefick wrote:
When you reproject a raster map to a new location does it
automatically re-sample it to the regions resolution.
The new map is created using the destination region. The original map
is read at its native resolution, regardless of the source region.
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?
For each 50x50m destination cell, r.proj reverse-projects the cell's
centre coordinate to the source projection, and samples the input map
at the projected coordinate. Exactly how it samples the map depends
upon the method= option (nearest, bilinear, cubic), but the sampling
or interpolation is based upon the raw source map, without any further
resampling.
--
Glynn Clements <glynn@gclements.plus.com>
--
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