[GRASS5] Re: GDAL, GRASS and AVHRR

[Bob Covill]

Markus & Frank,

Which i.rectify did you use? You may get better results with i.rectify3
and i.points3. I have found that these tend to produce less unusual
distortions than the original i.rectify.

This also brings up the question of which points / rectify programs are
more accurate, up to date, etc?


Markus Neteler wrote:
> 
> Hi developers,
> 
> in below mail Frank and me were discussing the geocoding of AVHRR
> satellite images. AVHRR data come with a nice set of GCPs.
> Those are extracted to i.* GRASS format when importing an AVHRR
> data set with r.in.gdal.
> 
> Unfortunately the i.rectify does not perform good rectification
> for this sort of "distortion". See below for details (and my
> call for help):
> 
> On Fri, Oct 11, 2002 at 09:17:39AM -0400, Frank Warmerdam wrote:
> > Markus Neteler wrote:
> > > In general i.rectify works well. But not for AVHRR rectifications...
> > > I used the standard approach: r.in.gdal -> GCPs POINTS file -> i.rectify
> > > from XY to LatLong LOCATION.
> > > The problem is that neighter 2nd nor 3rd order polynoms apply to rectify
> > > AVHRR data accurately.
> > >
> > > Find attached two tests (2nd + 3rd order polynoms): I have extracted the
> > > GCPs and just rectified them. There is a horrible overfitting at the
> > > "ends" of the AVHRR image:
> > >  irectify_2nd_order.gif
> http://mpa.itc.it/markus/tmp/irectify_2nd_order_avhrr.png
> 
> > >  irectify_3rd_order.gif
> http://mpa.itc.it/markus/tmp/irectify_3rd_order_avhrr.png
> 
> > > which results in a 20km shift (see the lake in white vector lines east
> > > from the dark lake in AVHRR):
> > > avhrr_geolocation.gif
> http://mpa.itc.it/markus/tmp/avhrr_geolocation.png
> 
> > > That's why I was wondering to incorporate the recification into
> > > GDAL itself (or write a better i.rectify).
> >
> > Markus,
> >
> > Ahh. We experienced this problem at PCI with some kinds of AVHRR data as
> > well.  I believe our solution there was to add a "thin plate splines"
> > transformation modelling in addition to the existing polynomial
> > transformations.  This is a transform with as many terms as there are
> > GCPs and gives increasing weight to gcps that are closer to the point
> > being warped.  Almost a piecewise warp.
> >
> > Implementing thin-plate splines, or some similar transformation
> > mechanism is not trivial.  It also doesn't specifically belong in
> > GDAL.
> >
> > I would suggest that this issue be brought back to the GRASS dev list,
> > and see if any of us can find a suitable source for a thin plate spline
> > algorithm (or another with similar properties).  This could then
> > be implemented in i.rectify or perhaps an i.rectify.tps.
> 
> So: Is there anyone willing to help with extending i.rectify to
> support also splines based rectification? Could we probably use functions
> from
> src/libes/rst_gmsl/
> ?
> 
> AVHRR is a pretty interesting and huge data base as this set of satellites
> was initially launched 1982. Data are available here (download):
> Satellite Active Archive (SAA)
> http://www.saa.noaa.gov/
> 
> Thanks in advance,
> 
>  Markus
> 
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-- 
Bob Covill

Tekmap Consulting
P.O. Box 2016
Fall River, N.S.
B2T 1K6
Canada

E-Mail: bcovill at tekmap.ns.ca
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