[GRASS5] Re: GDAL, GRASS and AVHRR

[Markus Neteler]

On Fri, Oct 11, 2002 at 11:17:24AM -0300, Bob Covill wrote:
> 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.

I was using i.rectify (so the former i.rectify2).
I should try i.rectify3, somehow I forgot about that...
 
> This also brings up the question of which points / rectify programs are
> more accurate, up to date, etc?

A good question. In 5.1 we should only have one i.rectify.

Markus


> 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
> > 
> > _______________________________________________
> > grass5 mailing list
> > grass5 at grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grass5
> 
> -- 
> Bob Covill
> 
> Tekmap Consulting
> P.O. Box 2016
> Fall River, N.S.
> B2T 1K6
> Canada
> 
> E-Mail: bcovill at tekmap.ns.ca
> Phone: 902-860-1496
> Fax: 902-860-1498

-- 
Markus Neteler

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