[GRASS-dev] New georectifying module in TclTk

[Michael Barton]

Thanks. That is helpful. It will be certainly easier if I only am doing rms
calculations using one equation. Still, I wonder can minimizing errors using
an affine transformation produce poorer results for polynomial
transformations?

Also, in responding to my own comment yesterday... I looked at gdalwarp last
night and I'm not sure it could replace i.rectify as is. If there is someway
to specify a set of GCP's that are not embedded in the raster file format, I
didn't see it in the docs. Also, I didn't see any way of using it to do
error calculations (though this might be moot). GDAL will also need to to
GRASS to GRASS file formats. I know this was a problem awhile back, but
perhaps it is not one now.

More interestingly, it might be more possible to wrap v.transform into the
georectifier so that it will do both raster and vector files, if this is of
any interest to people.

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton



> From: Glynn Clements <glynn at gclements.plus.com>
> Date: Tue, 6 Jun 2006 08:46:29 +0100
> To: Michael Barton <Michael.Barton at asu.edu>
> Cc: Helena Mitasova <hmitaso at unity.ncsu.edu>, GRASS developers list
> <grass-dev at grass.itc.it>
> Subject: Re: [GRASS-dev] New georectifying module in TclTk
> 
> 
> Michael Barton wrote:
> 
>> Shows how little I understand C. This will help to try and follow out the
>> various transformations. Mainly I need the affine transformation.
>> 
>> A question here. Is a single transformation used to generate the projected
>> points for RMS error calcuations? Or are different transformations used
>> depending on whether 1st, 2nd, or 3rd order is selected?
> 
> i.points only understands affine (first-order) transformations, so
> that's what the error estimates assume.
> 
> The 2nd- and 3rd-order transformations are specific to i.rectify, in
> imagery/i.rectify/crs.c.
> 
> The problem with higher-order transformations is that you can't
> readily invert them, so you can only calculate the error in one
> direction.
> 
> With regard to your subsequent message:
> 
>>> I_compute_georef_equations() and I_georef() are both defined in
>>> lib/imagery/georef.c.
> 
> -- 
> Glynn Clements <glynn at gclements.plus.com>