[GRASS-dev] New georectifying module in TclTk

[Michael Barton]

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?

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 05:52:54 +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:
> 
>> I will admit from the beginning that since I don't really know C, I may well
>> be missing something. But I think this calls some transformations in
>> i.rectify somehow. As I understand it (also in the i.points and i.rectify
>> docs), you first need to rectify the points using one of the rectification
>> transformations. Then you can go ahead with the compute_transformation()
>> procedure.
> 
> compute_transformation() starts by calling Compute_equation() (in
> imagery/i.points/equ.c), which calls I_compute_georef_equations().
> That computes the best-fit affine transformation (forward and inverse)
> from the control points.
> 
> It then goes on to call I_georef() for each pair of control points,
> measuring the difference between the projected source and the
> destination, and vice-versa.
> 
> I_compute_georef_equations() and I_georef() are both defined in
> lib/imagery/georef.c.
> 
> On entry, the only values which need to have been set are the control
> points.
> 
> Upon completion:
> 
> + group.equation_stat contains 1 if a transformation could be
> computed.
> 
> + group.{E,N}{12,21} contain the transformation coefficients (see
> I_georef() for their interpretation).
> 
> + xres, yres and gnd contain the x, y and diagonal (Euclidian)
> differences for each pair.
> 
> + xmax, ymax and gmax contain the indices of the control points with
> the greatest x, y and diagonal (Euclidian) differences.
> 
> + rms is set to the RMS diagonal error
> 
> -- 
> Glynn Clements <glynn at gclements.plus.com>