i.rectify2

[Gerald I. Evenden]

>Date: Wed, 05 Jan 1994 16:57:19 +0100
>From: olthof at ecn.nl (H. Olthof)
>Subject: Re:  i.rectify2
>response to:
>
>----------------------------------------------------------------------------------------------
	... old material deleted
>Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
>----------------------------------------------------------------------------------------------
>
>I have both vector data and rasterdata in my lat-lon mapset. I made the
>coordinate transformation from ll-->lcc for the vector data with
>v.proj.  For the raster data I used i.rectify2 as described.  Now, if
>you make an overlay in the lcc mapset. (the vector map over the raster
>data) they don't fit. They are shifted up to tens of kilometers. in
>some of the Greek islands, the 'vector island' and the `raster island`
>don't even overlap!  In order to check my results I have transformed a
>vector map (with v.proj) and a raster map which was derived from this
>vector map (In the lat-lon mapset, using v.to.rast) I would expect that
>both maps would fit exact in the lcc mapset, as they did in the lat-lon
>mapset. But no....  So either v.proj is not accurate (but since this is
>a coordinate transformation for which the transformation formulas are
>known, I have assumed that my vector data are correct) Or, i.rectify2
>is not accurate enough. (which I can understand, because the area is
>large and i.rectify only makes a fit, for the transformation matrix).
>Or, I am missing something and there is an other solution for this
>problem.

I suspect that your second  supposition is the problem---i.rectify2 .
To check v.proj you are better to compare it to proj of PROJ.4 or
GCTP but not against i.rectify.

The clue also seems buried in the "transformation matrix" but since
I have no experience with i.rectify I can't be of much help other
than to note that it must be *much* more than a simple scale-rotate-
shift operation.  That is, i.rectify2 must know all about lcc
to be able to create a complex non-linear transformation.

Any i.rectify2 gurus out there?

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027