Newsgroups: info.grass.user
Path: zorro.cecer.army.mil!shapiro
From: shapiro@zorro.cecer.army.mil (Michael Shapiro)
Subject: Re: problems with i.rectify
Message-ID: <C1J2H2.CAF@news.cecer.army.mil>
Sender: news@news.cecer.army.mil (Net.Noise owner)
Organization: US Army Corps of Engineers Construction Engineering Research Labs
References: <9301131904.AA11778@hanover-crrel.army.mil>
Date: Wed, 27 Jan 1993 19:36:37 GMT
Lines: 28
In <9301131904.AA11778@hanover-crrel.army.mil> samg@hanover-crrel.army.mil (Sam M. Gustman) writes:
I wrote in before asking for help with i.rectify. I have successfully
used i.target and i.points. I get an RMS error smaller then my pixel size.
However, when I perform i.rectify my image gets blown up to enormous
proportions. The region I am rectifying to fully contains my image. Are
there any other suggestions as to what could be happening?
Thanks, Sam Gustman...
I'm not sure from this description ("gets blown up to enormous
proportions") what the problem is. Is the image much larger than
the region? Does it have too many rows and cols in it? Is the
image distorted in some way?
I have seen odd behaviour from i.rectify if you don't pick enough
and properly spaced control points in i.points. i.rectify uses a
linear transformation (affine) which requires the determination
of 6 parameters (3 for the x direction, 3 for the y direction).
If you do not specify enough control points, or if the control
points tend to lie on a line, then you can get a distorted
rectification. 4 points is a technical minimum, but you should
use more (perhaps 10 or 15) and they should be spread out over as
much of the image as possible.
--
Michael Shapiro U.S. Army CERL
Environmental Division