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Newsgroups: info.grass.user
Path: zorro.cecer.army.mil!shapiro
From: shapiro at zorro.cecer.army.mil (Michael Shapiro)
Subject: Re: digitizing problems - request for advice
Message-ID: <C2543A.7MD at news.cecer.army.mil>
Sender: news at news.cecer.army.mil (Net.Noise owner)
Organization: US Army Corps of Engineers Construction Engineering Research Labs
References: <01GTZEUVA9WY006ANO at GW.AGR.CA>
Date: Mon, 8 Feb 1993 17:18:46 GMT
Lines: 48

In <01GTZEUVA9WY006ANO at GW.AGR.CA> JOHNSON at abrsle.agr.ca (Dan Johnson, 1-403 327-4561) writes:

>Dear Sir/Madam at GRASS HQ:

>We are having an unusual problem with GRASS 4.0 and we wonder if it has 
>been noticed by other users.  Calibration of the digitizer indicates that
>squares on graph paper come out like rectangles, but not always the same
>side length, and this only seems to happen below a side of 3 cm on the table!
>Dave Satnick in Ellensberg helped us out a bit, and thought that that the
>problem must lie with the digitizer (Altek Model AC30, table HS-243, Gentian 
>Electronics), and the digitizer people naturally say that the problem lies 
>with GRASS.  It may well be the table, but we wonder if there have been 
>any similar experiences?

>- Anne Smith; Dan Johnson

>smitha at abrsle.agr.ca
>johnson at abrsle.agr.ca

>Research Station
>P.O. Box 3000 Main     
>Lethbridge, AB Canada T1J 4B1
>phone (403) 327-4561, ext. 301            
>facs. (403) 382-3156     

The software in the GRASS digitizer program which converts digitizer
coordinates to map coordinates is an affine transformation.

	east  = Ax + By + C
	north = Dx + Ey + F

This is a linear transformation that allows the map to be scaled,
translated, and rotated. The scaling also allows the x scaling to be
different than the y.  Linear regression is used to determine the
A,B,C,D,E,F based on the registration points.

The transformation described here is one that the affine transformation
can't do - ie, apply different scales to different parts of the map.
If the coordinate grid on the map is square then the problem may lie
with either the digitizer or the registration points (or perhaps the
map has shrunk).


-- 


Michael Shapiro U.S. Army CERL
Environmental Division