hi
because with 3 control point, there is no control - you have just supplied
enough information for the transformation.
The minimum number of (good known and defined) point in prof use is four,
because then you get additional error information. The control points should
also be outside the area you want to digitizise.
When we are working with mapsheets, we are normaly using the four corner
point and look carefully at the accurracy - if the error is small (less than
3/10 mm i real world) - then we would use the setup for digitizing else we
would add more control points and/or examin the map/setup for errors
Ole Gregor
Viborg Amt, Milj� og Teknik
(45) 87 27 13 07
[EMAIL PROTECTED]
> -----Original Message-----
> From: [EMAIL PROTECTED] [SMTP:[EMAIL PROTECTED]]
> Sent: Monday, March 22, 1999 10:43 PM
> To: [EMAIL PROTECTED]
> Subject: MI registering maps on a digitizer
>
>
>
>
> Over the past two years, there have been numerous questions on inputting a
> paper map into mapinfo. Most answers indicated that if you had points with
> known coordinates, digitizing was a convenient method of input. In the
> most
> recent discussion, George Smith indicated that the more points with known
> coordinates the better (intuitively this sounds correct).
> I have found that just the opposite is true. I have had to input specific
> data from numerous USGS quad maps. I discovered that if I entered more
> than 3
> control points, the accuracy of the digitizer-to-mapinfo mapping was
> seriously
> degraded (both in the numeric calculations and in comparing puck location
> with
> the map on the screen). The more control points that were included, the
> greater the location errors. The best accuracy always occurred with only
> 3
> control points.
> why?
> S. Figuers
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