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Georeference Image Code

Posted on 2004-03-23
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Last Modified: 2010-10-05
I need to build a simple UI to georeference a jpg or gif image with a worldfile. Simple stuff if you are using ArcSomething or MapInfo. However, I can't seem to figure out how to do the affine transformation using more than 2 control points. Using a linear equation (top left coordinates and bottom right coordinates) seems easy enough, but what type of formula (and how) would I plug in more than 2 control points? I would hope the system could use control points that are not corners to make it easier on the user. The system is being developed in VB.net and yes, I realize applications like OziExplorer can do this and unfortunately, I need to reinvent the wheel. Any thoughts on how to get started on this?

Thanks in advance,
Michael
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Question by:mhaynie
6 Comments
 
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Expert Comment

by:Quash
ID: 10720575
As long as you build the grid up all you need is one point to work with.  Since you are inventing it, you just have to choose how you want to do it.  The deal is you have to know how you are going to reference things, I'd make a giant grid and use the lower left hand corner of the image as the reference, and snap it to the giant grid.  Another way of looking at it is you lay out a giant collage, and then put a grid on it, and label the grids.  

Quash, my ideas!
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Expert Comment

by:imarshad
ID: 10743473
I have developed a solution to that problem. Although it also requires the points to be as distant as possible on the image , but it still can be used with nearer points.
You may check it at as an answer from me at

http://www.experts-exchange.com/Programming/Wireless_Programming/PDA's_All/Q_20870574.html

I worked on another idea to Georeference Images (with as many points as available) but the results of it were not that much satisfying.
If you need it I can put it up here.
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Expert Comment

by:krisnackaerts
ID: 10744169
It comes down to a linear regression problem.

Xnew = a + bXold + cYold
Ynew = d + eXold + fYold

Fill in at least three know coordinate pairs and you can calculate the coefficients. Use more and you can calcuate the RMS which gices you an estimation of the accuracy.

Let me know if you need the exact formula's to calculate the coefficients.

Kris
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Accepted Solution

by:
f_rodriguez earned 125 total points
ID: 10842446
I developed a program to calibrate raster images using 2 points (which can be anywhere on the image) that conforms to the WorldFile format, which is the standard used for calibrating raster maps. If you're interested, I can e-mail you this program. Let me know...

fabricio@webmail.co.za
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Author Comment

by:mhaynie
ID: 10842750
Kris,
The formulas would be helpful for calculating the coefficients. If you have them please email or post them in this forum. My email is mhaynie(AT)caddistech(DOT)com. I've seen bits and pieces of the affine transformation, but can't seem to find how to solve for each part programatically. Thanks in advance, I really appreciate it.

Michael
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LVL 5

Expert Comment

by:mrfixit22
ID: 11019919
I think f rodriguez answered your question.  Please award points to the experts that help you.  Who knows maybe he can help you figure out the programmimg.
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