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Hi Stephen, If the images are not orthocorrected or at least geo-corrected then image differencing will not be accurate at all for change detection. So if you haven't already done so you will need to correct the photos first. I don't know if MapInfo has an add-on for image correction or not. For most image correction software you will need to know the camera calibration information (focal length, scale, resolution) and have photos that contain fiducial marks. PCI OrthoEngine is a great product and simple to use (probably expensive though). You can probably send your photos to a company who does orthocorrections and they will likely charge under $500 for 2 photos. You will have to provide a DEM (for orthocorrection) and Ground Control Points (GPS usually)in order for them to do a proper correction. Then it is simply a matter of taking one image and subtracting it from the other. If you aren't familiar with this technique then the company correcting your photos should do it as there are ways to reduce noise and select thresholds for specific types of changes. Hope this helps. Steve -----Original Message----- From: Stephen Christie [mailto:[EMAIL PROTECTED]] Sent: Thursday, October 04, 2001 7:47 AM To: [EMAIL PROTECTED] Subject: MI-L Difference between 2 images I have 2 aerial photos (format is black-and-white bitmap image files) of the same location taken on separate dates. If I can succesfully line up the images, can I subtract one image from the other to reveal the differences? If so, how? Suggestions appreciated. Thanks. Stephen Christie _______________________________________________________________________ List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, send e-mail to [EMAIL PROTECTED] and put "unsubscribe MapInfo-L" in the message body. _______________________________________________________________________ List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, send e-mail to [EMAIL PROTECTED] and put "unsubscribe MapInfo-L" in the message body.
