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Image Misregistration as Map Projection Changes

by Technical Evangelist on ‎03-08-2016 10:23 AM - edited on ‎04-06-2016 01:25 PM by Moderator (363 Views)

Symptoms

The effects of the affine math model on image geometry when the workspace coordinate system changes are discussed, along with the "Advanced image processing" option available in GeoMedia Desktop 2015

Diagnosis

In GeoMedia versions prior to 15.0, an image could only be displayed as a parallelogram. This was a limitation of the GeoMedia raster display.  This is no longer the case in GeoMedia Desktop 2015.

As an example of the problem, consider a GeoTIFF image with a UTM/WGS84 projection/datum. The image displays as a rectangle in a GeoWorkspace configured in the matching UTM system. If you digitize a rectangle around the image, it might help to visualize the problem. When converted the GeoWorkspace from the UTM projection, to a non-projected Geographic system, the rectangle transforms to a quadrilateral - opposite sides are not necessarily parallel to one another, and the length of each of the 4 sides changes independently of the others. In some cases, the straight sides of the rectangle may even become curved, if multiple points are digitized between corner points.

Solution

In earlier versions of GeoMedia, the affine transformation which was applied to the image could not display the irregular quadrilaterals or their curved sides. Opposite sides had to remain parallel and of identical length. There could be no curving of any of the sides. The four corners of the image were registered to the 4 points defined by the corners of the rectangle (its footprint) using the affine math model.

The amount of error seen with the affine transformation is a function of both the areal extent of the image and the difference between the projection of the GeoWorkspace and the image's native coordinate system. Images with large area coverage showed greater displacement than small coverages, and similar projections had less displacement than radically different projections.

To illustrate this misregistration due to the affine warp, open USSampleData.gws, and add USSampleImage.tiff to the legend. Change the workspace coordinate system to Geographic, and see the huge displacements - the image no longer registers properly.

To illustrate the capabilities of the new GeoMedia Desktop 2015 "Advanced Image Processing" option, follow the above steps and then go to the File pulldown and select the 'Options' command.  On the 'Map Display' tab, check the box labled 'Advanced image processing' and then OK the dialog.  You should now see the USSampleImage redisplay, this time it matches the geographic coordinates of the USSampleData representation of the continental United States.

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