Collaborating in geospatial context since 2000!

Tuesday, March 15, 2005

Multi-point Georegistration

One of the key abstractions we use is that we consider three coordinate systems: PDF, projection, and geodetic. From the publishing perspective, we assume that the georegistrant (georegistor?) knows the projection, parameters and datum used to create the map. Given that, we can do inverse projections, datum shifts, whatever. However, we don't worry about any of that during georegistration. All we're doing is registering two two-dimensional Cartesian coordinate systems*. For a wide variety of maps, and nearly all working maps produced today, this works well. However, you start to run into trouble with scanned images, sketchy underlying landbases, etc. One thing we don't do is munge the image, or the graphical representation of the map. So, what we're going to provide is a way to munge the linear mapping between the PDF and projection coordinate systems, via a multi-point georegistration process. We've played around with this internally, and I hope we can get it to you, soon.

* As I hinted in a previous post, georegistration is slightly preferable to georeference in this context, IMHO. Let me know what you think.


Anonymous MTUCache said...

Greetings again guys, just stopping by to see if you've got any cool new updates.

I'm liking the "multi-point" references here, as it's an especially good point for the instances you pointed out (aerial photos, sketchy maps, etc). In the GPS-end of things of the Surveying profession, we call this "rubber-sheeting", as we can pin several set points around the subject area to create a tight mesh for the photo to be "stretched" over.

Ideally, with our sites, we simply go out and take measurements on definate points to create a box around the site, so that any projection will be correct across that box. Unfortunately, with larger sites it becomes more and more important to have internal points included to account for anomolies inside of the site.

On the scale of larger maps, I can't even imagine how many points you'd need for a survey-grade mesh, but luckily I doubt you guys would have to worry about doing anything at survey-grade.

Anyway... enough rambling out of me. Again, keep up the good work, you've got at least one fan on the edge of his seat. ;)

8:31 AM EST

Blogger George Demmy said...

Hi again MTUCache,

JB is working to integrate the multi-point georegistration into a user interface. The core optimization engine is complete and in place. I probably need to write up a technical description of what we're doing...


8:42 AM EST

Blogger Patrick Graves said...

We've completed the implementation of multi-point registration and it will be included in the next release.

One thing to keep in mind is that our multi-point registration isn't rubber sheeting. We do not change the imagery of the PDF. What we do is best fit the control points into the map projection using a linear transformation matrix.

So if you have a ortho photo, you would need to rubbersheet it before creating the PDF. In the future we may allow for rubber-sheeting, but for now, the multi-point registration is most effective for reducing error associated with scanning distortions.

8:58 PM EST


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