Jumat, 08 Mei 2009

The Google Book Scanner and Mapping

In recent days there has been a lot of media coverage on Google's system for scanning books for it's online books database.  After taking a peek at the patent, it looks like there are a lot of similarities between the system devised by Google and what we do in the mapping business.

Mapping Problem: The surface of the earth is not flat: so when you take a picture of the earth from an airborne (or satellite) sensor,  the images will contain geometric distortion.  This effect is particularly acute in areas of high relief.  This is a problem because due to the distortion, accurate measurements cannot be made from the images.  This means they are not typically suitable for a GIS.

Mapping Solution: Aerial photography is captured in stereo.  After photogrammetric processing (triangulation), the images can be viewed in stereo and 3D measurements can be created - including a 3D model (digital elevation model) of the terrain surface.  The terrain surface is then applied to the original aerial photographs to create digital orthophotos.  If processed correctly, orthophotos will be relatively free of geometric distortion.  This is important because accurate measurements (e.g. distance between two locations) can then be made from them.

Well, it looks like Google ran into the same problem with regards to scanning books....

Book Scanning Problem: Because of their designs, the surface of a book is not flat when you open it.  Rather, it curves out from the spine.  This presents a problem for optical character recognition systems, which typically require the page being scanned to be on a flat surface (e.g. remove the spine of the book and run the pages through a flat-bed scanner).

Book Scanning Solution: According to the patent, Google uses a system of two cameras and infra-red light to capture a stereo image pair of the pages.  Then a "three-dimensional image" of the pages is created, which is then used to geometrically correct the original images.  The "three-dimensional surface" seems to be what we would call a digital elevation model.  The final product is a geometrically corrected image that is suitable for optical character recognition.

Sounds very similar to the orthorectification process, doesn't it?  Instead of an IR camera we have the sun, and instead of curving pages we have the terrain of the earth.  The underlying problem in both cases is similar.  The main difference is that instead of using two cameras, in photogrammetry the norm would involve the use of a single camera on a moving platform (aircraft or satellite).  But it is the same idea...

Here is a graphic of the Google scanning system:


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