On Google Earth discrepancies

Over at Spatially Adjusted, essential reading if you belong to the select group of people who are actually trying to use Google Earth as a productivity enhancement tool. (As opposed to the rest of us, for whom Google Earth is a productivity destruction tool without equal.)

I think I’ll contribute with a question that betrays my GIS ignorance: Could the location discrepancies exist because Earth is not perfectly round, whereas Google Earth might be?

2 thoughts on “On Google Earth discrepancies”

  1. No, I don’t think so. This is already corrected for in the WGS projection. Google ‘map projection’, there’s some good stuff out there, its an interesting topic to see the vastly complex calculations done to bring us fly by 3D viewing in GE.


  2. To clarify Rich’s comment- WGS 84 is a datum, basically the definition of the shape of a spheroid that approximates the shape of the earth. A projection is a mathematical operation used to translate points on a spheroid to a “flat” view and like wise, take flat images and map them to the spheroid.

    So, Google Earth is using the WGS 84 datum, which is not round, but is in fact correct to the actual shape of the earth +-40 meters. The method used to register individual satellite photos (which are fundamentally flat) to coordinates on the surface of the spheroid is not one that is free from error. You have to imagine the complexity involved in registering images that were not taken taken at a 90 degree angle and locating on there some control points which are at known coordinates, some of which are difficult to access on the ground with GPS. Each of those registrations produces a function that describes how coordinates in the image correspond to coordinates on the earth. Now, do that for imagery covering the whole earth, and you have a major project on your hands. It’s likely that there are image registration errors. It’s not likely that there would be anything wrong with the basis for their geographic model of the world.

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