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At first glance, this seems to be a simple matter. The latitude and longitude degrees in Google Maps URLs have six decimal places and about 10cm precision. Getting more precise appears to be as easy as adding more decimal places. But it turns out that these numbers by themselves are not very meaningful without metadata including the geodetic "datum", which defines the reference locations for these measurements. In the case of the WGS84 (World Geodetic System) datum, which is used by GPS, the "epoch" (date) also needs to be specified, because the exact coordinates of any given location change over time due to plate tectonic drift. The velocity varies by location, and is about 5cm/year in California. Several other datums including various realizations of NAD83 (North American Datum) are also common, but many providers of geospatial data, including official sources, do not provide sufficient metadata to clearly identify them. In addition, standard GIS software does not adequately handle the transformations between these datums. For the most common combination of datums in California this leads to errors of about 1.7 meters, which is very significant for many high precision applications.
This presentation analyzes this problem with respect to one such application: having avatars for GPS-tracked people appear during "after action reviews" (AARs) in a virtual terrain made from high-resolution imagery and elevation data and surveys of buildings. Live GPS tracking can have 1 meter accuracy in ideal conditions, and surveying can be done with 2 centimeter accuracy, so an unnecessary additional 1.7 meter error significantly reduces the potential fidelity of such AARs. It affects line of sight calculations and the positioning of the avatars with respect to significant terrain elements such as doors and windows. The exact causes of the problem are explained, and workarounds are suggested, but longer-term solutions require the cooperation of a larger community.
This paper was presented at the recent Fall 2009 SISO (Simulation Interoperability Standards Organization) conference. The proceedings of that conference are not yet available online. In the meantime the paper can be found here.
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