The mention of high-accuracy datums reminded me to ask about how large-scale geological processes affect the accuracy of GIS data through time.
For example, after a bit of research, I found that the North American Plate (including myself) is moving away from London (and thus the Greenwich Meridian) at about 4.4 cm/yr. The plate is also rotating, so that the Y axis of a projected coordinate system (say, a State Plane Coordinate System) no longer points due north. Local governments in my area have specified that their data have an accuracy of 0.1 ft (about 3 cm). But they're using high-accuracy GPS to capture other data years afterward. If we're going to use space-based reference systems such as GPS and ITRS, this will definitely degrade the accuracy of GIS data over time. I suppose that if you converted data back and forth between different realizations of datums, you could see the movement. Vertical accuracy would also be degraded by things such as isostatic rebound. My knowledge of geodesy is cursory at best. Does anyone know of any research that has been done as how such processes affect GIS data accuracy? Are movement rates really enough to matter? Spencer --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 11876
