EB4APL wrote:
A word of caution here: Don't trust Google maps coordinates for any technical / serious work, they can have errors in the 100´s meter class. And also don't use the copyright date of the maps and images as a time reference, they normally are older than that. If you live in an area under urban development you can check what I mean. Take in account that this is a beautiful thing for locating a restaurant or a route, and can be classified as a geomarketing tool, not a measuring one.
For that matter, what ellipsoid is google maps using? GPS is WGS84, but many (if not all) topographic maps in the US are still NAD27 (the corrections are in the bottom left corner). The difference in horizontal position around where I live is some 30-40 meters.
Google has to reconcile their imagery, their map data, and their topography somehow, and I imagine they take it all to some common ellipsoid, but it's possible they don't. (That is, nobody is using Google earth to fly a plane across oceans)
Especially if you are using digital elevation models (DEMs) (e.g. to do propagation path analysis), you can be off by 50-100 meters comparing the topography in the DEM to the feature on the ground. A "3 second" DEM has horizontal control comparable to a 1:250,000 map.
If you see that phrase at the bottom of a USGS map "Meets national map accuracy standards" it helps to know that the standard is basically "positions are accurate to the diameter of a pencil point or about 1/50th of an inch: 0.5 mm)" 0.5mm on a 1:250,000 map is 125 meters.
Google is my friend, and I turned up the following with respect to Google Earth (which is NOT the same as Google Maps, apparently)
"We represent the earth as a sphere (special case of an ellipsoid). The surface of our sphere corresponds to 0 meters sea level. As far as the KML coordinate system, we consider 0 meters altitude to be sea level, and we draw KML and terrain in a way that's consistent with that. Specifically, the EGM96 geoid is our sea level, a potato-like shape that's smoothly varying but not perfectly smooth, and represents mean sea level around the globe. The geoid (and therefore sea level) is offset from the ideal WGS84 reference ellipsoid by as much as 200 meters or so in some places. " _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
