[time-nuts] Loran-C Anthorn
jimlux at earthlink.net
Mon Jul 12 10:00:25 EDT 2010
> 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. "
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