On 5/10/12 9:09 AM, Chris Albertson wrote:
There is an error in your quoted text. The author must have though there was a difference between WGS84 and "true sea level". No that is not true. If you paper map that you bought from US Gological Survey says "WGS84" on it then THAT is the definition of sea level on that map. The altitudes of contour lines and peaks will be in WGS84 and should match what the GPS says. Many older maps use a different system so their saw level is defined differently. Almost all GPSes have away to select the elipoid. It defauls to WGS84 but you need to set it to match your paper map
It depends.. some GPS report MSL using a geoid model (all the lumps and bumps that differ from the ellipsoid).
USGS topo maps used to be NAD27 (which interestingly, has different horizontal and vertical datums).
If I look at the most recent 7.5 minute quad for THousand Oaks, CA... it says at the bottom "North American Vertical Datum of 1988" and *that* is what sets "0 elevation".
That level is a fit to a variety of observations in Canada, US, and Mexico. And is referenced to the primary tidal benchmark, which is in the great lakes, of all things, near Quebec.
I don't know if it happens to match the geoid. the contours come from the 1999 National Elevation Dataset. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
