[time-nuts] question about Thunderbolt geo acuracy

Jim Lux jimlux at earthlink.net
Thu May 10 19:49:15 UTC 2012


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.







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