[time-nuts] question about Thunderbolt geo acuracy

[Arthur Dent]

>Chris Albertson albertson.chris at gmail.com

>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.

  I'm not sure what point you're trying to make but it is a fact, as the OP pointed 
out, that there are differences between the empirical data of 'true elevation' and
what various GPS receivers will indicate based on whatever model they are 
using. Elevation errors associated with GPS are well known and I have observed 
these errors and the 2 sources I quoted pointed them out as well. I've known the 
error between well-established landmarks and the GPS to be significantly off 
(over 200') and the error will vary from location to location but the error will be 
essentially constant for any single location. 

  If you're saying that a GPS and a map based on the same model would agree 
and give the same incorrect elevation, then that is entirely possible but it would 
simply make both of them incorrect by the same amount. The example given in 
both references I quoted of having a GPS tell you are say 100 feet below 
the sea surface when you are standing on the beach high and dry is a situation 
where I'd believe my observations and doubt the model the GPS is using.  The 
point of the 2 references is to explain why there could be a difference between 
two GPS receivers reporting elevation and not to get us mired in the weeds 
near MSL at the sea shore.

-Arthur