[time-nuts] Another Trimble Tbolt question....

[Björn Gabrielsson]

On Sun, 2008-03-30 at 05:46 -0400, GandalfG8 at aol.com wrote:

> -----------------------------
> I think that's all rather optimistic, as mentioned previously height  
> estimation is the most innacurate parameter to be reported by GPS units.

Height is about 1.6 to 1.8 times worse than horizontal position, due to
geometry reasons and the need to solve for the local clock. 

Due to our flat earth, its much easier to have an independant idea of
you site height than xy-coordinates. Lake heights are often printed on
common maps. Anyone here that would spot a 30m horizontal offset of your
favorite walk trail?  (watching lat/lon without a map overlay)

With most low end OEM receivers qouting 1us accuracy of the PPS output
-- I wote for "time" as the worst accuracy measurement reported by GPS
units.
 
> It's also important to remember that the "mean" sea level reference is not  
> necessarily the same as "local" sea level.

Its very important to know the difference between ellipsoid height and
geoid or Mean Sea Level (MSL). The ellipsoid is the idealized earth
model used in WGS84 to approximate the geoid (zero MSL). Because the
earth is no perfect ellipsoid (or sphere...). The "normal" height for a
GPS receiver would be ellipsoid height, but with a model of the geoid
imperfections it might also tell you geoid (MSL) height. 

--

   Björn