[time-nuts] question about Thunderbolt geo acuracy

[Chris Albertson]

On Thu, May 10, 2012 at 10:23 AM, Arthur Dent <golgarfrincham at yahoo.com> wrote:
>>Chris Albertson albertson.chris at gmail.com
>

>   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.


Sorry if not clear.   My point was that (1)  the wgs84 reference his
GPS uses is not "wrong".  It can't be.  It is the definition.  It may
not be the definition he wants
and (2) GPS just is not good at altitude and he'd be better off using
a paper map, they are free now so why not.

"mean sea level" is not meaningful any more.  What shape is the ocean
and what if you live in Kanas?   How to extrapolate the ocean level to
Kanas?  The answer is to use a model of some kind

Here where I live I can walk down to the beach and pound a stake in
the sand and mark the water level.  People actually do that (in a more
sophisticated way with tide misting stations up and down the coast)
But in Kanas you need some kind of model that tell you what the ocean
level would be if there were an ocean in Kanas.     But BIG PROBLEM.
No one knows how to do make such a model. So they simply DEFINE the
height of the ocean in Kanas.  One definition is wgs84.     The
trouble with a defining it is that it will not match what you measure
with your stick in the sand.    So there are any number of local
definitions that are closer matches to measured heights

The root of the problem is that the earch has a very complex shape.
It is "lumpy" in random ways and you can't model this, you have to
measure it and then look it up.
Chris Albertson
Redondo Beach, California