[time-nuts] question about Thunderbolt geo acuracy

Jim Lux jimlux at earthlink.net
Thu May 10 20:01:48 UTC 2012


On 5/10/12 10:46 AM, Chris Albertson wrote:

> "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
>

mean sea level, these days, is a name for a particular height that 
matches the long term average of the ocean, where there is ocean to be 
measured, and which smoothly varies in between those points.


    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

WGS84 won't match the stick in the sand, but one of the modern reference 
geoids most certainly will match it, within a few cm.  It's important 
these days, where there are property boundaries referenced to things 
like "mean high tide line".

Measuring sea height (the actual height) to an accuracy of cm over a 
global scale is pretty straightforward these days (that's what 
TOPEX/JASON is all about).  After that, it's a matter of choosing an 
appropriate averaging technique to remove the effect of tides  (which 
you need to do on solid land, as well)

WGS84, is pretty much the "simplest" model, and is more about defining 
the directions of X,Y, and Z (or lat/lon) than where the surface of the 
earth is.


>
> 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.

You CAN model it.. and that's what the EGM model is.. using multihundred 
order spherical harmonics.  The model isn't simple, but neither is it 
just a table lookup of measured data.   And, as mentioned in an earlier 
set of posts.. since the bumps aren't huge, if you're only interested in 
meter scale uncertainties, a fairly small table will give you the local 
variation between WGS84 ellipsoid (no bumps at all) and EGM.


> Chris Albertson
> Redondo Beach, California
>
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