On Wed, 13 Feb 2008, Tom Anderson wrote:

On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need to
determine the spheroid and geoid separation in relation to the grid used
and so on. Remember that many countires use by the different versions of
these for their mapping and with different origins.

If you want to know the height above local sea level, then yes, you need a
map of the geoid. But nobody uses that. In the UK, we use height above the
OSGB36 datum,

Hang on, no, that's rubbish. We do use the local sea level, aka Ordnance
Datum Newlyn.

So yes, you're right.

Clearly, the solution is just to switch to using geometric rather than
gravitational heights.

The use of an irregular height datum kind of freaks me out. It's fine for
going up and down without moving across the planet, but it means that you
can't relate a height in one place to a height in another, in terms of
position in space, without knowing the shape of the datum. It means our
coordinate system isn't really a coordinate system.

But if you used geometric coordinates, then you'd find that sometimes,
walking along a contour was walking up or down the gravity well. And the
maths for working out distance is still hard, because it's all on the
surface of a spheroid!

Cartography is hard.

tom

--
Me ant a frend try'd to WALK the hole unterrgrand but was putting off -
sometime we saw a trane! -- Viddler Sellboe