On Fri, 15 Feb 2008 00:56:55 +0000, Tom Anderson wrote:

On Thu, 14 Feb 2008, R.C. Payne wrote:

Tom Anderson wrote:
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.

Well whenever I am using GPS these days [1], I can find my altitude by
reference to my watch and a copy of Reed's Almanac. And that leads me
to the question, what sea level are you taking? Certainly most charts
I've found (Admiralty and Imray) use LAT [2] as their datum for points
below MHWS [3], and MHWS for heights on dry land.

Really? I know about LAT, but i'm surprised to hear that land heights are
measured from MHWS. OS maps use the Newlyn datum, which is the mean sea
level at Newlyn back in 1915 or something; that's carried through the
country by levelling, so the datum is an gravitational isopotential
surface. MHWS is not only a high, not mean, tide, but is something that's
affected by local seabed topography, and so is not an isopotential
surface. That means it won't be parallel to the Newlyn datum, so not only
will Admiralty heights be different to OS heights, but the difference will
vary across the country!

Horses for courses, though. Nautical charts use LAT as a datum because
depths are there so you can work out if you're going to run aground and
that lets them have tide values which are always positive. Plus, it means
that when you see a blue bit on a chart, you know it's always underwater.
You couldn't use LAT for land heights, because it's not defined on land. I
suppose they use MHWS on land because it has a similar property - anything
with a positive height is always above water.

Hang on, how do they determine MHWS on land? Are you sure they don't use
ODN?

It irks me that the Newlyn datum is a mean sea level, and not LAT. But
then i suppose it's natural to define an isopotential surface that way,
because it's the sea level you'd have if you got rid of the moon. Except
it's not, because of topographic effects. I think.

In conclusion, geomatics is hard.

Anyway, my proposal is for *all* heights to be measured as distance from
the centre of mass of the earth. SOLVED!

The thing to appreicaite is that the purpose of a chart is not to record
what the sea is like, it is a tool to allow you to sail around on it
safely. By charting depths below LAT, you are in the position that if you
navigated entirely ignoring tides, only considering your draught and the
chart depth, you will not run aground.

If you consider what a mariner might want heigts above MHWS for, there are
only two uses: air draught under bridges and power lines, or using the
hieght of something to determine distance (eg dipping lights). For air
draught, if you apply the same principel as with depths, if you ignore
tides, only consider your mast height and the charted clearance, you will
be OK with a height above MHWS.

Robin

PS I've never come across HAT, as the opposite of LAT. Both MHWS and MLWS
are talked about, as well as MHWN and MLWN.