On Fri, 15 Feb 2008, R.C. Payne wrote:

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.

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.

I didn't think of air draft, but that makes a lot of sense.

For heights for sighting, though, you really want height above LAT, so you
can work out the height difference between you and the object by
subtracting the current tide height from its height. With MHWS, you need
to know the local value for MHWS as well, although i suppose this is in
your tide tables anyway.

There's still an issue with deciding what MHWS is inland; do they just use
MHWS at the nearest point on the coast, or the nearest standard port or
something? Do they indicate which MHWS heights are measured relative to?
Do they pick one reference MHWS for each chart, and indicate it in the
margin? Looking at an Imray chart, i can't see any indication of how MHWS
is defined.

For sighting, really you want GPS-style purely geometric coordinates, as
local variations in gravity and topography, as affect ODN and LAT/MHWS
heights, don't come in to sighting.

The more i think about it, the more all this annoys me. Okay, how about a
single datum that's based on finding the lowest LAT around the British
Isles, and carrying that height elsewhere via levelling? You get an
isopotential, geoid-based datum, like ODN, but with the nautical
advantages of LAT. It would be more conservative than LAT, though, as the
datum would be below actual LAT in most places (i don't know how much by).
But then, chart datum is only approximately LAT anyway, and this would
mean that tidal heights would still be positive.

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.

Ditto. The argument for using MHWS for air drafts is really an argument
for using HAT.

Aha! It seems this is exactly what the UKHO are now doing - see the last
item he

http://easytide.ukho.gov.uk/Easytide...pport/faq.aspx

tom

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