Tom Anderson wrote:

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

Why not measure heights from the centre of mass of the earth-moon system?
That would abolish the need for tide tables because the tide would be at a
fixed height... although the land would go up and down.

;-)

On Thu, Feb 14, 2008 at 12:13:05PM -0000, John Rowland wrote:

Have you downloaded the latest software? My latest download introduced an
option under "Safety preferences" to choose a sound for when you go over the
speed limit. I've never actually tried it, though.

Oh, cool, didn't know about that - thanks!

--
David Cantrell | Nth greatest programmer in the world

Irregular English:
you have anecdotes; they have data; I have proof

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.

In uk.transport.london message ,
Fri, 15 Feb 2008 01:33:44, John Rowland
n.co.uk posted:
Tom Anderson wrote:

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

Why not measure heights from the centre of mass of the earth-moon system?
That would abolish the need for tide tables because the tide would be at a
fixed height... although the land would go up and down.


Earth radius 4000 miles, Moon distance 240000 miles. Ratio of masses
81. The barycentre is therefore 3000 miles from the Earth's centre,
1000 miles down. So you're suggesting a tidal range of 6000 miles from
high to low.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 IE 6.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
I find MiniTrue useful for viewing/searching/altering files, at a DOS prompt;
free, DOS/Win/UNIX, URL:http://www.idiotsdelight.net/minitrue/ unsupported.

On Fri, 15 Feb 2008, John Rowland wrote:

Tom Anderson wrote:

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

Why not measure heights from the centre of mass of the earth-moon system?
That would abolish the need for tide tables because the tide would be at a
fixed height...

SPLENDID IDEA.

although the land would go up and down.

A minor detail.

A slightly bigger 'although' is that it wouldn't work - sea level isn't
just a constant distance from the earth-moon barycentre. If it was,
there'd only be one tide a day, wouldn't there?

tom

--
natural disasters, unexplained phenomena, chaos, chance, tattooing,

In uk.transport.london message Pine.LNX.4.64.0802150035150.18870@urchin
..earth.li, Fri, 15 Feb 2008 00:39:03, Tom Anderson
posted:

Am i right in thinking that you couldn't calculate height if the
satellites were all equidistant from you?
Yes, if they are co-planar; not, I think, otherwise.

But then you wouldn't be able to calculate position at all.
No. For example, if more than two satellites are equidistant from you
and randomly placed the surface of a sphere, then you must be at the
centre of the sphere.

Is there a configuration where you can get a fix in XY but not Z?



S1 .. S2








___________________TA______________ - You X -






If the satellites are co-planar, you cannot determine the sign of your
height measured from that plane; and if you are either very near to or
very far from that plane your height measurement becomes inaccurate.

In terrestrial use : At sea, the altitude is already known to within a
few metres. On land, the altitude is already known to within about
+-5km. In the air, the altitude is already known to within about
+-15km. In those cases, the usable satellites are always above a
horizontal plane through the receiver.

AIUI, commercially-available GPS receivers will refuse to give an answer
if above some specific altitude; that is to annoy D-I-Y ICBM makers.
There could also be a speed limit.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Proper = 4-line sig. separator as above, a line exactly "-- " (SonOfRFC1036)
Do not Mail News to me. Before a reply, quote with "" or " " (SonOfRFC1036)

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

--
natural disasters, unexplained phenomena, chaos, chance, tattooing,

In article ,
John Rowland wrote:
Tom Anderson wrote:

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

Why not measure heights from the centre of mass of the earth-moon system?
That would abolish the need for tide tables because the tide would be at a
fixed height... although the land would go up and down.

Notice the way in which the sea stays steady as a rock whilst the
buildings keep washing up and down.

Nick
--
Serendipity: http://www.leverton.org/blosxom (last update 4th February 2008)
"The Internet, a sort of ersatz counterfeit of real life"
-- Janet Street-Porter, BBC2, 19th March 1996

On Thu, Feb 14, 2008 at 12:13:05PM -0000, John Rowland wrote:
David Cantrell wrote:
I use Tomtom on a Palm Treo 680.
Mine turns the speed display from white to red when I exceed it
(possibly when I exceed it by a certain amount, not sure). Trouble
is, it's *far* too small to even see what colour the text is without
taking my eyes off the road for longer than I want to. I've not
found any way of getting an audible warning out of it.
Have you downloaded the latest software? My latest download introduced an
option under "Safety preferences" to choose a sound for when you go over the
speed limit. I've never actually tried it, though.

The self-update thingy seems to think that I do indeed have the latest
version. No such option though.

--
David Cantrell | Enforcer, South London Linguistic Massive

Featu an incorrectly implemented bug