GPS to grid reference

Peter Clinch said:

There are lots of datums because there are lots of scales, locations,
co-ordinate systems and functional uses that just work better in
practice with different datums. Would you really want OS Landrangers to
be marked with lines of latitude and longitude rather than the OS UK
National Grid, and supplied slightly curved rather than flat?

Sometimes it can be useful but the lines would be curved as the map
would be flat with curved lines of latitude and longitude.

ie I have a 292 Withernsea and Spurn point map from the millenium
project and this marks the meridian for anyone trying to visit it as a
curved line. In fact the OS made a point of going up the country
producing first all the maps all the way up to Sand Le Mere at
Tunstall E Yorks so that people could visit the meridian where they
lived.

We did walk the line Sept 2000 for the millenium from SLM to the
Humber 24.5 miles and good it was too.
--
Gwyn. [email hidden]
Remove rem if replying

The message <[email hidden]>
from Gwyn <g@> contains these words:

Quoted message said:

Quoted message said:

I think there would be too much loss of face for the OS ever to admit
that they had got it wrong these last 60 odd years.

Quoted message said:

Roger I think your missing the point. Who cares if it means the SW
corner of a square or a point as long as your intelligent enough to be
able to convert a 10 figure grid ref into a six figure one so you can
point the mountain rescue services where you are. I think its
intelligent of the OS to mean a square so you can just truncate and
not have to round your coordinates.
In any case a 10 figure grid ref is just too small to matter.

I rather think you are missing my point Gwyn which is a point of
principle rather than practice. You obviously don't care about accuracy
either in your language or on the ground. If you did you wouldn't cite
intelligence as a prerequisite for dumbing down a grid reference before
giving it as your position to someone who could make better use of the
more accurate one, nor give the OS the credit of planning 50 years ahead
to the day when the ill informed would have 10 figure grid references to
reduce to 6 figures, and you certainly wouldn't have wanted to say "In
any case a 10 figure grid ref is just too small to matter".

OTOH the OS with good reason prides itself on the accuracy of its maps
and is thus likely to be very embarrassed by being wrong.

--
Roger Chapman
Nearest Marilyn still to be visited - Great Orme.
89 miles as the crow flies,
considerably more as the walker drives.

Roger said:

The message <[email hidden]>
from Gwyn <g@> contains these words:

Quoted message said:

Quoted message said:

I think there would be too much loss of face for the OS ever to
admit that they had got it wrong these last 60 odd years.

Quoted message said:

Roger I think your missing the point. Who cares if it means the SW
corner of a square or a point as long as your intelligent enough to
be able to convert a 10 figure grid ref into a six figure one so you
can point the mountain rescue services where you are. I think its
intelligent of the OS to mean a square so you can just truncate and
not have to round your coordinates.
In any case a 10 figure grid ref is just too small to matter.

I rather think you are missing my point Gwyn which is a point of
principle rather than practice. You obviously don't care about
accuracy either in your language or on the ground. If you did you
wouldn't cite intelligence as a prerequisite for dumbing down a grid
reference before giving it as your position to someone who could make
better use of the more accurate one, nor give the OS the credit of
planning 50 years ahead to the day when the ill informed would have
10 figure grid references to reduce to 6 figures, and you certainly
wouldn't have wanted to say "In any case a 10 figure grid ref is just
too small to matter".

OTOH the OS with good reason prides itself on the accuracy of its maps
and is thus likely to be very embarrassed by being wrong.

But not the accuracy of the pictures on the front it seems.

Sorry Roger I couldn't resist. ;O)

--

Regards
Nick

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

The basic problem comes down to this: maps are flat, but they portray a
surface which isn't. That is actually very difficult to do
consistently, usably and accurately.

That's neither true, nor the point.

The point is why they are not using the one, true definition of Lat and
Long? Maybe it's historical, but I'd like to know the history. The
brevity of the web page is like saying "there are different definitions
of the metre" and not saying why.

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

Quoted message said:

Normal maps don't even have lat/long :-(

Would you want them to? OS maps have it marked on the borders for
reference, but I've yet to come across any isntance of anyone actually
/using/ that on the ground

BBC weather forecast for your town. The nearest weather station is given
in lat/long.

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Dominic Sexton
<[email hidden]> writes

Quoted message said:

In article <[email hidden]>, Bernard
Hill <[email hidden]> writes

Quoted message said:

In article <[email hidden]>, Dominic Sexton
<[email hidden]> writes

Quoted message said:

In article <[email hidden]>, Bernard
Hill <[email hidden]> writes
>As a mathematician, I know there is one axis of rotation (it may
>move slowly over millennia but for now there is one). There is one
>centroid of the earth, and together these define the equator and the
>

You have assumed that the earth is a perfect ellipsoid or sphere. It
isn't.

No I have not. Whatever the shape, there is one centroid and one axis
of rotation.

How do you determine the centroid of the earth? Is it the mathematical
centre of the body formed by the shape of the surface of the earth?
Does it take into account the uneven distribution of mass in the earth?

A centroid is the centre of gravity. It's vertically downwards for every
person on earth.

Quoted message said:

Originally the shape, rotation and centroid location were determined by
celestial observations. The observations were obviously made from the
ground and those from different observatories were combined to model
the size, shape, rotation and location of the earth. These models were
specific to the locations for the observations. That is why when they
were made elsewhere they got different results.

As observations improved the models were refined. There have been a
number of realisations of the zero degree meridian going through
Greenwich observatory. Some history:

http://gpsinformation.net/main/greenwich.htm
http://www.nmm.ac.uk/server/show/conWebDoc.416

Surveyors need a model of the earth to make meaning from their
measurements. The models determined by the astronomers were in that
country / region were used because they naturally fitted the shape of
the earth in that area.

The model will most probably not tie in with the reality for the whole
earth but it does tie in with the local shape of the earth.

I hope that explains why we have a number of geodetic datums.

It does, and is the first response I've had that makes sense. Phrases
like

"This is because within any country or region the maps are referred to a
spheroid that best fits the sea level or mean land level of that region.
This is necessary in order to give a sensible height system, in which
heights are zero at the coast, and in which water flows downhill".

Thank you.

Although it's a great shame that everyone does not use the British
definition, as they should.

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

Bernard Hill said:

But they can't be. So don't assume they must and define them in a
more logical way.

Well what is your "more logical way"?

It's a mathematical fact that the axis of rotation of any body contains
the centroid (centre of gravity). Furthermore the axis is easy to
define. So the logical way is that each position is defined in what
mathematicians call spherical polar coordinates: distance from the
centre+latitude+longitude.

These are defined and unique only on the assumption that an arbitrary
zero of latitude is chosen. And that's some particular spot in Greenwich
observatory, at some particular point in the past.

Quoted message said:

Quoted message said:

Quoted message said:

so given angular distances along it are constant,

??

x degrees along a parallel, angular distance should always be a
constant c km lomg. But if the surface is irregular (as it is), that
won't be the case. This is Not Good in a deterministic measurement
system...

Why? We don't expect that a distance on a map is the distance we walk -
we have to go up and down too. It seems good to me to have agreement
across the world...

Quoted message said:

Quoted message said:

Of course. But that doesn't mean to say that there isn't a set of
coordinates which describe points on the surface. In mathematical
terms it's called Spherical Polar Coordinates: radius, latitude,
longitude. Conventionally called R, theta, phi. All you need to
define are where the zeroes of theta and phi are.
This isn't new stuff: it's centuries old.

It isn't new, but what it isn't is much use in practice on the Earth.
WGS84 gives a much better account of the real shape of the place than
an ellipsoid, which is what you need for sperical polars to work
consistently, accurately and usefully across the surface. Since maps
are of the surface, and not some reference ellipsoid, it makes a lot of
sense to use a reference model that more accurately reflects the
surface you're mapping.

Quoted message said:

As I said, there can be defined one unique set of lat/long.

The issue isn't whether you can do it, it's is it actually useful in
practice. Lat/Long breaks down as a fully consistent system because
its assumed geometry is not the same as the earth's.

That doesn't make it break down at all! It gives a unique, true value
which we could use on maps. I can see that each country may want its own
grid reference - like Britain - but not that they each want their own
definition of what can be uniquely defined. That's why we agree on the
definition of metre (etc).

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Roger
<[email hidden]> writes

Quoted message said:

The message <[email hidden]>
from Bernard Hill <[email hidden]> contains these words:

Quoted message said:

So why more than one datum?? If there is more than one system of
lat/long then they are all (with possible exception of 1) in error.

<With respect to turning the Greenwich Meridian into a movable feast.>

Because the French are still smarting for not getting the Prime Meridian
in the first place and the Americans don't want to give precedence to
anywhere outside the USA.

Good grief. Logic has been subverted by local pressures. What is the
world coming to?

Hey, we use the metre, don't we? And A4 paper.

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Bernard Hill
<[email hidden]> writes

Quoted message said:

In article <[email hidden]>, Dominic Sexton
<[email hidden]> writes

Quoted message said:

How do you determine the centroid of the earth? Is it the mathematical
centre of the body formed by the shape of the surface of the earth?
Does it take into account the uneven distribution of mass in the earth?

A centroid is the centre of gravity. It's vertically downwards for
every person on earth.

In truth there is no one point which is vertically down from everywhere
on earth. Vertically down is orthogonal to the equipotential surface
(known as the geoid) and that undulates due to gravity anomalies from
terrain and variations in subsurface density. This is one of the
problems.

Given that how do you determine the location of the centroid?

--

Dominic Sexton

Bernard Hill said:

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

The basic problem comes down to this: maps are flat, but they portray
a surface which isn't. That is actually very difficult to do
consistently, usably and accurately.

That's neither true, nor the point.

It /is/ true, and assuredly so. And it is also a lot of the point
of multiple datums.

Quoted message said:

The point is why they are not using the one, true definition of Lat and
Long?

Because it is of very limited *actual use* on an irregular globe
where the things you're locating move around relative to one
another. Aside from which, see Dominic's points about the "true
definition": he put it better than I have.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Bernard Hill said:

BBC weather forecast for your town. The nearest weather station is given
in lat/long.

It's pretty spurious though: I don't /use/ that info for anything,
and I doubt anyone much else does either. More a case of "we have
this info, we might as well hand it out".

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Bernard Hill said:

That doesn't make it break down at all! It gives a unique, true value
which we could use on maps.

That in itself does not make it useful. We use National Grid
because it's much easier to use than lat/long.

What most people want from maps is relative to other things on the
same maps. Lat/Long is a dog's dinner for that at a local scale,
especially as you get further towards the poles.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

Bernard Hill said:

BBC weather forecast for your town. The nearest weather station is
given in lat/long.

It's pretty spurious though: I don't /use/ that info for anything, and
I doubt anyone much else does either. More a case of "we have this
info, we might as well hand it out".

Well my point was that I *did* use that information to find out where
the weather station way (35 miles away!).

The other time I've used it is to calculate the time of the sunrise/set
at my location.

--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

Well my point was that I *did* use that information to find out where
the weather station way (35 miles away!).

It's not typically the easiest way though. If you're online it's easier
to just put the name into Streetmap and ask "where is this", for
example. Or if you had a GR of where you were and where the station is
the maths is trivial too.

Quoted message said:

The other time I've used it is to calculate the time of the sunrise/set
at my location.

A GPS (even a simple, base model like the eTrex) will give you that at
the touch of a button. As well as Lat/Long and an OS grid ref.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Dominic Sexton said:

In article <[email hidden]>, Bernard Hill
<[email hidden]> writes

Quoted message said:

Quoted message said:

A centroid is the centre of gravity. It's vertically downwards for
every person on earth.

In truth there is no one point which is vertically down from everywhere
on earth. Vertically down is orthogonal to the equipotential surface
(known as the geoid) and that undulates due to gravity anomalies from
terrain and variations in subsurface density. This is one of the problems.

Given that how do you determine the location of the centroid?

Wot 'e says... the Geoid article on the Wiki is quite informative.

It is, in fact, more of a "problem" in mountains, Schiehallion having
been famously used for an early and remarkably successful geophysical
experiment to estimate the mass of the earth by assessing how much the
mountain pulled a plumb away from "vertical".

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Peter Clinch said:

It is, in fact, more of a "problem" in mountains, Schiehallion having
been famously used for an early and remarkably successful geophysical
experiment to estimate the mass of the earth by assessing how much the
mountain pulled a plumb away from "vertical".

This experiment is also touted as having caused the contour line to be
invented.
--
Phil Cook, last hill: Angletarn Pikes via Beda Fell.
http://www.p-t-cook.freeserve.co.uk/May07/martin9.htm

Phil Cook said:

This experiment is also touted as having caused the contour line to be
invented.

I never realised that before, thanks for the note!

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Quoted message said:

It is, in fact, more of a "problem" in mountains, Schiehallion having
been famously used for an early and remarkably successful geophysical
experiment to estimate the mass of the earth by assessing how much the
mountain pulled a plumb away from "vertical".

Quoted message said:

Pete.

The current edition of the "Scottish journal of Geology" , has a paper
on this experiment.
Sorry no ref, I'm away from home.

--
Hbunnet : (replace "nospam" with "athome" to reply)
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

Bernard Hill said:

Well my point was that I *did* use that information to find out where
the weather station way (35 miles away!).

It's not typically the easiest way though. If you're online it's
easier to just put the name into Streetmap and ask "where is this", for
example. Or if you had a GR of where you were and where the station is
the maths is trivial too.

Quoted message said:

The other time I've used it is to calculate the time of the
sunrise/set at my location.

A GPS (even a simple, base model like the eTrex) will give you that at
the touch of a button. As well as Lat/Long and an OS grid ref.

I'm doubtful. Look at NavMan W300, specifically for walkers. For
instance

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=300113698266

Also available for £40 on Amazon. Of all the features it offers, nowhere
does it say "where you are". All the details I've found on the internet
say things like this:

--------------quote------------
NEW NAVMAN W300 GPS HIKING WALKING RAMBLING SPORTS TOOL
Designed specifically to measure and monitor walking hiking health and
interest factors.
Strap on the arm band and monitor your health factors.
Make sure you get maximum benefit and fun from exercise.
Navman Sport Tool is a modern performance measuring device.
Sport Tools are using the latest GPS satellite technology. They give
walkers, hikers, ramblers all the necessary ultra-precise information.
When walking, it is important to work out at the right rate.
The W300 has special features to make walk training even more
health-promoting and effective.
This helps you to avoid over exertion and gets you more fun when
walking.

Main functions:
- Measures speed and distance
- Current and average pace
Calories burned
----------------------end quote---------------------

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Peter Clinch
<[email hidden]> writes

Quoted message said:

Dominic Sexton said:

In article <[email hidden]>, Bernard
Hill <[email hidden]> writes

Quoted message said:

Quoted message said:

A centroid is the centre of gravity. It's vertically downwards for
every person on earth.

In truth there is no one point which is vertically down from
everywhere on earth. Vertically down is orthogonal to the
equipotential surface (known as the geoid) and that undulates due to
gravity anomalies from terrain and variations in subsurface density.
This is one of the problems.
Given that how do you determine the location of the centroid?

Wot 'e says... the Geoid article on the Wiki is quite informative.

It is, in fact, more of a "problem" in mountains, Schiehallion having
been famously used for an early and remarkably successful geophysical
experiment to estimate the mass of the earth by assessing how much the
mountain pulled a plumb away from "vertical".

You are right. I should have had more sense than to write that, as a
mathematician. I apologise.

However locating the "true" lat/long is still straightforward as long as
you can find the centre of the earth. You can halve the distance between
the two poles, that will do to get the centre of the earth. And the
poles can be located astronomically.

--
Bernard Hill
Selkirk, Scotland