GPS to grid reference

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

Quoted message said:

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:

> 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.

And how do you decide where the south pole is? The ice shelf surface?
The land surface beneath the ice or the projection of the geoid under
both?

The multiple datums you dislike so much generally agree about the
location of the centroid to within a couple of hundred metres. Unless
you use the geoid your method will be further out than that.

You seem to have a real bee in your bonnet about this being much simpler
than is the case. The astronomers and cartographers of the last few
hundred years have carefully taken their measurements, made their
calculations and come up with the multiple datums which have been
usefully applied in the area of applicability. Do you really think that
they were unable to think of and apply your simple ideas and all come up
with the same answer?

There is no one 'true' lat/long system for the earth. The closest, on
average, is WGS-84 whose origin is the centroid of the earth as
determined by highly accurate measurements of satellite orbits.

--

Dominic Sexton

Bernard Hill said:

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

No need, the point is to get the grey matter exercising. In
discussions like this I often have to change or abandon answers
half way through composing them, as they get the grey matter
exercising...

Quoted message said:

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.

As Dominic points out, that sort of thing will give you /a/ centre,
not *the* centre.

Lat/long certainly /started off/ simple, but global co-ordinate
systems have got progrssively complex the more that's been found
out about the true shape and nature of the earth. Nobody actually
/wants/ them to be complicated, but if you're going to have a
universal, consistent and accurate system then it has to be complex
as the simple systems break down. The devil, as is often the case,
is in the detail. It's the difference between applied and pure
science. Pure tells you how it should be, applied is diluted with
practical realities :-(

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

Quoted message said:

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

Quoted message said:

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:

That is not a conventional recreational GPS receiver it is aimed at
logging exercise. There are several other receivers aimed at the
'fitness' market which also don't do grid refs.

If you want grid refs but no frills you are probably limited to a basic
recreational receiver from Garmin, Magellan or Lowrance.

From Garmin anything in the eTrex or Geko ranges will do (but the Geko
101 does not have a PC interface which is not just useful for
transferring data but also upgrading / reloading the firmware if
necessary).

The Magellan eXplorist range is starts with no frills but like the Geko
101 the eXplorist 100, 200 and 300 models have no PC interface.

The Lowrance iFinder has similar capabilities to the others.

If you want one under £50 I expect that you will need to patiently watch
the second hand offerings on eBay. You might get lucky...

--

Dominic Sexton

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

Quoted message said:

Quoted message said:

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.

And how do you decide where the south pole is? The ice shelf surface?
The land surface beneath the ice or the projection of the geoid under
both?

the same way as you do the N pole. By geometry and the stars. They are
the points on the earth's surface where the stars go in circles. And you
can tell one from the other by looking for the polar bears <g>

Quoted message said:

The multiple datums you dislike so much generally agree about the
location of the centroid to within a couple of hundred metres. Unless
you use the geoid your method will be further out than that.

You seem to have a real bee in your bonnet about this being much
simpler than is the case. The astronomers and cartographers of the last
few hundred years have carefully taken their measurements, made their
calculations and come up with the multiple datums which have been
usefully applied in the area of applicability. Do you really think that
they were unable to think of and apply your simple ideas and all come
up with the same answer?

No. We have better instruments these days.

Quoted message said:

There is no one 'true' lat/long system for the earth. The closest, on
average, is WGS-84 whose origin is the centroid of the earth as
determined by highly accurate measurements of satellite orbits.

That sounds good.

But why do you ask me how it's done if you already know the answer? And
if the answer is WGS84 then let's all use it!!!

--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

In article <[email hidden]>, Dominic Sexton

Quoted message said:

Quoted message said:

And how do you decide where the south pole is? The ice shelf surface?
The land surface beneath the ice or the projection of the geoid under
both?

the same way as you do the N pole. By geometry and the stars. They are
the points on the earth's surface where the stars go in circles.

But that doesn't answer the question Dominic asked (read it again),
because your method isn't a *singular place*, but at some as yet
undefined point a line extending from the rotation axis. And since
you're using a ploy of half the distance between two points, that
doesn't cut it as a precise definition.
You can't really do the stars from the North Pole on the sea bed, so I
guess you're looking at sea level, so that'll be the geoid. That's not
necessarily going to be where you can see the stars from the south pole,
as you're on a land mass with a thick ice sheet on top, which will throw
the value of the answer you get for half the distance between them.

Quoted message said:

Quoted message said:

You seem to have a real bee in your bonnet about this being much
simpler than is the case. The astronomers and cartographers of the
last few hundred years have carefully taken their measurements, made
their calculations and come up with the multiple datums which have
been usefully applied in the area of applicability. Do you really
think that they were unable to think of and apply your simple ideas
and all come up with the same answer?

No. We have better instruments these days.

Which is precisely why we don't have to stick to overly simplistic
models like a regular ellipsoid any more: we can use better descriptions
of earth shape now, with our better instruments.

Quoted message said:

But why do you ask me how it's done if you already know the answer? And
if the answer is WGS84 then let's all use it!!!

It's the most common datum by far, but not necessarily the best one in
any particular locality. Especially one where a datum has been selected
for a local best fit, rather than a global best fit as WGS84 has. And
even if that wasn't the case, do you want to throw away all your OS maps
because they use the "wrong" datum? I doubt it!
OS use their UK national grid datum, the Norwegian mapping agency use
WGS84. What's the problem of them using different datums for their
local maps? None that I can see, as there's no point where the agencies
are trying to map the same localities.

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

Quoted message said:

But why do you ask me how it's done if you already know the answer? And
if the answer is WGS84 then let's all use it!!!

Well that kind of ignores history. OS maps are based on a grid that is
projected on a datum from 1936 - OSGB36. Now the OS have redefined the
heights and the National Grid with respect to WGS-84. That's a good
thing but it is natural for organisations / governments to be wary of
going to considerable lengths to update all their maps / charts never
mind the potential for confusion when changing from one system to
another.

--

Dominic Sexton

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

Quoted message said:

Bernard Hill said:

In article <[email hidden]>, Dominic Sexton

Quoted message said:

Quoted message said:

And how do you decide where the south pole is? The ice shelf
surface? The land surface beneath the ice or the projection of the
geoid under both?

the same way as you do the N pole. By geometry and the stars. They
are the points on the earth's surface where the stars go in circles.

But that doesn't answer the question Dominic asked (read it again),
because your method isn't a *singular place*, but at some as yet
undefined point a line extending from the rotation axis. And since
you're using a ploy of half the distance between two points, that
doesn't cut it as a precise definition.
You can't really do the stars from the North Pole on the sea bed, so I
guess you're looking at sea level, so that'll be the geoid. That's not
necessarily going to be where you can see the stars from the south
pole, as you're on a land mass with a thick ice sheet on top, which
will throw the value of the answer you get for half the distance
between them.

You have not understood at all. We don't need to get the position right
as long as we all agree. The centre of the earth (however defined - as
long as it's on the axis of rotation) is to be the centre of the
coordinate system. With that we have the zero of latitude, the equator.
Then we all agree one point which is our zero of longitude. QEF.

Now if you can do things better by using orbits (because that gives the
centre of gravity, the centroid) then even better. But my whole point is
that there IS ONE exact centroid and axis of rotation. So once we agree
on the zero of longitude then we have a universal system.

Quoted message said:

Quoted message said:

But why do you ask me how it's done if you already know the answer?
And if the answer is WGS84 then let's all use it!!!

It's the most common datum by far, but not necessarily the best one in
any particular locality. Especially one where a datum has been
selected for a local best fit, rather than a global best fit as WGS84
has. And even if that wasn't the case, do you want to throw away all
your OS maps because they use the "wrong" datum? I doubt it!
OS use their UK national grid datum, the Norwegian mapping agency use
WGS84. What's the problem of them using different datums for their
local maps? None that I can see, as there's no point where the
agencies are trying to map the same localities.

You speak of "what works best" in one country. A very strange concept it
seems to me. This defined coordinate system gives harmony across the
world, we can use it to find out where the geoid is, and how it changes.
We can do continental drift to see where Greenwich has moved to and so
on.

And, note, I have never rubbished the OSGR system. Each country can -
and probably does - define it's own grid reference, and that's fine and
good. But the relationship between that and a Universal Lat/Long can
quite happily change with time either because the country is moving or
because the standard is being refined. We're all happy with the
declination changing, as indicated by the note at the top of the map:
there is no reason to reprint any maps.

You seem to be unable to imagine my horror as a scientist at finding
there are apparently different systems of such a basic thing as the
coordinate system. That's why we standardised the metre, the second, the
coulomb (etc) and are continually refining them.

--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

You have not understood at all. We don't need to get the position right
as long as we all agree.

But we all need to agree, and it's down to an arbitrary decision. Why
not decide on a more accurate model? (well, "we" pretty much have, of
course).

Quoted message said:

Now if you can do things better by using orbits (because that gives the
centre of gravity, the centroid) then even better. But my whole point is
that there IS ONE exact centroid and axis of rotation.
So once we agree on the zero of longitude then we have a universal system.

The point isn't whether it's possible to do, it's whether it's as useful
as possible. And it isn't, which is why we don't do that!

Quoted message said:

You speak of "what works best" in one country. A very strange concept it
seems to me. This defined coordinate system gives harmony across the
world, we can use it to find out where the geoid is, and how it changes.
We can do continental drift to see where Greenwich has moved to and so on.

It's not strange at all. Even if your defined co-ordinate system does
all of the above wonderfully... but I want to know how far is it from
this summit to that pub then I really am not best off working in your
long/lat system, I'm better off working on a square grid. Which is why
we use a (distorted) projection onto a square grid for that sort of job.

Quoted message said:

You seem to be unable to imagine my horror as a scientist at finding
there are apparently different systems of such a basic thing as the
coordinate system.

It's not a basic thing though. It's only "basic" if the shape you're
mapping (and the construction underneath the surface too) is simply and
predictably regular, and there is no dichotomy between portraying its
surface as it really is and on a flat map. The earth isn't like that
though.
This has been worked out by scientists, rather more high powered than
either of us I suspect. They haven't made it complicated on a whim,
they've made it complicated because that works best in describing and
portraying the earth.

Quoted message said:

That's why we standardised the metre, the second, the
coulomb (etc) and are continually refining them.

Look at what the metre was originally defined as, and what it is defined
as now, and that might give you a clue as to the problems of pretending
the earth is a nice and easy shape!

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:

You speak of "what works best" in one country. A very strange concept
it seems to me. This defined coordinate system gives harmony across
the world, we can use it to find out where the geoid is, and how it
changes. We can do continental drift to see where Greenwich has moved
to and so on.

It's not strange at all. Even if your defined co-ordinate system does
all of the above wonderfully... but I want to know how far is it from
this summit to that pub then I really am not best off working in your
long/lat system, I'm better off working on a square grid. Which is why
we use a (distorted) projection onto a square grid for that sort of job.

We are continually not making contact here. I have maintained I am happy
with local grids which make sense locally: OSGR being our own example.
It's the local **Latitude and Longitude** which makes me shudder.

Quoted message said:

Quoted message said:

You seem to be unable to imagine my horror as a scientist at finding
there are apparently different systems of such a basic thing as the
coordinate system.

It's not a basic thing though. It's only "basic" if the shape you're
mapping (and the construction underneath the surface too) is simply and
predictably regular, and there is no dichotomy between portraying its
surface as it really is and on a flat map. The earth isn't like that
though.

Of course not! That's not my point at all.

No roads are straight yet we measure them in metres. It's the **basic
unit of measurement** (metre) which we agree on because THAT is basic.
To disagree on the basic unit of measurement (Lat/Long) is what
horrifies me. It's worse than US/UK gallons because those are different
units: it's like dealing in French metres or British metres and worrying
about conversion between units purporting to be basic in each system.

Quoted message said:

This has been worked out by scientists, rather more high powered than
either of us I suspect. They haven't made it complicated on a whim,
they've made it complicated because that works best in describing and
portraying the earth.

Quoted message said:

That's why we standardised the metre, the second, the coulomb (etc)
and are continually refining them.

Look at what the metre was originally defined as, and what it is
defined as now, and that might give you a clue as to the problems of
pretending the earth is a nice and easy shape!

Of course! I am aware of all that - but that has been sorted now (and
for a long time actually). And the argument has nothing to do with the
shape of the earth (I keep saying that don't I?).

Lat/Long/altidude is a coordinate system and can be applied to
absolutely any shape of body provided the zeroes are agreed. And the
lack of agreement about zeroes astonished me when all this started.

--
Bernard Hill
Selkirk, Scotland

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

Quoted message said:

Lat/Long/altidude is a coordinate system and can be applied to
absolutely any shape of body provided the zeroes are agreed.

It can but to translate relative lat/long co-ordinates into something
useful (i.e. distances and bearings) requires a model based on the
earth's size and shape.

It is entirely understandable that the astronomers determining the model
of the earth would come up with answers that fitted their measurements.
Had they wanted to determine the actual centroid of the earth I'm they
would have gone about it a different way and there may well have been
consensus earlier. However they were not so much interested in the
'real' centroid as determining a useful model of the earth that would
make the co-ordinates match the reality in their area of interest.

Perhaps they knew that the centroid of their model did not match reality
but that didn't matter because their model was a good match for the land
surface where they wanted it to be.

--

Dominic Sexton

Bernard Hill said:

No roads are straight yet we measure them in metres. It's the **basic
unit of measurement** (metre) which we agree on because THAT is basic.
To disagree on the basic unit of measurement (Lat/Long) is what
horrifies me.

But what is the meaning of the "basic unit"? You came up with a means
of describing the centre of the Earth as a reference which might not
quite work accurately, which will mean (since it will push things South
of where they should be) that the N hemisphere is bigger than it ought
to be and the value of a nautical mile (1 minute of latitude) will
actually change according to where you are. Thus, what you *won't* have
is a standardised measure, and people using long/lat can't tune in to
ground values with meters, because one changes and the other doesn't.

Quoted message said:

Of course! I am aware of all that - but that has been sorted now (and
for a long time actually). And the argument has nothing to do with the
shape of the earth (I keep saying that don't I?).

Lat/Long/altidude is a coordinate system and can be applied to
absolutely any shape of body provided the zeroes are agreed. And the
lack of agreement about zeroes astonished me when all this started.

See the above example: what is the point of knowing that something is at
x,y if you can't accurately say from that if x, y+1 is exactly 1 unit
away from it?

It is important where you put the zero for a given application. The
given application means selecting the right tool for the job. Which
would you prefer, a single adjustable spanner for all possible nuts and
bolts, or a large set of different spanners optimised for different
sizes? And answer the question not for a notional magic ideal case, but
one using actual adjustable/static spanners available form the local
ironmonger. The adjustable is the simplest and most elegant solution,
people moving lots of nuts and bolts tend to prefer the other one though

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:

No roads are straight yet we measure them in metres. It's the **basic
unit of measurement** (metre) which we agree on because THAT is basic.
To disagree on the basic unit of measurement (Lat/Long) is what
horrifies me.

But what is the meaning of the "basic unit"? You came up with a means
of describing the centre of the Earth as a reference which might not
quite work accurately, which will mean (since it will push things South
of where they should be) that the N hemisphere is bigger than it ought
to be and the value of a nautical mile (1 minute of latitude) will
actually change according to where you are.
Thus, what you *won't* have is a standardised measure, and people using
long/lat can't tune in to ground values with meters, because one
changes and the other doesn't.

I don't follow that at all. The earth is an *oblate* spheroid (to one
level of approximation) and so the n.m. to latitude-minute already
varies depending on your latitude. Similarly metres to lat/long.

Quoted message said:

Quoted message said:

Of course! I am aware of all that - but that has been sorted now (and
for a long time actually). And the argument has nothing to do with the
shape of the earth (I keep saying that don't I?).
Lat/Long/altidude is a coordinate system and can be applied to
absolutely any shape of body provided the zeroes are agreed. And the
lack of agreement about zeroes astonished me when all this started.

See the above example: what is the point of knowing that something is
at x,y if you can't accurately say from that if x, y+1 is exactly 1
unit away from it?

Why can you not?

Quoted message said:

It is important where you put the zero for a given application. The
given application means selecting the right tool for the job. Which
would you prefer, a single adjustable spanner for all possible nuts and
bolts, or a large set of different spanners optimised for different
sizes? And answer the question not for a notional magic ideal case,
but one using actual adjustable/static spanners available form the
local ironmonger. The adjustable is the simplest and most elegant
solution, people moving lots of nuts and bolts tend to prefer the other
one though

You're swimming against the tide here if you really believe what you
have written. Over the last 100 years (or whatever) we have been
redefining *international* standards of distance so that they all agree.
The fact that we end up with a time unit which forces our day length to
drift by a second every so many months (and so eg we have 7 pips not 6
in the Greenwich Time Signal) is a consequence of this standardisation.
And the metre is no longer defined as 10^-7 of the equatorial distance -
it's not as convenient but in the interests of scientific accuracy we
are used to it.

--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

I don't follow that at all. The earth is an *oblate* spheroid (to one
level of approximation) and so the n.m. to latitude-minute already
varies depending on your latitude. Similarly metres to lat/long.

So it's even worse! But at least if it was /just/ an oblate spheroid
with an unarguable centre the effects would be systematic and easily
predictable, but it doesn't even manage that.

Quoted message said:

Quoted message said:

See the above example: what is the point of knowing that something is
at x,y if you can't accurately say from that if x, y+1 is exactly 1
unit away from it?

Why can you not?

Because the earth is an awkward shape and you're laying on a co-ordinate
system that doesn't come up on the ground in nice regular places as is
handy for them to do. Where does the next minute of latitude come up
relative to this one I'm on? Will it be /exactly/ 1 Nautical Mile (as
expressed as a constant in meters) due North or South? You can't tell
me with your system.

Quoted message said:

You're swimming against the tide here if you really believe what you
have written. Over the last 100 years (or whatever) we have been
redefining *international* standards of distance so that they all agree.

This is not actually relevant. A unit of distance is simply a set
length: it is not job-specific at all. A co-ordinate system, on the
other hand, is almost always job specific and the choice of system in
use is determined by what will work best to describe the environment to
hand. We'd be quite daft, for example, to use polar co-ordinates on
something like a Landranger sheet, or to try and draw a wind-rose using
Cartesians. That's why we use grids for Landrangers, and polars for
wind roses.
If I use a different unit to a meter for some reason it's just a simple
scalar to change it to the standard unit, but co-ordinate systems don't
work that neatly. They are chosen as suitable tools to describe certain
environments they're particularly suited to.

Remember, the co-ordinate systems used have been developed by scientists
who (a) knew what they were doing and (b) know the ins and outs of
what's needed and what breaks down and stands up much better than either
of us. These people would like life to be as simple and refined as
possible, just like you, but there are times when simplification makes
something simplistic, rather than better.

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

Quoted message said:

The fact that we end up with a time unit which forces our day length to
drift by a second every so many months (and so eg we have 7 pips not 6
in the Greenwich Time Signal) is a consequence of this standardisation.

The predominant reason for leap seconds is actually the slowing down of
the earth's rotation not a difference between the standard second and
the ephemeris second.

Were the predominant factor the difference in seconds there would be a
need for leap seconds at regular intervals. However there were eight in
the 1990s and thus far we have only had one since the start of 2000.

As the earths rotation is unpredictable far in advance leap seconds are
decided upon less than a year in advance.

For the background to leap seconds see:

http://tycho.usno.navy.mil/leapsec.html

--

Dominic Sexton

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

Quoted message said:

Bernard Hill said:

I don't follow that at all. The earth is an *oblate* spheroid (to one
level of approximation) and so the n.m. to latitude-minute already
varies depending on your latitude. Similarly metres to lat/long.

So it's even worse! But at least if it was /just/ an oblate spheroid
with an unarguable centre the effects would be systematic and easily
predictable, but it doesn't even manage that.

Quoted message said:

Quoted message said:

Quoted message said:

See the above example: what is the point of knowing that something
is at x,y if you can't accurately say from that if x, y+1 is exactly
1 unit away from it?

Why can you not?

Because the earth is an awkward shape and you're laying on a
co-ordinate system that doesn't come up on the ground in nice regular
places as is handy for them to do. Where does the next minute of
latitude come up relative to this one I'm on? Will it be /exactly/ 1
Nautical Mile (as expressed as a constant in meters) due North or
South? You can't tell me with your system.

Quoted message said:

You're swimming against the tide here if you really believe what you
have written. Over the last 100 years (or whatever) we have been
redefining *international* standards of distance so that they all agree.

This is not actually relevant. A unit of distance is simply a set
length: it is not job-specific at all. A co-ordinate system, on the
other hand, is almost always job specific and the choice of system in
use is determined by what will work best to describe the environment to
hand. We'd be quite daft, for example, to use polar co-ordinates on
something like a Landranger sheet, or to try and draw a wind-rose using
Cartesians. That's why we use grids for Landrangers, and polars for
wind roses.

You are losing sight of my main point and knocking down straw men.

I have never been opposed to local units. The OSGR is good for what it's
good for, and I use it all the time. I suspect that other countries have
their own good systems.

My horror arose when someone said that there were several versions of
latitude and longitude. For instance that the N pole is not 90°N in all
systems.

That's horrendous because in my book the N pole is *defined* as 90°N and
similarly the equator and S pole. They (in my opinion) define the
coordinate system for the earth and are the way in which we would
compare for instance the movement of the UK islands and the N American
mainland.

Quoted message said:

If I use a different unit to a meter for some reason it's just a simple
scalar to change it to the standard unit, but co-ordinate systems don't
work that neatly. They are chosen as suitable tools to describe
certain environments they're particularly suited to.

Remember, the co-ordinate systems used have been developed by
scientists who (a) knew what they were doing and (b) know the ins and
outs of what's needed and what breaks down and stands up much better
than either of us. These people would like life to be as simple and
refined as possible, just like you, but there are times when
simplification makes something simplistic, rather than better.

The coordinate systems evolved, just as the metre evolved. They were not
designed with the knowledge which we have today... which is why the
basic units of measurement were redefined by scientists.

And to say the lat/long was "defined by scientists" is invidious because
you have to ask "what scientists?" and "using what tools?". Modern tools
are radically different. I strongly suspect that NASA know where the
centroid of the earth is so that they can calculate the course for their
unmanned spacecraft. In the same way as the metre, second, coulomb (etc)
were put on a different footing only last century it's time we agreed on
latitude and longitude: indeed I suspect that a system is already there
which we can adopt. Star positions are quoted in Right Ascension and
Declination - a different way of saying latitude and longitude, and
stars are accurately recorded, in fact we measure movement.

--
Bernard Hill
Chartered Scientist
Chartered Mathematician
Chartered IT Professional
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:

The fact that we end up with a time unit which forces our day length
to drift by a second every so many months (and so eg we have 7 pips
not 6 in the Greenwich Time Signal) is a consequence of this
standardisation.

The predominant reason for leap seconds is actually the slowing down of
the earth's rotation not a difference between the standard second and
the ephemeris second.

Were the predominant factor the difference in seconds there would be a
need for leap seconds at regular intervals. However there were eight in
the 1990s and thus far we have only had one since the start of 2000.

As the earths rotation is unpredictable far in advance leap seconds are
decided upon less than a year in advance.

For the background to leap seconds see:

http://tycho.usno.navy.mil/leapsec.html

Thank you for that. However I don't think it's relevant to the
discussion. My point is that in the interest of consistency and
uniformity we have changed the definition of basic units, and that if we
have many definitions of latitude and longitude then this is overdue for
a change.

--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

My horror arose when someone said that there were several versions of
latitude and longitude. For instance that the N pole is not 90°N in all
systems.

That's horrendous because in my book the N pole is *defined* as 90°N and
similarly the equator and S pole.

But that's your book. Why should it be the same as everyone else's, if
other people have a slightly different job in mind?

Quoted message said:

The coordinate systems evolved, just as the metre evolved. They were not
designed with the knowledge which we have today... which is why the
basic units of measurement were redefined by scientists.

But the same scientists have *not* seen fit to propose an ISO
co-ordinate system for planetary co-ordinate systems! The definitions
of unit distances are not really the same thing at all as co-ordinate
systems, which are a certain way of using measurements, not the
measurements themselves.

Quoted message said:

And to say the lat/long was "defined by scientists" is invidious because
you have to ask "what scientists?" and "using what tools?".

The ones given the task of finding a way around a big ocean and getting
back to where you came from, using whatever tools they could find or
devise. What of it?

Quoted message said:

Modern tools
are radically different. I strongly suspect that NASA know where the
centroid of the earth is so that they can calculate the course for their
unmanned spacecraft. In the same way as the metre, second, coulomb (etc)
were put on a different footing only last century it's time we agreed on
latitude and longitude:

But to what end? As long as we know where we are using whatever datum
we are using there is no need to create a "standard" lat/long. The
nearest it's been to being agreed is WGS84, but as has been pointed out
repeatedly, WGS84 is not all things to all men so there's no point in
imposing it on those for whom that is the case. If you like to work in
inches rather than meters you can, and it's just a scalar conversion to
go to the formally defined measurement unit but it doesn't work anything
like that with terrestrial co-ordinate systems, so the comparison to ISO
fundamental units is irrelevant. If you have a co-ordinate system
forced on you that doesn't work as you need it to you can't just
multiply by a constant to make it all right.

Quoted message said:

indeed I suspect that a system is already there
which we can adopt. Star positions are quoted in Right Ascension and
Declination - a different way of saying latitude and longitude, and
stars are accurately recorded, in fact we measure movement.

They are recorded using a system developed for that specific use.
Lat/long was itself developed for a specific use. Standardising it as
an ISO fundamental does not make any real sense though.

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:

Thank you for that. However I don't think it's relevant to the
discussion. My point is that in the interest of consistency and
uniformity we have changed the definition of basic units, and that if we
have many definitions of latitude and longitude then this is overdue for
a change.

The difference between a cubit and a light year is just a scalar
conversion though. Different co-ordinate systems work differently
well for different situations. For example, if you want a good
overall fit to the whole earth surface WGS84 is probably the best
we have at present, but it isn't necessarily the best for localised
portions, so in those areas you'd maybe be better off using
something else. And something else would not be "WGS84 multiplied
by constant scalar n". That is why there are, and continue to be,
multiple datums.

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:

My horror arose when someone said that there were several versions of
latitude and longitude. For instance that the N pole is not 90°N in
all systems.
That's horrendous because in my book the N pole is *defined* as 90°N
and similarly the equator and S pole.

But that's your book. Why should it be the same as everyone else's, if
other people have a slightly different job in mind?

You don't give up do you? <sigh>

What do you think these "different jobs" could be? We've agreed that a
local grid reference system is not what we are talking about, and "my
book" is locked into an astronomical system, so what other job is there
which needs Latitude and Longitude?

Quoted message said:

Quoted message said:

The coordinate systems evolved, just as the metre evolved. They were
not designed with the knowledge which we have today... which is why
the basic units of measurement were redefined by scientists.

But the same scientists have *not* seen fit to propose an ISO
co-ordinate system for planetary co-ordinate systems!

It wasn't their job of course.

Quoted message said:

The definitions of unit distances are not really the same thing at all
as co-ordinate systems, which are a certain way of using measurements,
not the measurements themselves.

I agree they are not the same. But the principal of agreeing on stated
values is identical for both. We no longer have "French metres" versus
"British metres" so we should not have "French latitude" versus "British
Latitude".

Quoted message said:

Quoted message said:

And to say the lat/long was "defined by scientists" is invidious
because you have to ask "what scientists?" and "using what tools?".

The ones given the task of finding a way around a big ocean and getting
back to where you came from, using whatever tools they could find or
devise. What of it?

Because modern tools are radically different... oh, I see I wrote it
before and you missed it:

Quoted message said:

Quoted message said:

Modern tools are radically different. I strongly suspect that NASA
know where the centroid of the earth is so that they can calculate
the course for their unmanned spacecraft. In the same way as the
metre, second, coulomb (etc)
were put on a different footing only last century it's time we agreed
on latitude and longitude:

But to what end? As long as we know where we are using whatever datum
we are using there is no need to create a "standard" lat/long. The
nearest it's been to being agreed is WGS84, but as has been pointed out
repeatedly, WGS84 is not all things to all men so there's no point in
imposing it on those for whom that is the case.

No-one says it's all things to all men... that's why we have GR systems.
How often do I have to clarify this?

Quoted message said:

If you like to work in inches rather than meters you can, and it's
just a scalar conversion to go to the formally defined measurement unit
but it doesn't work anything like that with terrestrial co-ordinate
systems, so the comparison to ISO fundamental units is irrelevant. If
you have a co-ordinate system forced on you that doesn't work as you
need it to you can't just multiply by a constant to make it all right.

In what sense does it "not work"?

In any case it *is* easy to convert - I'm told that cheap GPS machines
do it at a touch of a button, just as calculators do metres to inches at
a touch of a button. It's only mathematics: lat and long are simply the
same system with different zeroes so simple trig is all that's involved.

Quoted message said:

Quoted message said:

indeed I suspect that a system is already there which we can adopt.
Star positions are quoted in Right Ascension and Declination - a
different way of saying latitude and longitude, and stars are
accurately recorded, in fact we measure movement.

They are recorded using a system developed for that specific use.
Lat/long was itself developed for a specific use. Standardising it as
an ISO fundamental does not make any real sense though.

Obviously no sense to you, but to me it's an annoyance. Much the same as
the US gallon versus UK gallon fools many motorists. There is no good
reason for it when we can all quote in litres.

I'm tired of saying the same things to you over and over. Can we agree
not to continue this pointless thread?
--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

You don't give up do you? <sigh>

Or looking at it from the other way, nor do you...! ;-/

We have a situation where there are numerous datums and no attempt at a
single standard. Don't you think there might be a genuine reason for
that state of affairs?

Quoted message said:

What do you think these "different jobs" could be?
We've agreed that a
local grid reference system is not what we are talking about, and "my
book" is locked into an astronomical system, so what other job is there
which needs Latitude and Longitude?

Your "book" is locked on one of several possible astronomical systems
that actually have different effects from one another for the job in
hand. For example, your suggested definition of a centroid as halfway
between the poles puts the equator in a different place to the greatest
possible diameter line of latitude, which is actually more sensible from
some points of view and would work better for many mapping purposes.
OTOH, the direct centre of mass is probably going to be more useful for
others. So you use the best one for the job at hand.

Quoted message said:

Quoted message said:

But the same scientists have *not* seen fit to propose an ISO
co-ordinate system for planetary co-ordinate systems!

It wasn't their job of course.

And they don't do it now, within the accepted scientific discipline of
geodesy, where it /is/ their job *if it would make life easier for
anyone, especially themselves*, because it won't actually help anyone much.

Quoted message said:

I agree they are not the same. But the principal of agreeing on stated
values is identical for both. We no longer have "French metres" versus
"British metres" so we should not have "French latitude" versus "British
Latitude".

We don't. We have functional definitions based on different functional
datums. Where these are for local systems it makes good sense to have
e.g. national datums like OSGB, but the point is it's really there for
geographical information purposes, not "my absolute units are better
than your absolute units" purposes.

Quoted message said:

Because modern tools are radically different... oh, I see I wrote it
before and you missed it:

No, I saw it, and pointed out it isn't relevant because you still use
what works best in a given situation and that still means there's not a
clear reason or place for an ISO geodetic single datum.

Quoted message said:

No-one says it's all things to all men... that's why we have GR systems.
How often do I have to clarify this?

Every time you say we really ought to have one and it's ridiculous that
we don't.. You still haven't come up with why such a thing would be
useful, aside from some vague notion of purity and we're having single
values of distances, charges etc. quantified.

Quoted message said:

Quoted message said:

If you like to work in inches rather than meters you can, and it's
just a scalar conversion to go to the formally defined measurement
unit but it doesn't work anything like that with terrestrial
co-ordinate systems, so the comparison to ISO fundamental units is
irrelevant. If you have a co-ordinate system forced on you that
doesn't work as you need it to you can't just multiply by a constant
to make it all right.

In what sense does it "not work"?

You didn't notice what was pointed out to you that different geometries
used don't easily map onto one another?

Quoted message said:

In any case it *is* easy to convert - I'm told that cheap GPS machines
do it at a touch of a button, just as calculators do metres to inches at
a touch of a button. It's only mathematics: lat and long are simply the
same system with different zeroes so simple trig is all that's involved.

Not for an exact translation of one to the other. There are ambiguous
points in some system conversions, for example. Doesn't really matter
with GPS, it's within the accuracy of the unit anyway, but shouldn't an
exact scientific system be exact?

Quoted message said:

Obviously no sense to you, but to me it's an annoyance. Much the same as
the US gallon versus UK gallon fools many motorists. There is no good
reason for it when we can all quote in litres.

But they're just volume measurements and are used where their meaning is
backed by familiarity with the units at hand. Ultimately a litre is no
different from a gallon as far as gauging how much volume you have as
long as you know how big it is. But with a co-ordinate system, if you
want to stand on the equator and the system you use puts it in a
different place to another, that is a clear and genuine difference. And
that's more than just "an annoyance".

Quoted message said:

I'm tired of saying the same things to you over and over. Can we agree
not to continue this pointless thread?

Fine by me. Go away and read up on geodesy and then take it up directly
with someone who works in the field professionally. They'll give you
better, clearer answers than I've been able to. Remember, if it was
clearly advantageous to come up with a single geodetic reference datum
then someone would have done it, or at least proposed it, inside a
specialised and important field of considerable importance to man.

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/