GPS to grid reference

"Roger" <[email hidden]> schreef in bericht
news:[email hidden]...

Quoted message said:

The message <[email hidden]>
from Peter Clinch <[email hidden]> contains these words:

There are 2 issues at stake. The first is the commonsense one that you
don't expect to be deliberately misinformed by purveyors of facts. The
second is that when you are aiming at something approximate you are
better off if your target is at the middle of the expected range, not
the extreme end.

This is really a non-issue. How far is it to London ? 45 miles accordong to
the motorway sign. Is that to the nearest part of London, the centre (the
old one or the geografically one?) or to the other side of London ? Like I
said, it's bollocks.

--
"Beannachd leibh"

Theo
www.theosphotos.fotopic.net

"Theo" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"Roger" <[email hidden]> schreef in bericht
news:[email hidden]...

Quoted message said:

The message <[email hidden]>
from Peter Clinch <[email hidden]> contains these words:

There are 2 issues at stake. The first is the commonsense one that you
don't expect to be deliberately misinformed by purveyors of facts. The
second is that when you are aiming at something approximate you are
better off if your target is at the middle of the expected range, not
the extreme end.

This is really a non-issue. How far is it to London ? 45 miles accordong
to the motorway sign. Is that to the nearest part of London, the centre
(the old one or the geografically one?) or to the other side of London ?
Like I said, it's bollocks.

/pedant

I believe distances to London are measured to Charing Cross. Well, they used
to be, anyway. Well maybe to the SW corner of a 100m square with Charing
Cross in the middle (sorry, couldn't resist :o) )

/pedant

Well it's not really bollocks, as I said before, in theory (or principle)
Roger is correct. Maps are sold for a practical purpose, however and I (and
Pete, I think) don't believe that, for the purposes to which maps are put,
it matters at all, nor that the OS need to do more (or that they have done
anything bad) and, with that in mind, if it makes it easier for them, why
not?

Bob Mannix said:

Well it's not really bollocks, as I said before, in theory (or principle)
Roger is correct. Maps are sold for a practical purpose, however and I (and
Pete, I think) don't believe that, for the purposes to which maps are put,
it matters at all, nor that the OS need to do more (or that they have done
anything bad) and, with that in mind, if it makes it easier for them, why
not?

And it also makes it easier IMHO for the great majority of users, as
instructions should always be as simple as possible to get the job in
hand done. For the isolated cases where the job might not be done the
folk in question should be savvy enough to work around in any case: it
ain't broke, so I don't see a need to fix it.

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]>, Paul Saunders
<[email hidden]> writes

Quoted message said:

Bernard Hill said:

Quoted message said:

They do have some navigation features (goto and following saved
tracks) but don't do grid refs only lat / long in WGS-84.

Exactly. I can't find one whose adverts say "grid reference". You'd
think that was pretty basic, but it seems not...

Don't forget that most GPS units are designed for the world market, and most
countries seem to have different datums and coordinate systems, so the unit
has to be loaded with hundreds of datums and grid systems.

So it's not simply a matter of switching from lat/long to grid ref. It would
be, if a GPS was designed to be used only in the UK, but then you'd be
stuffed if you wanted to take it on holiday to another country.

Well of course I don't expect GR to be the *only* scale! Just one of the
options. And it's not mentioned in *any* item description I've seen so
far so I presumed it was not available.

--
Bernard Hill
Selkirk, Scotland

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

Quoted message said:

"Bernard Hill" <[email hidden]> schreef in bericht
news:[email hidden]...

Quoted message said:

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

Quoted message said:

In article <[email hidden]>, Craig
Wallace <craigw84@fast_NOSPAM_mail.fm> writes

They do have some navigation features (goto and following saved tracks)
but don't do grid refs only lat / long in WGS-84.

Exactly. I can't find one whose adverts say "grid reference". You'd think
that was pretty basic, but it seems not...

I don't see a problem. My Geko 201 gives me a grid reference and I suppose
the cheapest 101 does the same. Second hand it'll come even cheaper.

Thanks for that information. It was exactly the reason I posted the
original question because that reply is not available anywhere in the
adverts as far as I can see.
--
Bernard Hill
Selkirk, Scotland

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

Quoted message said:

Bernard Hill said:

I'll look out for that. But I was hoping to get under £50 since the
electronics obviously comes that sort of price.

eBay is probably your friend here.

Quoted message said:

From a quick trawl it's possible to get a basic unit (say, an eTrex or
Explorist) for sensible quantities of money.

Exactly. But finding out if they do OS GR is why I asked the question.
--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, Paul Saunders
<[email hidden]> writes

Quoted message said:

If by "co-ordinate" you mean a 6 figure GR, then no, that's not a
point, it's a 100m wide square.

Yes, but where is the centre of that square?

It seems to me it's an inbuilt problem with the design of the
coordinates. 472 850 is both a point and a square. Tell me to go to 472
850 and I have to ask whether you mean 47200 85000 or the centre of the
472 850 square. They are 71m apart.

--
Bernard Hill
Braeburn Software
Author of Music Publisher system
Music Software written by musicians for musicians
http://www.braeburn.co.uk
Selkirk, Scotland

In article <[email hidden]>, Alan Murphy
<[email hidden]> writes

Quoted message said:

Bernard Hill said:

What's a "datum"? Is the L/Long not unique in converting to OSGB?

No. Lat/Long is not unique. The position
N50°00.0000' W001°00.0000' (in WGS84 datum)
is exactly the same position as
N49°59.9647' W000°59.9126' (in Ord Srvy Grt Britn datum)

See a layman's (non-technical) explanation of datums at
http://www.gpsu.co.uk/datums.html

Um. What a hopeless explanation. He refers to relative coordinates then
just says there is more than one datum!

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 two poles.
So we have well-defined latitude lines and one longitude line which goes
through a particular spot in Greenwich.

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.

--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

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

Quoted message said:

"Bernard Hill" <[email hidden]> schreef in bericht
news:[email hidden]...

Quoted message said:

In article <[email hidden]>, Dominic Sexton
<[email hidden]> writes
>In article <[email hidden]>, Craig
>Wallace <craigw84@fast_NOSPAM_mail.fm> writes
>
>They do have some navigation features (goto and following saved tracks)
>but don't do grid refs only lat / long in WGS-84.
>

Exactly. I can't find one whose adverts say "grid reference". You'd think
that was pretty basic, but it seems not...

I don't see a problem. My Geko 201 gives me a grid reference and I suppose
the cheapest 101 does the same. Second hand it'll come even cheaper.

Thanks for that information. It was exactly the reason I posted the
original question because that reply is not available anywhere in the
adverts as far as I can see.

They used to make a thing about the grids they did. Perhaps the list
got too long-winded.

--
Phil Cook, last hill: Angletarn Pikes via Beda Fell.
http://www.p-t-cook.freeserve.co.uk/May07/martin9.htm

Bernard Hill said:

There has to be a cheap GPS which has a readout as a grid ref.

I don't need maps, got those. I don't need track saving, I got maps. I
don't need direction, I got a compass.

But just now and again I get "lost" on a featureless flat top in the
cloud and would love to know where I am as a simple grid reference.

--
Bernard Hill
Selkirk, Scotland

Geko 201!

David
--
http://www.fenrunfellwalk.co.uk

Craig Wallace craigw84@fast_NOSPAM_mail.fm said:

Paul mentioned wrist mounted GPS devices. The Garmin Foretrexs can
display a grid reference (they are much the same as the Geko, but in a
different form factor), but I don't think the Forerunner series can give
a grid ref (they are designed more for training, so lacking in
navigation features).

For running I've got a Forerunner 205 and it displays location in
degrees not in grid ref.

David
--
http://www.fenrunfellwalk.co.uk

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

Quoted message said:

In article <[email hidden]>, Alan Murphy
<[email hidden]> writes

Quoted message said:

Bernard Hill said:

What's a "datum"? Is the L/Long not unique in converting to OSGB?

No. Lat/Long is not unique. The position
N50°00.0000' W001°00.0000' (in WGS84 datum)
is exactly the same position as
N49°59.9647' W000°59.9126' (in Ord Srvy Grt Britn datum)

See a layman's (non-technical) explanation of datums at
http://www.gpsu.co.uk/datums.html

Um. What a hopeless explanation.

Not at all.

Quoted message said:

He refers to relative coordinates then just says there is more than one
datum!

As there are.

Quoted message said:

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 two poles.

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

Quoted message said:

So we have well-defined latitude lines and one longitude line which
goes through a particular spot in Greenwich.

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.

They are all in error to a more or less extent. The earth is not a
perfect geometrical shape. It simply does not fit an ellipsoid
everywhere no matter how you specify that ellipsoid. However it is
mathematically advantageous to model the earth as though it were an
ellipsoid.

Therefore the various ellipsoid based datums were developed (with
different centres and sizes) to try and fit the shape of the earth in
the area of application.

With the advent of the space age it became possible to determine datums
with reasonable fit globally such as the native datum of the GPS WGS-84.
The average global fit is good but the fit in any particular area might
be poor. This combined with vast quantities of historical data referred
to local datums around the world means that the local datums have not
suddenly been replaced by a 'superior' global datum.

--

Dominic Sexton

Bernard Hill said:

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 two poles.
So we have well-defined latitude lines and one longitude line which goes
through a particular spot in Greenwich.

So why more than one datum??

Because the Earth is an awkward shape. In practice a parallel should be
a constant distance from the next parallel, and it should be the same
length as it's counterpart in the other hemisphere so given angular
distances along it are constant, but if the planet isn't a convenient
shape (and it isn't) then such assumptions will break down to some degree.

Lat/Long really assumes a reference ellipsoid. Those are smoother than
the Geoid (surface of equal gravitational potential), which in turn is
smoother than the actual surface. The problem comes in using a system
designed for an ellipsoid on something that isn't /quite/ an ellipsoid.

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/

Roger said:

Using a gps doesn't necessarily help overmuch. In entering a 6 figure
reference do you add 00 to each co-ordinate or 50?

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

I use Simon's site and download the full 10 figure reference!

BTW on the Howgills last week at some stages the GPS stated that it's
accuracy was + or - 2m!!!

David
--
http://www.fenrunfellwalk.co.uk

Paul Saunders said:

I think you can get some kind of wristwatch GPS models. Maybe they're
featureless enough for you? Although I don't think your wrist is the best
place to wear a GPS. Don't know any details about them, sorry, but someone
here can probably advise.

For running it is perfect having the antenna in the strap of the
wristwatch GPS!

David
--
http://www.fenrunfellwalk.co.uk

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]>, Alan Murphy
<[email hidden]> writes

Quoted message said:

On Mon, 30 Jul 2007 16:45:58 +0100, Bernard Hill
<[email hidden]> wrote:

>What's a "datum"? Is the L/Long not unique in converting to OSGB?
No. Lat/Long is not unique. The position
N50°00.0000' W001°00.0000' (in WGS84 datum)
is exactly the same position as
N49°59.9647' W000°59.9126' (in Ord Srvy Grt Britn datum)

See a layman's (non-technical) explanation of datums at
http://www.gpsu.co.uk/datums.html

Um. What a hopeless explanation.

Not at all.

Quoted message said:

He refers to relative coordinates then just says there is more than
one datum!

As there are.

Quoted message said:

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 two poles.

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. Slice the earth with planes perpendicular to the axis of
rotation and you have parallels of latitude. The one going through the
centroid you can define as the equator, the rest can be divided up into
parts for the other ones.

Quoted message said:

Quoted message said:

So we have well-defined latitude lines and one longitude line which
goes through a particular spot in Greenwich.

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.

They are all in error to a more or less extent. The earth is not a
perfect geometrical shape. It simply does not fit an ellipsoid
everywhere no matter how you specify that ellipsoid. However it is
mathematically advantageous to model the earth as though it were an
ellipsoid.

It doesn't need to be a perfect anything. A stony asteroid can have
latitude and longitude.

Quoted message said:

Therefore the various ellipsoid based datums were developed (with
different centres and sizes) to try and fit the shape of the earth in
the area of application.

With the advent of the space age it became possible to determine datums
with reasonable fit globally such as the native datum of the GPS
WGS-84. The average global fit is good but the fit in any particular
area might be poor. This combined with vast quantities of historical
data referred to local datums around the world means that the local
datums have not suddenly been replaced by a 'superior' global datum.

But how are datums (ugh! data is plural of datum) defined? That's why
it's a lousy article. It says there are lots of them but not why.

--
Bernard Hill
Selkirk, Scotland

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

Quoted message said:

Bernard Hill said:

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 two poles.
So we have well-defined latitude lines and one longitude line which
goes through a particular spot in Greenwich.
So why more than one datum??

Because the Earth is an awkward shape. In practice a parallel should
be a constant distance from the next parallel,

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

Quoted message said:

and it should be the same length as it's counterpart in the other
hemisphere

ditto

Quoted message said:

so given angular distances along it are constant,

??

Quoted message said:

but if the planet isn't a convenient shape (and it isn't) then such
assumptions will break down to some degree.

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.

Quoted message said:

Lat/Long really assumes a reference ellipsoid. Those are smoother than
the Geoid (surface of equal gravitational potential), which in turn is
smoother than the actual surface. The problem comes in using a system
designed for an ellipsoid on something that isn't /quite/ an ellipsoid.

As I said, there can be defined one unique set of lat/long. In fact I
fail to see how any other method could work.

--
Bernard Hill
Selkirk, Scotland

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.

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

The message <[email hidden]>
from "Theo" <[email hidden]> contains these words:

Quoted message said:

This is really a non-issue. How far is it to London ? 45 miles accordong to
the motorway sign. Is that to the nearest part of London, the centre (the
old one or the geografically one?) or to the other side of London ? Like I
said, it's bollocks.

And bollocks to you too.

AFAIK distances from London are always measured from Charing Cross.

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

The message <[email hidden]>
from "Bob Mannix" <[email hidden]> contains these words:

Quoted message said:

Quoted message said:

Quoted message said:

> Would you care to point me towards the place where the OS advertises
> the
> fact that they expect the general public to truncate their co-ordinates
> rather take their best guess in order to confirm to the OS belief that
> a
> grid reference absolutely relates to the Square to the NE of the Point.

Quoted message said:

http://www.ordnancesurvey.co.uk/oswebsite/gi/nationalgrid/nationalgrid.pdf

I did ask for a place for the general public to be informed, not
Internet nerds, but I can't see anything in the cited site that actually
does identify a grid reference as a square. It doesn't help that the
site seems aimed at children with a reading age of 10 and simplified to
the detriment of both good English and detail.

Good English? If I see "Unique reference system" I expect a unique
system not one that will generate a unique reference.

You didn't ask for something with good English or acceptable detail. One
might also say that the only people with such an overwhelming interest in
something that clearly isn't a problem in real life (I am with Pete on this
one) might just be "map nerds" and dismiss them in a similarly cavalier
fashion :o)

I have an interest in the meaning of words that is a good deal broader
than just map notation. One of my favourite TV programs is Balderdash
and Piffle and not just because it has an attractive presenter. :-)

Quoted message said:

Nowhere (as far as I can see) do they claim they are using exact
coordinates
rather than the SW corner of a square, so they are NOT deliberately
misinforming (whatever other failings they may have, and I'm sure thay are
failings). Most people (including the OS) don't think it matters in
the real
lives of their map users but, if you ask them to clarify the GR system, I
have no doubt they would explain it without misinformation!

But it remains a fact that the natural meaning of the words used
describe a point, not a square, and what is more a point that might not
lie within the OS square. If the OS want us to believe a GR is a square
they should make some attempt to publicise their convention. It would
only take the addition of a few words so the cost would be infinitesimal
even to the bean counters at The Treasury.

For everyone who knows and accepts that there is a discrepancy between
what the words say and what they are supposed to mean there will be a
dozen or more who will take as gospel the untrue version because that is
what they were taught at an impressionable age and will dismiss with
arrogance and contempt any attempt to highlight the truth.

Quoted message said:

It would seem to be the case that you care and others, even if they didn't
already know and have it carefully explained to them, don't. There is,
therefore, unlikely to be a resolution!

I care about the truth as a matter of principle. There is no excuse for
needlessly misinforming people however limited the consequences are.

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.

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