GPS to grid reference

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

Quoted message 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.

Of course there is.

--
Bill Grey

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.

That's a bit like saying you want to buy a car but you don't want one with
seats, windows or doors! I mean, if you're going to get a GPS, where's the
harm in having the extras? Okay, you don't need maps, I'll grant you that,
but even the most basic models give you track saving and direction.

Being pedantic...

Quoted message said:

I don't need maps, got those.

So have I, but that hardly makes GPS maps redundant. Having them in my GPS
means I don't have to stop and get out my map to cross reference the grid
ref to see where I am. I can just look at the GPS. Far simpler and quicker.

Quoted message said:

I don't need track saving, I got maps.

Eh? Since when can maps save tracks?

Don't tell me you can just draw your route on the map because you know
exactly where you've been. It's not the same thing. When you record a track
you find out where you *really* went, rather than where you thought you
went. If you don't want track saving, fine, but having a map doesn't
substitute for it.

Quoted message said:

I don't need direction, I got a compass.

So even if you had a GPS, you're telling me you'd rather stop, get out your
map, determine your position, lay your compass on the map to work out the
direction, factor in the magnetic variation, put the map away, then use the
compass to navigate, rather than simply glancing at the GPS to see that it's
"that way". The first method can take minutes, the second takes seconds and
doesn't even require stopping. In other words, you'd rather do it the hard
way?

Don't get me wrong, I know that navigation can be challenging and fun, and
if you want that then that's fine, use a map and compass, but if you just
want to know which way to go with the minimum possible hassle, what's wrong
with simply looking at the GPS? It's not as if you're suddenly going to
forget how to navigate overnight. I haven't used map and compass for many
years, but I still know how. It's like riding a bike, you never forget. If
all the satellites stopped working tomorrow, it would be inconvenient, but
it wouldn't stop me going out for a walk.

Even if you buy the most basic model, you're going to have those features
anyway, so why not use them?

Quoted message said:

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.

Well if you're lost in hill fog, wouldn't you also like to know which way to
go as a simple "that way"? I don't understand why so many people seem to
want to make things harder for themselves than it needs to be. Maybe you've
still got that "map and compass" mindset and haven't yet allowed yourself to
believe that GPS isn't the work of the devil after all. Maybe once you've
used one for a while you'll change your mind.

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.

Paul
--
http://www.wilderness-wales.co.uk/
http://www.wilderness-wales.co.uk/weblog/

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.

Cheapest and simplest option would be a basic Garmin eTrex or a Geko
201. Both have much the same features, the main difference is the Geko
is smaller and uses AAA batteries. Both are available for less than £100.
Or there's the new eTrex H, which has much the same features, plus a new
high sensitivity receiver. I'm not sure if it has actually been released
yet.

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).

--
Craig Wallace
http://craig.neogeo.org.uk
http://www.neogeo.org.uk

Paul Saunders said:

Quoted message said:

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.

That's a bit like saying you want to buy a car but you don't want one with
seats, windows or doors!

No. It's a bit like saying you want a vehicle to save time to get the
1000 yds each moring to the train station. A bicycle is the solution.

Bernard Hill said:

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

Most of the ones I've seen do that, amongst other things. My eTrex
certainly does, and they're quite cheap.

Quoted message said:

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.

So don't bother with them! Get a cheap unit and just look at the grid
ref. if that's all you're interested in. On an eTrex, for example,
power up the unit and press the OK button, it will pop up a screen with
where you are and ask if you want to waymark it. Whether you waymark or
not, it will say where you are.

Quoted message said:

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.

As above. It will be a 10 figure reference rather than a 6 figure one
(i.e., down to a 1m square rather than a 100m square) but it's still
pretty straightforward.

You may feel that it should be cheaper if it doesn't have a track log
etc., but memory is cheap, the software is done and it doesn't get any
cheaper spending more money cutting bits out of it, and the spare
capacity in the processors is free. What you're really paying for as
the bulk of a low-end machine is the rugged multi-channel receiver and
processing power to turn the signals into your position. You get the
rest because it's basically no-cost anyway. But it doesn't /weigh/ any
more, and it doesn't hurt if you don't use it.
I never use Dual Time Zone on my watch, but it wouldn't make the watch
cheaper if it didn't do it: it knows about time and the power to cope
with displaying an offset from that is there, and the software's done
and dusted. Same with GPS. If mechanical engineering isn't required
then reducing features doesn't necessarily make the unit any cheaper.
In fact, by reducing production runs it might actually make things more
expensive!

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]>, Craig
Wallace <craigw84@fast_NOSPAM_mail.fm> writes

Quoted message said:

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.

Cheapest and simplest option would be a basic Garmin eTrex or a Geko
201. Both have much the same features, the main difference is the Geko
is smaller and uses AAA batteries. Both are available for less than £100.

I have both and usually take the Geko out in preference to the eTrex.
They do have very similar features but I prefer the lighter and smaller
Geko.

Quoted message said:

Or there's the new eTrex H, which has much the same features, plus a
new high sensitivity receiver. I'm not sure if it has actually been
released yet.

It is supposed to be out in the next month or so. Bernard if you are
wanting to use it in forested areas this would probably be your best bet
at the cheaper end of the market. Older receivers (prior to the new
generation high sensitivity GPS chipsets) can be a bit iffy under heavy
tree cover. If you only expect to be using it out in the open you will
probably not notice any difference.

Quoted message 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).

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

--

Dominic Sexton

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

Quoted message said:

It will be a 10 figure reference rather than a 6 figure one
(i.e., down to a 1m square rather than a 100m square)

I think we discussed this some years ago and the conclusion was that
with a gps the co-ordinates were of a point rather than the SE corner of
a square which the OS apparently thinks is the way to go. That is that
the gps averages the numbers rather than truncating them. I know it is a
bit of a moot point given that the resolution exceeds the discrimination
but I still think it matters.

I still haven't found out why the OS switched to the square system from
the point system previously in use by the military who had a national
grid a decade or more before the revised version appeared on civilian
maps. Or indeed why they go to such great lengths to keep the general
public in ignorance of their meaning of a grid reference.

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

Roger said:

I think we discussed this some years ago and the conclusion was that
with a gps the co-ordinates were of a point rather than the SE corner of
a square which the OS apparently thinks is the way to go. That is that
the gps averages the numbers rather than truncating them. I know it is a
bit of a moot point given that the resolution exceeds the discrimination
but I still think it matters.

That the resolution exceeds the discrimination is why I can switch
freely between the two assumptions and not have it affect my navigation
at all, so as you say, very much a moot point. In fact, I can do worse
than that and still get away with it... if something is just inside a
square I'll typically treat it differently at either side of the square,
giving 1s rather than technically correct 0s on one side, and giving
consistently proper 9s on the other side.

Quoted message said:

I still haven't found out why the OS switched to the square system from
the point system previously in use by the military who had a national
grid a decade or more before the revised version appeared on civilian
maps.

My guess is OS use a square system as the accuracy and resolution go
hand in hand. A 4 figure ref gives you a 1km square, and you know it's
that because it's a 4 figure reference. If you give a co-ordinate then
it's always a point and you can't tell from a point what sort of
accuracy or resolution you're talking about.
Once you get down to 10 figures it's effectively the same thing in any case.

Quoted message said:

Or indeed why they go to such great lengths to keep the general
public in ignorance of their meaning of a grid reference.

It's described on every map, so it's not /that/ secret. And people
manage to use the system because of the nature of mapped data and what
they give grid refs for. If I say "the summit at [censored] yyy" folk will go
to more or less [censored] yyy and look around intelligently for summits,
rather than take out the verniers and ascertain that actually there is
no summit at that exact location. While this is done on the fly by
human data processing, whether or not you consciously think of squares
or points, it actually fits the square model better: go to an /area/ and
look inside it. It works with a map because it's a highly selective
presentation of data, and often where the symbols take up far more space
than the feature really does on the ground.

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:

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

Every model I've come across has the option to change Lat/Long to OSGB.
Make sure the datum is set correctly though. You can test all this in
your local park long before you need it in the hills.

Probably one of the most reliable and affordable is the "Yellow" Garmin
Etrex. You should be able to get one of the original models fairly
reasonably now as there is a new "H" range coming on the market.

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

Quoted message said:

Quoted message said:

I think we discussed this some years ago and the conclusion was that
with a gps the co-ordinates were of a point rather than the SE corner of
a square which the OS apparently thinks is the way to go. That is that
the gps averages the numbers rather than truncating them. I know it is a
bit of a moot point given that the resolution exceeds the discrimination
but I still think it matters.

Quoted message said:

That the resolution exceeds the discrimination is why I can switch
freely between the two assumptions and not have it affect my navigation
at all, so as you say, very much a moot point. In fact, I can do worse
than that and still get away with it... if something is just inside a
square I'll typically treat it differently at either side of the square,
giving 1s rather than technically correct 0s on one side, and giving
consistently proper 9s on the other side.

Which in practice is part way to how any newcomer to the system would
proceed if they were not in possession of the big secret.

Quoted message said:

Quoted message said:

I still haven't found out why the OS switched to the square system from
the point system previously in use by the military who had a national
grid a decade or more before the revised version appeared on civilian
maps.

Quoted message said:

My guess is OS use a square system as the accuracy and resolution go
hand in hand. A 4 figure ref gives you a 1km square, and you know it's
that because it's a 4 figure reference. If you give a co-ordinate then
it's always a point and you can't tell from a point what sort of
accuracy or resolution you're talking about.

You can tell the accuracy by the length of the co-ordinates given. Using
the square system means that the target progresses consistently SW as
the resolution increases.

Quoted message said:

Once you get down to 10 figures it's effectively the same thing in any case.

Quoted message said:

Quoted message said:

Or indeed why they go to such great lengths to keep the general
public in ignorance of their meaning of a grid reference.

Quoted message said:

It's described on every map, so it's not /that/ secret.

You may think that, but that is not actually the case. The natural
meaning of "Estimate tenths Eastward ..." is to make the most accurate
guess at the actual distance, not to stop at the invisible grid lines
just prior to the point. That it also once actually meant what it says
but doesn't anymore is the secret the OS shows no desire of sharing. The
wording on OS maps of how to give a Grid Reference has changed very
little since it first appeared on military maps well before civilian
maps had dumped their A-Z type references.

I have a pre-war (1939) copy of the Army "Manual of Map Reading, Photo
Reading, and Field Sketching" which is an updated version of manual
first issued in 1929. This describes 2 Grid Reference systems. The first
"The British System" was in use between 1919 and 1927 when it was
superceded by the "Modified British System". The later system is exactly
like the one we use today other than the position of the false and real
origins of The Grid. Grid references in the earlier system repeated
periodically.

"Points are described by their co-ordinates in kilometres ..."

And on areas:

"As long as it is clear and unmistakable the reference may be given by
the co-ordinates of the S.W. corner of the square - e.g. 3207 or Q.70,
etc., according to scale.

It is often necessary to refer to areas for tactical purposes. The area
of a square may be indicated by the S.W. corner - e.g Bivouac area
squares Q.70 and Q.71 - or squares 7363 and 7463 (according to scale)."

It would have been ridiculously easy to maintain the distinction between
GRs and GSs but the opportunity has been lost. I presume the military
has by now changed its tune, possibly to fall in with the Americans who
have dictated NATO specifications since it was first set up.

Quoted message said:

And people
manage to use the system because of the nature of mapped data and what
they give grid refs for. If I say "the summit at [censored] yyy" folk will go
to more or less [censored] yyy and look around intelligently for summits,
rather than take out the verniers and ascertain that actually there is
no summit at that exact location. While this is done on the fly by
human data processing, whether or not you consciously think of squares
or points, it actually fits the square model better: go to an /area/ and
look inside it. It works with a map because it's a highly selective
presentation of data, and often where the symbols take up far more space
than the feature really does on the ground.

Funnily enough I don't see it that way. I go to a point and look around
it. With a 6 figure grid ref the maximum displacement error on the
square system is about 140m while that with the point system is half
that.

It doesn't help that anyone quoting a GR will without question refer to
'the co-ordinates' when strictly speaking they mean the co-ordinates of
a point at the SW corner of their chosen square. Such sloppy thinking
doesn't bode well for accuracy in other quarters.

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

Roger said:

Which in practice is part way to how any newcomer to the system would
proceed if they were not in possession of the big secret.

What's secret here? People manage to get useful stuff out of GRs
whichever assumption they use, quite interchangeably. That is a good
indication that it's Not A Problem.

Quoted message said:

You may think that, but that is not actually the case. The natural
meaning of "Estimate tenths Eastward ..." is to make the most accurate
guess at the actual distance, not to stop at the invisible grid lines
just prior to the point.

In practice you make an estimate in the ballpark, and the natural
processing of the user does the rest. And it works the same way
/whichever assumption you use/ as to how they work. In other words,
actual usage is sufficiently fuzzy that the exact theory behind isn't
actually a Big Issue.

Quoted message said:

It would have been ridiculously easy to maintain the distinction between
GRs and GSs but the opportunity has been lost.

But in practice since it doesn't make any difference which one you use,
maintaining the distinction would just add to text density and potential
confusion and not actually achieve much that's useful.
I use both interchangeably and I find what I'm looking for and I don't
get lost. So does someone who assumes co-ordinate points alone, so does
someone assuming bounded squares alone. Surely that we all find what we
need is the /actual point/?

Quoted message said:

Funnily enough I don't see it that way. I go to a point and look around
it. With a 6 figure grid ref the maximum displacement error on the
square system is about 140m while that with the point system is half
that.

But as I've already pointed out several times, nobody actually measures
out such things so the resolution "problem" is a non-issue. Folk make
an eyeball guess, with considerably more potential for inaccuracy than
140m, and it's still perfectly adequate.

Quoted message said:

It doesn't help that anyone quoting a GR will without question refer to
'the co-ordinates' when strictly speaking they mean the co-ordinates of
a point at the SW corner of their chosen square. Such sloppy thinking
doesn't bode well for accuracy in other quarters.

Maps are symbolic representations, not completely spatially accurate
models. Is a typical station a circular structure, roughly 100m across
in all dimensions? Of course not, so there must necessarily be
considerable spatial distortion going on around them, so there goes your
accuracy. Similarly, are all churches with towers the same exact size
and shape? Are dual-carriageways /really/ as wide on the ground as the
exact 1:50K (or whatever scale) suggests?
Maps work by throwing away all the information you don't need and
leaving the (spatially often inaccurate) symbol for what you do need in
enough unambiguous open space relative to stuff around it that you can
find it on the ground. Because of the way things are left unambiguously
distinct from stuff around them, in practise an approximate location
mechanism is all you need. And that people persist in being able to
find things with GRs /or/ GSs even when they have no idea the two are
even distinct shows that it just doesn't matter.

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:

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.

That's a bit like saying you want to buy a car but you don't want one with
seats, windows or doors! I mean, if you're going to get a GPS, where's the
harm in having the extras? Okay, you don't need maps, I'll grant you that,
but even the most basic models give you track saving and direction.

That's fine if the basic models do. But they don't. They come as low as
£40 but only tell me distance travelled and so on.

I'm obviously looking to spend about that sort of price.

Quoted message said:

Being pedantic...

Quoted message said:

I don't need maps, got those.

So have I, but that hardly makes GPS maps redundant. Having them in my GPS
means I don't have to stop and get out my map to cross reference the grid
ref to see where I am. I can just look at the GPS. Far simpler and quicker.

Quoted message said:

I don't need track saving, I got maps.

Eh? Since when can maps save tracks?

In my head. I know where I've been because I've used a map.

Quoted message said:

Don't tell me you can just draw your route on the map because you know
exactly where you've been. It's not the same thing. When you record a track
you find out where you *really* went, rather than where you thought you
went. If you don't want track saving, fine, but having a map doesn't
substitute for it.

Why would I want to know exactly where I went, to the metre?

Quoted message said:

Quoted message said:

I don't need direction, I got a compass.

So even if you had a GPS, you're telling me you'd rather stop, get out your
map, determine your position, lay your compass on the map to work out the
direction, factor in the magnetic variation, put the map away, then use the
compass to navigate, rather than simply glancing at the GPS to see that it's
"that way". The first method can take minutes, the second takes seconds and
doesn't even require stopping. In other words, you'd rather do it the hard
way?

You're kidding, right? I have a watch compass, also one on my rucksack.
They already have declination factored in.

Quoted message said:

Don't get me wrong, I know that navigation can be challenging and fun, and
if you want that then that's fine, use a map and compass, but if you just
want to know which way to go with the minimum possible hassle, what's wrong
with simply looking at the GPS? It's not as if you're suddenly going to
forget how to navigate overnight. I haven't used map and compass for many
years, but I still know how. It's like riding a bike, you never forget. If
all the satellites stopped working tomorrow, it would be inconvenient, but
it wouldn't stop me going out for a walk.

Even if you buy the most basic model, you're going to have those features
anyway, so why not use them?

Ah, but they don't as I indicated earlier.

Quoted message said:

Quoted message said:

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.

Well if you're lost in hill fog, wouldn't you also like to know which way to
go as a simple "that way"? I don't understand why so many people seem to
want to make things harder for themselves than it needs to be. Maybe you've
still got that "map and compass" mindset and haven't yet allowed yourself to
believe that GPS isn't the work of the devil after all. Maybe once you've
used one for a while you'll change your mind.

Maybe.

Quoted message 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.

But from the adverts I've seen they don't give grid reference...
--
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]>, Craig
Wallace <craigw84@fast_NOSPAM_mail.fm> writes

Quoted message said:

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.

Cheapest and simplest option would be a basic Garmin eTrex or a Geko
201. Both have much the same features, the main difference is the Geko
is smaller and uses AAA batteries. Both are available for less than £100.
Or there's the new eTrex H, which has much the same features, plus a
new high sensitivity receiver. I'm not sure if it has actually been
released yet.

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).

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

However I don't really want a wrist mount, although I can always put it
away I suppose. I use a watch, you see.

--
Bernard Hill
Selkirk, Scotland

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...

Good grief.

--
Bernard Hill
Selkirk, Scotland

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

Quoted message said:

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

Quoted message said:

It will be a 10 figure reference rather than a 6 figure one
(i.e., down to a 1m square rather than a 100m square)

I think we discussed this some years ago and the conclusion was that
with a gps the co-ordinates were of a point rather than the SE corner of
a square which the OS apparently thinks is the way to go. That is that
the gps averages the numbers rather than truncating them. I know it is a
bit of a moot point given that the resolution exceeds the discrimination
but I still think it matters.

I still haven't found out why the OS switched to the square system from
the point system previously in use by the military who had a national
grid a decade or more before the revised version appeared on civilian
maps. Or indeed why they go to such great lengths to keep the general
public in ignorance of their meaning of a grid reference.

Good grief. It's obviously part of the dumbing down.

Grist to my GOM mill.

--
Bernard Hill
Selkirk, Scotland

In article <[email hidden]>, dino
<di-no@//REMOVETHIS//utvinternet.ie> writes

Quoted message said:

Bernard Hill said:

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

Every model I've come across has the option to change Lat/Long to OSGB.
Make sure the datum is set correctly though. You can test all this in
your local park long before you need it in the hills.

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

Quoted message said:

Probably one of the most reliable and affordable is the "Yellow" Garmin
Etrex. You should be able to get one of the original models fairly
reasonably now as there is a new "H" range coming on the market.

Thanks.
--
Bernard Hill
Selkirk, Scotland

Bernard Hill said:

Quoted message said:

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

http://en.wikipedia.org/wiki/Datum

I'm not 100% up on the technical side but AIUI in layman's terms it's a
way of measuring the Earth's shape. Lat/Long tends to use WGS84 datum
whereas British National Grid uses the OSGB datum. Most GPS receivers
come with Lat/Long as the default and automatically change the datum
when you change to British National Grid. Others don't so you need to be
careful. Once changed it'll stay that way unless you tell it otherwise.

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

Quoted message said:

Quoted message said:

Which in practice is part way to how any newcomer to the system would
proceed if they were not in possession of the big secret.

Quoted message said:

What's secret here? People manage to get useful stuff out of GRs
whichever assumption they use, quite interchangeably. That is a good
indication that it's Not A Problem.

The secret is what they actually mean by Grid Reference. A secret you
apparently weren't in on and now suggest doesn't matter. If the OS want
users to accept their way of doing things rather than the natural way
they should make what they mean clear, not rely on the 'it really
doesn't matter' gambit.

Quoted message said:

Quoted message said:

You may think that, but that is not actually the case. The natural
meaning of "Estimate tenths Eastward ..." is to make the most accurate
guess at the actual distance, not to stop at the invisible grid lines
just prior to the point.

Quoted message said:

In practice you make an estimate in the ballpark, and the natural
processing of the user does the rest. And it works the same way
/whichever assumption you use/ as to how they work. In other words,
actual usage is sufficiently fuzzy that the exact theory behind isn't
actually a Big Issue.

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.

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

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

Alan Murphy