Re: geo microformat - possible extension for waypoint lists

Neil Pugh said:

I always use the National Grid when geocaching. The OS coordinates are
given on geocaching.com as well as lat & long. It has the advantage of
telling you approx haw many metres you are from the cache.

So does Lat&Long if you have your GPS set to the correct datum! How do
you think those outside of GB manage 🙂

Also AFAIK the OS coords given on the cache pages aren't completely
correct but there is a Greasemonkey script that will show them properly.
IIRC it's to do with a conversion algorithm that's used but as I live in
Ireland I can't tell you how correct that is.

dino said:

Neil Pugh said:

I always use the National Grid when geocaching. The OS coordinates are
given on geocaching.com as well as lat & long. It has the advantage of
telling you approx haw many metres you are from the cache.

So does Lat&Long if you have your GPS set to the correct datum! How do
you think those outside of GB manage 🙂

Perfectly well. As do we in these islands. The pain is switching
between datums and grids. I find it far easier to keep my GPS on Ord
Svy GB and OSGB whilst in Great Britain. If I was to hop on a ferry to
Ireland I would change it to whatever I needed to use with OSNI/OSI
maps. The only use I have for WGS 84 and Lat Long is where there are
no good maps.
--
Phil Cook, last hill: Heron Pike via Nab Scar.

dino said:

So does Lat&Long if you have your GPS set to the correct datum!

Be warned that the same lat/long position will give you two different
locations depending on whether you use the WGS84 or Ordnance Survey datum.
Technically both are correct, but I suspect that WGS84 is probably the one
you should use.

Quoted message said:

Also AFAIK the OS coords given on the cache pages aren't completely
correct

Quoted message said:

IIRC it's to do with a conversion algorithm

Could be, or it could be down to which of the above datums you're using, or
both!

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

On Tue, 27 Mar 2007 19:02:23 +0100, "Paul Saunders"
<[email hidden]> wrote:

|!dino wrote:
|!
|!> So does Lat&Long if you have your GPS set to the correct datum!
|!
|!Be warned that the same lat/long position will give you two different
|!locations depending on whether you use the WGS84 or Ordnance Survey datum.
|!Technically both are correct, but I suspect that WGS84 is probably the one
|!you should use.
|!
|!> Also AFAIK the OS coords given on the cache pages aren't completely
|!> correct
|!
|!> IIRC it's to do with a conversion algorithm
|!
|!Could be, or it could be down to which of the above datums you're using, or
|!both!

The problem in fact is due to the shape of the earth, it is not a perfect
sphere or a perfect geoid, or a perfect anything :-( . Different datums
use different assumptions about the shape of the earth, and so give
different positions. In the UK OSGB is the most accurate, as it is
reproducable, with the correct equipment, to IIRC less than a centimetre,
and is used on almost all GB maps. The hand held, or vehicle bourn,
equipment, which are available to the public are only accurate to a few
metres, soon to be improved to 1-2m when EGNOS starts working properly.
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

In article <[email hidden]>, Dave Fawthrop
<[email hidden]> writes

Quoted message said:

On Tue, 27 Mar 2007 19:02:23 +0100, "Paul Saunders"
<[email hidden]> wrote:

|!dino wrote:
|!
|!> So does Lat&Long if you have your GPS set to the correct datum!
|!
|!Be warned that the same lat/long position will give you two different
|!locations depending on whether you use the WGS84 or Ordnance Survey datum.
|!Technically both are correct, but I suspect that WGS84 is probably the one
|!you should use.
|!
|!> Also AFAIK the OS coords given on the cache pages aren't completely
|!> correct
|!
|!> IIRC it's to do with a conversion algorithm
|!
|!Could be, or it could be down to which of the above datums you're using, or
|!both!

The problem in fact is due to the shape of the earth, it is not a perfect
sphere or a perfect geoid, or a perfect anything :-( . Different datums
use different assumptions about the shape of the earth, and so give
different positions. In the UK OSGB is the most accurate, as it is
reproducable, with the correct equipment, to IIRC less than a centimetre,
and is used on almost all GB maps. The hand held, or vehicle bourn,
equipment, which are available to the public are only accurate to a few
metres, soon to be improved to 1-2m when EGNOS starts working properly.

In actual fact the OS grid has errors in the implementation which is why
the official OSTN02 transformation uses a table of shifts to make the
conversion. See:

http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/sur
veying_osnet.html#6

--

Dominic Sexton

Dave Fawthrop said:

The problem in fact is due to the shape of the earth, it is not a
perfect sphere or a perfect geoid, or a perfect anything :-( .

Yeah, bloody Earth! That's the problem! Who the hell designed it?

I reckon the simplest solution to all these datum issues would be to correct
the shape of the earth to a perfect sphere and while they're at it, to fix
the magnetic north pole so that it matches the actual north pole and stays
there! ;-)

Quoted message said:

public are only accurate to a few metres, soon to be improved to 1-2m
when EGNOS starts working properly.

As I mentioned recently, I've been getting 1-2m accuracy lately. But not
with the atitude though... :-(

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

Phil Cook said:

dino said:

Neil Pugh said:

I always use the National Grid when geocaching. The OS coordinates are
given on geocaching.com as well as lat & long. It has the advantage of
telling you approx haw many metres you are from the cache.

So does Lat&Long if you have your GPS set to the correct datum! How do
you think those outside of GB manage 🙂

Perfectly well. As do we in these islands. The pain is switching
between datums and grids.

I agree with using one system but you made it sound in your first post
that Lat & Long isn't as accurate as OSGB but it is and if you're using
info from the cache pages then you need to be careful using the OSGB
coords that are quoted as they're not always accurate.

Quoted message said:

I find it far easier to keep my GPS on Ord
Svy GB and OSGB whilst in Great Britain. If I was to hop on a ferry to
Ireland I would change it to whatever I needed to use with OSNI/OSI
maps. The only use I have for WGS 84 and Lat Long is where there are
no good maps.

That would be Ireland then 😉

On Tue, 27 Mar 2007 19:47:18 +0100, Dominic Sexton
<[email hidden]> wrote:

|!In article <[email hidden]>, Dave Fawthrop
|!<[email hidden]> writes
|!>On Tue, 27 Mar 2007 19:02:23 +0100, "Paul Saunders"
|!><[email hidden]> wrote:
|!>
|!>|!dino wrote:
|!>|!
|!>|!> So does Lat&Long if you have your GPS set to the correct datum!
|!>|!
|!>|!Be warned that the same lat/long position will give you two different
|!>|!locations depending on whether you use the WGS84 or Ordnance Survey datum.
|!>|!Technically both are correct, but I suspect that WGS84 is probably the one
|!>|!you should use.
|!>|!
|!>|!> Also AFAIK the OS coords given on the cache pages aren't completely
|!>|!> correct
|!>|!
|!>|!> IIRC it's to do with a conversion algorithm
|!>|!
|!>|!Could be, or it could be down to which of the above datums you're using, or
|!>|!both!
|!>
|!>The problem in fact is due to the shape of the earth, it is not a perfect
|!>sphere or a perfect geoid, or a perfect anything :-( . Different datums
|!>use different assumptions about the shape of the earth, and so give
|!>different positions. In the UK OSGB is the most accurate, as it is
|!>reproducable, with the correct equipment, to IIRC less than a centimetre,
|!>and is used on almost all GB maps. The hand held, or vehicle bourn,
|!>equipment, which are available to the public are only accurate to a few
|!>metres, soon to be improved to 1-2m when EGNOS starts working properly.
|!
|!In actual fact the OS grid has errors in the implementation which is why
|!the official OSTN02 transformation uses a table of shifts to make the
|!conversion. See:
|!
|!http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/sur
|!veying_osnet.html#6

No they have just discovered more irregularities in the shape of the earth
as the accuracy of measurement improves. The GPS WGS84 system has errors
because it does not agree with measurements on the ground, which must be
the Gold Standard for mapping. WGS84 just assumes one geoid which is far
from perfect. OSGB in the UK, as you say, corrects for errors in its
geoid.

A *summary* of the problem is given in the 46 page PDS file
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf

--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

In article <[email hidden]>, Dave Fawthrop
<[email hidden]> writes

Quoted message said:

On Tue, 27 Mar 2007 19:47:18 +0100, Dominic Sexton
<[email hidden]> wrote:

|!In article <[email hidden]>, Dave Fawthrop
|!<[email hidden]> writes
|!>The problem in fact is due to the shape of the earth, it is not a perfect
|!>sphere or a perfect geoid, or a perfect anything :-( . Different datums
|!>use different assumptions about the shape of the earth, and so give
|!>different positions. In the UK OSGB is the most accurate, as it is
|!>reproducable, with the correct equipment, to IIRC less than a centimetre,
|!>and is used on almost all GB maps. The hand held, or vehicle bourn,
|!>equipment, which are available to the public are only accurate to a few
|!>metres, soon to be improved to 1-2m when EGNOS starts working properly.
|!
|!In actual fact the OS grid has errors in the implementation which is why
|!the official OSTN02 transformation uses a table of shifts to make the
|!conversion. See:
|!
|!http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/sur
|!veying_osnet.html#6

No they have just discovered more irregularities in the shape of the earth
as the accuracy of measurement improves. The GPS WGS84 system has errors
because it does not agree with measurements on the ground, which must be
the Gold Standard for mapping. WGS84 just assumes one geoid which is far
from perfect.

The Geoid is the theoretical equipontential surface (akin to mean sea
level extended under the land mass) and only affects vertical
measurements.

Quoted message said:

OSGB in the UK, as you say, corrects for errors in its
geoid.

A *summary* of the problem is given in the 46 page

Page 46 is the Glossary so I am unsure what you are referring to.

Quoted message said:

PDS file
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate
_Systems_in_Great_Britain.pdf

No Dave there are very real errors in the implementation of the OS grid
on the ground compared to the theoretical grid.

See the first line of section 6.3 of the above guide

"To cope with the distortions in the OSGB36 TRF, different
transformations are needed in different parts of the country. For this
reason, the national standard datum transformation between OSGB36 and
ETRS89 is not a simple Helmert datum transformation. Instead, Ordnance
Survey has developed a ‘rubber-sheet’ style transformation which
works with a transformation grid expressed in easting and northing
coordinates."

Or as html:

<http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesyst
emsinfo/guidecontents/guide6.html>

--

Dominic Sexton

On Wed, 28 Mar 2007 09:59:13 +0100, Dominic Sexton
<[email hidden]> wrote:

|!>A *summary* of the problem is given in the 46 page
^^^^^^^^^^^
|!
|!Page 46 is the Glossary so I am unsure what you are referring to.
|!
|!> PDS file
|!>http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate
|!>_Systems_in_Great_Britain.pdf

I referred to the *whole* 46 page pdf file, reading or even skimming 46
pages not an insignificant effort :-(
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

In article <[email hidden]>, Dave Fawthrop
<[email hidden]> writes

Quoted message said:

On Wed, 28 Mar 2007 09:59:13 +0100, Dominic Sexton
<[email hidden]> wrote:

|!>A *summary* of the problem is given in the 46 page
^^^^^^^^^^^
|!
|!Page 46 is the Glossary so I am unsure what you are referring to.
|!
|!> PDS file
|!>http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate
|!>_Systems_in_Great_Britain.pdf

I referred to the *whole* 46 page pdf file,

Sorry I mis-read that.

Quoted message said:

reading or even skimming 46
pages not an insignificant effort :-(

Had you read section 6.3 you might have understood that there really are
errors in the OS grid. Although the traditional surveying methods were
rigorously used when laying out the OS grid it is entirely
understandable that there are errors in it when checked with a system
that has an accuracy in the order of centimetres.

--

Dominic Sexton

dino said:

Phil Cook said:

dino said:

Neil Pugh wrote:

> I always use the National Grid when geocaching. The OS coordinates are
> given on geocaching.com as well as lat & long. It has the advantage of
> telling you approx haw many metres you are from the cache.
So does Lat&Long if you have your GPS set to the correct datum! How do
you think those outside of GB manage 🙂

Perfectly well. As do we in these islands. The pain is switching
between datums and grids.

I agree with using one system but you made it sound in your first post
that Lat & Long isn't as accurate as OSGB

Huh? Where did I say that?

Quoted message said:

but it is and if you're using
info from the cache pages then you need to be careful using the OSGB
coords that are quoted as they're not always accurate.

They might not be accurately converted. GIGO
--
Phil Cook, last hill: Heron Pike via Nab Scar.

Phil Cook said:

Huh? Where did I say that?

Oops! Mixed your reply up with Neil Pugh and Dave Fawthrop posted! You
replied to a reply of mine to Neil and I didn't check the author.

A thousand apologies 🙂

The message <[email hidden]>
from Dominic Sexton <[email hidden]> contains these words:

Quoted message said:

Or as html:

Quoted message said:

<http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesyst
emsinfo/guidecontents/guide6.html>

Er, I cannot find any of this interesting kit - the OS web site today
seems to be very thin on information, and strong on sales pitch. What am
I missing?

Richard

On Wed, 28 Mar 2007 18:31:44 GMT, Richard Spencer
<[email hidden]> wrote:

|!The message <[email hidden]>
|!from Dominic Sexton <[email hidden]> contains these words:
|!
|!
|!> Or as html:
|!
|!> <http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesyst
|!> emsinfo/guidecontents/guide6.html>
|!
|!Er, I cannot find any of this interesting kit - the OS web site today
|!seems to be very thin on information, and strong on sales pitch. What am
|!I missing?

Re-assemble the above URL, or start at
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/
There is more technical information than you would care to read
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

In article <[email hidden]>, Richard Spencer
<[email hidden]> writes

Quoted message said:

The message <[email hidden]>
from Dominic Sexton <[email hidden]> contains these words:

Quoted message said:

Or as html:

Quoted message said:

<http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesyst
emsinfo/guidecontents/guide6.html>

Er, I cannot find any of this interesting kit - the OS web site today
seems to be very thin on information, and strong on sales pitch. What am
I missing?

Yes it does seem to have been driven by the marketing department. There
is still good stuff to be found but it isn't always easy to find it.

The above link can be got to via:

http://preview.tinyurl.com/2fa2rc

The links on the left of the target page go to some of the more useful
technical parts of the site.

--

Dominic Sexton

In message <[email hidden]>, dino
<[email hidden]> writes

Quoted message said:

Phil Cook said:

Huh? Where did I say that?

Oops! Mixed your reply up with Neil Pugh and Dave Fawthrop posted! You
replied to a reply of mine to Neil and I didn't check the author.

A thousand apologies 🙂

I didn't say that either. Lat & long are perfectly good if your maps are
in lat & long, when abroad that's how I set my GPS. Maps in this country
tend to be based on the OSGB grid & datum, therefore it makes sense to
have your GPS set to OSGB grid & datum.

Any errors in conversion between the two formats for geocaching purposes
are probably less than the accuracy of the instrument, therefore fairly
irrelevant.

Regards,
--
Neil Pugh

Neil Pugh said:

Lat & long are perfectly good if your maps are
in lat & long, when abroad that's how I set my GPS. Maps in this country
tend to be based on the OSGB grid & datum, therefore it makes sense to
have your GPS set to OSGB grid & datum.

Any errors in conversion between the two formats for geocaching purposes
are probably less than the accuracy of the instrument, therefore fairly
irrelevant.

Provided the datum is set and specified correctly.

OS Lat Long is different from WGS84 Lat Long.
--
Phil Cook, last hill: Heron Pike via Nab Scar.

The message <[email hidden]>
from Neil Pugh <[email hidden]> contains these words:

Quoted message said:

Lat & long are perfectly good if your maps are
in lat & long, when abroad that's how I set my GPS. Maps in this country
tend to be based on the OSGB grid & datum, therefore it makes sense to
have your GPS set to OSGB grid & datum.

I am not into geo caching but I would have thought that navigating to
the approximate area of the cache was a map reading exercise for which
lat/long would be little use. Unless you are at 2 degrees West the only
lat/long data on OS maps is to be found in the margins.

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