"David A Stocks" wrote in message

"Graculus" wrote in message
...
(No, GPS isn't accurate enough to say if its on the up fast/down
slow/etc track.)
Should be - I get accuracy down to about a metre on a very old
hand-held GPS box.

How can you be sure that it's so accurate? I took my Garmin wrist GPS
to Greenwich Observatory, and was crestfallen to discover that my GPS
wasn't nearly as accurate as it claimed. As I recall, it claimed to be
accurate to something like 15', but I had to move about 100' east from
the brass strip before it thought it was at logitude 0 degrees exactly.
I also have doubts about the vertical elevation it reports, though
that's not an issue for trains.

Andy wrote:
I understood it was because the GPS system, which controls the
selective door opening, can't see any of its satellites when it is
under the building built on top of the platforms. The conductor has to
wait for the system to time out allowing it to be overriden. It only
really seems to affect the Southern platforms, the South-Eastern
platforms (1-7) generally seem to have normal opening times.

However Southeastern's 375s have similar problems at Cannon Street,
although they've been better of late, perhaps because the office
buildings above the station are being redeveloped so maybe the GPS
signals are getting through now?

--
Roy

On Fri, 23 Jan 2009, Recliner wrote:

"David A Stocks" wrote in message

"Graculus" wrote in message
...
(No, GPS isn't accurate enough to say if its on the up fast/down
slow/etc track.)
Should be - I get accuracy down to about a metre on a very old
hand-held GPS box.

How can you be sure that it's so accurate? I took my Garmin wrist GPS
to Greenwich Observatory, and was crestfallen to discover that my GPS
wasn't nearly as accurate as it claimed. As I recall, it claimed to be
accurate to something like 15', but I had to move about 100' east from
the brass strip before it thought it was at logitude 0 degrees exactly.

That might be because GPS is giving you a position on the WGS84 ellipsoid,
and the strip at Greenwich is zero longitude on some other datum (Airy
1830?), and the two don't coincide there.

Might be - i don't know the details of the Greenwich strip, or whether
ellipsoids are all defined so as to align at zero longitude, or whether
any difference might be 100 feet.

In any case, if GPS consistently puts the meridian exactly 100 feet east
of the strip, that would be accurate enough for trains, provided that the
coordinates they work with were as measured by GPS, rather than by eg
brass strips and Victorian gentlemen.

I also have doubts about the vertical elevation it reports, though
that's not an issue for trains.

Yes, i believe the altitude is typically much less accurate than the
horizontal position. Something to do with geometry.

tom

--
sh(1) was the first MOO

Tom Anderson wrote:

On Fri, 23 Jan 2009, Recliner wrote:

"David A Stocks" wrote:

(snip)

[...] - I get accuracy down to about a metre on a very old
hand-held GPS box.

How can you be sure that it's so accurate? I took my Garmin wrist GPS
to Greenwich Observatory, and was crestfallen to discover that my GPS
wasn't nearly as accurate as it claimed. As I recall, it claimed to be
accurate to something like 15', but I had to move about 100' east from
the brass strip before it thought it was at logitude 0 degrees exactly.

That might be because GPS is giving you a position on the WGS84 ellipsoid,
and the strip at Greenwich is zero longitude on some other datum (Airy
1830?), and the two don't coincide there.

Might be - i don't know the details of the Greenwich strip, or whether
ellipsoids are all defined so as to align at zero longitude, or whether
any difference might be 100 feet.

In any case, if GPS consistently puts the meridian exactly 100 feet east
of the strip, that would be accurate enough for trains, provided that the
coordinates they work with were as measured by GPS, rather than by eg
brass strips and Victorian gentlemen.


The brass strip dates from the 1970s, so no Victorian gentlemen were
directly involved in that endeavour, and what's more it has since been
replaced by a stainless steel strip.

But basically Tom's analysis is right - Airy's Prime Meridian has been
usurped by those goddamn yanks without so much as an International
Meridian Conference in a typical display of unilateralism and general
lack of gentlemanly behaviour. I speak of course of WGS 84, which
basically was a thorough overhaul of earlier World Geodetic Systems
that were developed by the US Department of Defense and forms the
foundations of the satellite-based Global Positioning System.

You can read more about all this in this entertaining article by Tom
Standage...
http://www.tomstandage.com/FEEDlongitude.html
....in which he notes that the de-facto 'new' prime meridian lies 334
feet to the east of that shown on the ground and runs through a tree
and a rubbish bin in Greenwich Park. He also ponders on an interesting
idea of there being a "meridian boulevard". I would however take issue
with his notion that there can be a "true dividing line between east
and west" - when it comes to longitude, there can be no "true" 0° -
where that meridian lies is always going to be arbitrary.


I also have doubts about the vertical elevation it reports, though
that's not an issue for trains.

Yes, i believe the altitude is typically much less accurate than the
horizontal position. Something to do with geometry.


Past discussions on here that turned to GPS left me in slight
wonderment at the understanding you and others had about it all... and
then you go and ruin it all by casually saying it's "something to do
with geometry"! Splendid ;-)

By the by, I've never owned a GPS device - what fun and games am I
missing out on?

In uk.transport.london message
, Fri, 23 Jan 2009 20:57:06, Recliner
posted:
"David A Stocks" wrote in message

"Graculus" wrote in message
...
(No, GPS isn't accurate enough to say if its on the up fast/down
slow/etc track.)
Should be - I get accuracy down to about a metre on a very old
hand-held GPS box.

How can you be sure that it's so accurate? I took my Garmin wrist GPS
to Greenwich Observatory, and was crestfallen to discover that my GPS
wasn't nearly as accurate as it claimed. As I recall, it claimed to be
accurate to something like 15', but I had to move about 100' east from
the brass strip before it thought it was at logitude 0 degrees exactly.
I also have doubts about the vertical elevation it reports, though
that's not an issue for trains.

See
http://www.absoluteastronomy.com/topics/Royal_Observatory,_Greenwich
and http://en.wikipedia.org/wiki/Prime_Meridian.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

"Tom Anderson" wrote in message

Might be - i don't know the details of the Greenwich strip, or whether
ellipsoids are all defined so as to align at zero longitude, or whether
any difference might be 100 feet.

If Google Earth is anything to go by, the WGS84 zero long position at
Greenwich is about 100 metres east of the Transit observatory position.