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

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.

--
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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.
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On Jan 22, 6:25*pm, MarkVarley - MVP
wrote:
I've often wondered and was reminded today, why is there a delay in
opening the doors for trains arriving at Victoria?
At other terminating stations the doors are released as soon as the
train has stopped, but at Victoria there seems to be a delay, anyone
know why?
--

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.

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