On Sat, 12 Apr 2008, Mr Thant wrote:

On 12 Apr, 19:06, Paul Corfield wrote:

Thanks for the technical explanation - what was odd about the example I
witnessed was that we'd just crossed the Lea Valley reservoirs where
there are no tall buildings for miles and the bus is fully exposed to
the sky so had every chance to receive its signals over a longish
distance. *Still it was the first installation so perhaps it was just
one in a long series of bugs.

Might be that the programmed positions of the stops was off:
http://www.tfl.gov.uk/assets/downloa...tions_v00e.pdf

(long paper on TfL trying to figure out exactly where there bus stops
are)

Eight pages!

Summary:

- GPS doesn't work well enough in cities (31.53 metre 95% error)

- plotting on a map based on the textual description they already have can
work well, but depends on the description being adequate, which not all
are (11.48 metre 95% error)

- going round bus-stops with a map and marking their location on it can
work very well (they did this with electronic maps on laptops, rather than
a paper map), but needs the plotter to be computer-literate do so (6.45
metre 95% error, 28.22 if computer illiterate!)

- stops and routes (which there is already a database of, based on OS road
data) can be mutually validated by trying to place stops on routes, and
looking for various cases indicative of error - this bit is quite clever

- they're working on using GPS logs from the buses to further validate and
refine the coordinates

tom

--
London has a suburb for every emotion. -- Cliff Laine