[OSM-talk] Choice of GPS for bicycle use in London

[Andy Robinson]

Erik,

Your picture of Stockholm is a very good example. The "backs" in Manchester
are very similar and where inexperienced gps loggers ended up producing poor
tracks. With additional knowledge and experience things may have been
better.

A GPS receiver only works properly when it can see at least 4 satellites. If
it can see 3 then a horizontal fix with some level of accuracy is possible
but not ideal, even 4 does not generally get you to the 10m accuracy we
would all like for OSM. So really GPS tracks along these streets are not the
answer for mapping in OSM. ie the gathering platform and method is ill
formed in this instance. We should seek a better answer and get it on the
wiki so that the quality of OSM data is not brought into question in the
important central urban settings.

Incidentally I'm not so comfortable now about using out of copyright maps to
trace these urban jungles. The trail run I did for Manchester at the weekend
showed up the limitations of this in so much as the map may or may not have
been accurate in 1911 and is certainly less than ideal in 2006. Much better
to keep the data for OSM coming in from physically proven sources. I'll
still possible look to use old mapping for the centre of Birmingham but I
shall make sure that each bit I do trace into OSM that way I personally
verify beforehand.

Cheers,

Andy


Andy Robinson
Andy_J_Robinson at blueyonder.co.uk 

>-----Original Message-----
>From: talk-bounces at openstreetmap.org [mailto:talk-
>bounces at openstreetmap.org] On Behalf Of Erik Johansson
>Sent: 22 May 2006 15:59
>To: Talk Openstreetmap
>Subject: Re: [OSM-talk] Choice of GPS for bicycle use in London
>
>On 5/22/06, Nick Hill <nick at nickhill.co.uk> wrote:
>> There has been a lot of talk about the sirf star III chipset being so
>> much more useful. They can after all pick up GPS signals in situations
>> where other chipsets will not receive a signal.
>>
>> This is not always a good thing. If you are in a concrete canyon with a
>> sirfstar III chipset, the fact that it can pick up a GPS signal shows
>> the signals it is using are bouncing off buildings. It is receiving
>> multipath signals. Such multipath signals are bad for accuracy.
>
>Ah, I too have wonder about the accuracy, but I believe that it's not
>a big problem. It gets to get a good position even though I'm in
>canyons such as these:
>
>Old town in Stockholm, (Gamla Stan)
>http://holk.fonky.org/holksidor/057.html
>http://members.virtualtourist.com/m/p/m/293d85/
>
>Sure I don't get a good enough lock to know on what street I'm on, but
>the streets are ~20meters apart. So it's ok.
>
>/emj
>
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