[OSM-talk] Choice of GPS for bicycle use in London
Graham Wall
g.hamwall at gmail.com
Mon May 22 15:31:20 BST 2006
I had come to similar conculsions when I tried mapping the centre of
Bath. The city is surrounded by hills as well as having predominently
sandstone building amidst its narrow streets. Often a lock would be
reported, but upon examination the track would veer off course.
In places, I ended up dropping waypoints at street intersections and
adding the streets between them, the streets being fairly straight
between intersections. Doing this allows you to 'average' between
various waypoints - which is harder to do with tracks.
Graham
On 22/05/06, Andy Robinson <Andy_J_Robinson at blueyonder.co.uk> wrote:
> All of this discussion shows how import it is, for heavy built-up urban
> areas, that we (that is OSM) find a way for making some comparisons with a
> set of GPS units doing the same thing in the same location. The Mapchester
> weekend clearly demonstrated the limited capability of the current
> technology (pre sirf star III at least) in the narrow streets of central
> Manchester which are bounded by moderately high buildings. We don't know for
> sure whether bounce or the lack of sight was the major issue or whether both
> in tandem were enough to confuse the largely eTrex set of units (bearing in
> mind we had around a dozen of them in all). There is also the issue of user
> experience and perhaps better methods of filtering the track data.
>
> My personal belief is that we should look to map these difficult areas
> differently. Much more akin to traditional surveying where you pick your
> points and verify position rather than walking a track. I'll certainly be
> looking more to the latter for the centre of Birmingham as a small sample
> area previously done did not produce good results and was difficult to edit.
>
> As has already been pointed out, this problem affects a tiny proportion of
> locations, limited to the very heavy high rise urban landscape found in the
> very centre of major cities. Almost all other places are ok with even the
> basic entry level gps receivers. Best to say that for those wishing to map,
> especially on a limited budget, leave these complex city centres alone for
> now.
>
> 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 Nick Hill
> >Sent: 22 May 2006 13:43
> >To: Erik Johansson
> >Cc: Talk Openstreetmap
> >Subject: Re: [OSM-talk] Choice of GPS for bicycle use in London
> >
> >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.
> >
> >I say it is better to receive no signal rather than receive multipath
> >(unless you are stuck down a mountain cravass and need a rough fix).
> >
> >IMO, a receiver which is only sensitive enough to receive a line of
> >sight signal has advantages in urban areas. ie for OSM.
> >
> >I have seen tracks go way off course in my area, which I beleieve to
> >have been captured with a SirfStar III. My Geko, OTOH, rarely lays
> >off-course tracks.
> >
> >The on-screen stated accuracy of my Geko is normally between 15ft-23ft.
> >Actual lat/lon accuracy is better than that. It is nearly always clear
> >which side of a small urban road I am on, even on different days, so
> >long as I keep it still when I turn it on, until it gets a good fix.
> >Don't bother turning the Geko on whilst moving.
> >
> >My tests show Etrex Yellow=Geko 201.
> >
> >Difference is that Etrex is bigger, has bigger batteries so lasts longer
> >on a charge.
> >
> >I highly recommend the Geko. It is small, light, tends to lay accurate
> >GPX traces, and is not sensitive enough to receive most multipath. The
> >down side is battery life. In normal mode, with 900mAh Uniross
> >rechargeables, I get 9.5Hr. In battery save mode (some loss of accuracy)
> >I expect 14 hr.
> >
> >
> >
> >
> >
> >Erik Johansson wrote:
> >> On 5/22/06, Christian van den Bosch <cjb at cjb.ie> wrote:
> >>
> >>> Emil Vaughan wrote:
> >>> > Would a "usb mouse" type gps receiver
> >>> > connected to my zaurus be a more accurate than a geko? And would it
> >>> > work ok under linux?
> >>>
> >>> I haven't tested it yet, but I believe that a Sirf Star III should be
> >>> /much/ more accurate than a Garmin log (because it won't lie,
> >>
> >>
> >> Actually I'm not so sure if my GPS puck^W mouse is lying. It has just
> >> happend once:
> >>
> >>
> >http://www.openstreetmap.org/edit.html?lat=59.33833563680978&lon=18.0867943
> >7637329&zoom=16
> >>
> >>
> >> There you have a diagonal line that starts between "Kommendörsgatan"
> >> and "Karlavägen", and then drifts up north east. The only explanations
> >> I have is that it's extrapolating.
> >>
> >> This is done with a sirf III and a sirf II the sirf II just lost it's
> >> GPS lock. And the SIRF III PDOP is at 50 during that line.
> >>
> >> /emj
> >>
> >> _______________________________________________
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> >>
> >
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>
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