Active 10 OS Mapping Anomaly

[TimO]

I've noticed on my commute, from Croydon to South Ken, that my position on the 1:50000 mapping of London is often off by a noticeable amount.  Looking at cycling along Chelsea Bridge for example, the position I see on the screen has the track at something like the position of the red line on this image (not quite the same mapping, but fairly close) ie about a roads width off:



I'd accept that this was due to the GPS being off a bit on a given day (Ionospheric conditions etc), but if I zoom in some more, so that the 1:50000 is replaced by the base map (I guess this happens because I don't have 1:25000 mapping installed), the track is pretty much on top of the road.

Similarly if I export the track as a GPX file, and import it into Bikely, it appears to be pretty much where I expect it to be, ie on most of the roads.

It looks like there is a bit of an offset of the mapping from where it should be, which is liveable with, but a little odd.  You do have to be fairly zoomed in to notice it.

I wondered if anyone else with an Active 10 had seen anything similar?

[andrew_s]

OS 1:50,000 and 1:25,000 isn't always spot on. Things are moved so that they aren't drawn on top of each other and you can see both.
A single carriageway A road (1 wideish lane each way) would normally be about 7m wide, but it is drawn at about 40m wide (which is about the width of a 3-lane motorway), so a building adjacent to the road will get pushed over by 15m or so. Add in other nearby roads, railways, rivers, buildings etc, and you can find that the map is up to 50m off.

To get a map where everything is the true size/scale, you've got to go to OS MasterMap mapping (digital vector, but the successor to 1:2,500 and 1:1,250 scale paper maps). Licencing costs will put this outside the range of the private individual.

[TimO]

It's not so much that things are not quite on top, it's that there is a distinct offset, on the 1:50000 mapping, which isn't on the base map (vector map I think), or Google Maps (via Bikely).  I noticed yesterday on the way home, that cycling along some London streets, where there are a bunch of parallel roads, the GPS in  effect said I was on a road adjacent to the one I was on.  It was fairly obvious, since I know the road, and the one I was actually on has a turning that the one the GPS said I was on doesn't, but conceivably you could come a bit unstuck if you didn't know the area.  Of course, the Active 10 isn't really meant for use in an Urban environment, and is more at home in the open countryside, where this would probably be less of an issue.

[andrew_s]

It could be that a whole bunch of parallel roads have been pushed to one side, domino fashion.

If you don't get the same offset on different days, it's most likely multipath - signals reflecting off buildings.

If you consistently get a similar offset across a wide area, it's most likely a map projection/coordinate system setting that's not right somewhere.

[TimO]

If you consistently get a similar offset across a wide area, it's most likely a map projection/coordinate system setting that's not right somewhere.

That's my best guess, and it may well also be a local thing, it'll be interesting when I'm a bit further afield (next Friday hopefully) to see if it's the same.  Although that won't be much further.  It's a shame I didn't haven't it around a couple of weekends ago, when I was in Devon.

[Rob S]

My guess would also be that as has been pointed out it's best away from very tall built up areas.