[Openstreetmap] GPS track oddities

[Andy Robinson]

Just for info, I checked the location of your roundabout centre on Mapsource
which I have found to pretty reliable with respect to positioning in
relation to the Garmin GPS for which the maps are designed. It computed from
the map as N52.64531 E1.27638. Near as damn it the same on Google Earth.

The Latsat images are way off though. I don't think I have drawn a single
segment line (and it's been a lot over the last week or so) that has been
remotely related to the underlying map. I've travelled a lot of route where
Alex (Moriati) has travelled and out GPS plots are within the expected gps
errors. In fact we generally have very good correlation.

I assume the Lansat image frames are similar to aerial photo frames? These
need to be stretched to fit whatever projection/transformation is used so
may be that's where the errors comes from too. May not be anything related
to OSM at all.

Of course we could all be using the wrong datum :^) ...WGS 84 in the case of
OSM. If you ever find you are about 100m east of where you think you are you
can bet there is a translation to the wrong datum figuring somewhere (eg WGS
72).

Finally there is continental drift. We (Europe that is) are heading east
away from the USA by 2.5cm per year they say.... so how long will it be
before OSM needs to be "corrected" I wonder :)

The bottom line is that precision is all relative. Provided all the roads
meet the roundabout I hope everyone is happy. I'm sure that OSM will never
reach the level where it becomes a safety critical application.

Andy Robinson
Andy_J_Robinson at blueyonder.co.uk 

-----Original Message-----
From: openstreetmap-bounces at vr.ucl.ac.uk
[mailto:openstreetmap-bounces at vr.ucl.ac.uk] On Behalf Of Jonathan McDowell
Sent: 14 October 2005 18:13
To: openstreetmap at vr.ucl.ac.uk
Subject: [Openstreetmap] GPS track oddities

I've been trying to produce some tracks around Norwich, but have been
having problems with my tracks appearing out compared to the Landsat
maps. At first I thought this was due to my cheap USB GPS receiver, but
I tried with a Magellan hand held device as well with similar problems.

So I tried something else. I used Streetmap to look up co-ordinates of a
roundabout (52.645225 1.276237) and a bit further up that road
(52.645992 1.273559).

Looking at the points with Google Maps shows them as you'd expect:

http://maps.google.co.uk/maps?q=52.645225+1.276237&iwloc=A&hl=en
http://maps.google.co.uk/maps?q=52.645992+1.273559&iwloc=A&hl=en

So I created a GPX file with these 2 points in it:

<gpx version="1.0" creator="A quick Perl hack"
	xmlns="http://www.topografix.com/GPX/1/0">
<trk>
<trkseg>
<trkpt lat="52.645225" lon="1.276237">
<time>2005-10-14T17:40:02Z</time>
</trkpt>
<trkpt lat="52.645992" lon="1.273559">
<time>2005-10-14T17:41:02Z</time>
</trkpt>
</trkseg>
</trk>
</gpx>

and fed this into osm-editor. Tell it to pull the Landsat data and I get
an image like:

http://the.earth.li/~noodles/osm-wrong.png

(248k)

This seems to me that I'm not going insane and that there's something
wrong with either the LandSat data or the way osm-editor and OSM deal
with it. Can anyone shed any light on this?

J.

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