Uploaded My First Trace - Am I Expecting Too Much?

Joined OSM 10 days ago and, after much reading of the wiki, went for a short walk near where I live to start the process of making my first trace. I have been disappointed with the results and wonder where I have gone wrong.

My GPS log was created with a TomTom Bluetooth GPS feeding an old HP iPAQ running VisualGPSce. The log file was converted to the OSM format at www.gpsvisualizer.com and uploaded to OSM without error. Editing OSM and displaying my trace, places it in approximately the right area but far enough away from where I actually walked to make it seem useless for mapping. I need help working out where I went wrong!!

You can see my work area at the UK postcode CH60 9LJ. My walk was out and back along Pipers Close (already mapped), Pipers End (unmapped yet) to the Postbox (point 507070655) in Pipers Lane (already mapped). I walked out and back keeping on the left side of the roads.

A couple of things with GPS units:

They vary in accuracy by brand and model.

Units that are 10 years old or more have less accuracy.

The more sattelites the unit is receiving, the more accurate.
(Trees and buildings can block signals. So can any metal above the GPS.)

Need to make a few traces along the path, preferably at different times of the day and/or different days. This will give you readings from different sattelites from different angles, and then average your traces.

If you bring up previously loaded traces from others, you will see quite often the traces are “wobbly” but if there are enough traces, you can see a fairly accurate line from the average of all the traces.

As for the software side, I can’t help you there.

AFAIK the bulk of the Wirral has been mapped from Yahoo aerial photography. It is entirely possible that there is a systematic error in the registration of this imagery, although a look at nearby traces suggests that the bias if anything to the yahoo images being too far S. Good quality GPS traces will really help improve the quality for this area.

To me your trace looks OK: dimensionally the distances seem similar and in most places the tracks are reasonably close. However, it does not seem to be in the right place (roughly 30 metres off). There’s a bit of drift on one leg.

I use a Garmin and I usually monitor its accuracy before doing any serious tracing. I find I get significantly better traces holding it level in front of me rather than having it in a pocket or even dangling round my neck. Its best to give the device 4-5 minutes whilst stationary to get a good lock on the satellites. Its also useful to waypoint certain objects several times to get an idea of accuracy/drift (e.g, the Post Box). Also when walking I usually try to walk down one side of a street and back-up the other side: with good traces it’s much easier to draw with the editor and if there’s a glitch its usually obvious.

Try in another location, perhaps with less tree cover: the combination of time of day etc may have affected accuracy.

Last thought: the GPS gadget doesn’t have a lock-on road option does it? This can affect how data are logged.

All the best.


SK53 said:

That is what I would like to contribute, although at present I am limited to adding detail/corrections that are only apparent ‘on the ground’

I put my GPS unit in the top flap pocket of my rucksack or, if I don’t have the rucksack, under my shirt on my shoulder! I did walk out and back on opposite sides of the road for my first trace. I think I need to repeat the trace several times to see what works and what doesn’t.

The software I use reports the PDOP value and I recently found out what it is and its significance as an indicator of quality of fix.

VisualGPSce does not have this option as it knows nothing about roads or anything else. When I occasionally use the TomTom software it puts the current position on a road but when you record the current GPS position as a POI, the POI indicator is often some way away from the current position icon. I infer from this that the POI records the unadjusted GPS position in the OV2 file from which I can extract the recorded location.

Could your GPS be configured to report positions on the wrong datum? If it has a datum option, set it to WGS84, which is the datum used by OSM.

I am using VisualGPSce <http://www.visualgps.net/VisualGPSce/default.htm>. There does not appear to be a datum option in the software.

I don’t know your hardware/software, but with state of the art receivers (sirfstar III or similar) the
precision of your track points can reach from 3 meters (free sight to the sky) to 10 meters
(dense forest) or even 30 meters and more (inner city with high buildings around).

It’s usefull to have units with a EPE (estimated precision) value display. If your offset is always
to same direction then you are probably using the wrong map-datum.


Took delivery yesterday of a GlobalSat BT-368i. Unfortunately it is faulty (won’t charge) but did operate when the USB cable was plugged in. Left it sitting on my window sill facing West through double glazed windows. It tracked up to 12 satellites using up to 7 of them. Logging the position gave a wandering trace (plus or minus 6 metres) centred correctly when displayed using JOSM with OSM mapping. This looks much better! Just need to get my RMA agreed so that I can send it back for a replacement!!

To test your old satnav you could use GPSBabel to convert your GPX file into a KML file and view it in cough Google Earth cough. You’ll have a better idea then.

I’ve also found with mine (a Nokia N79) that trees will cause the trace to wander and keeping it in your pocket is hopeless.