[OSM-talk] Strange GPS trace - drifting?

[Nick Hill]

I have noticed this occasionally to a few tens of feet. Yours seem extreme.

Is the area on a northern slope of a hill? I ask this as the mis-alignment is 
mainly N-S. E-W alignment seems OK. Satellites high overhead on an east-west 
plane could give enough information for your GPS to track your longitude 
accurately, but not your latitude. Many satellites are on the southern horizon.

Other important factors include how your GPS was mounted. If it is on a lanyard 
around your neck, it will at best ever see half the available satellites. 
Usually much less. Situation only a little better if on the dashboard of the 
car. When you turn a corner, a different set of satellites come into view, which 
your receiver will need to train on. Takes time, and in that time, your GPS can 
loose track where you are.

If driving around, I mount it on the roof of my car with blu-tack and a back-up 
lanyard around the aerial. This way, it can nearly always maintain a fix on 
enough satellites to give reasonable accuracy. As you turn an obstructed corner, 
it is more likely to 'see' both new and old satellites for long enough to 
maintain a fix.






Barry Crabtree wrote:
> Hi
> 
> I've been out mapping round a local residential area. Everything seemed 
> ok plenty of satellites in view, but when I look at the trace it seems 
> that over the space of a few minutes it has shifted what looks like 
> about 50/100 metres. I've put a screenshot of the trace at 
> http://wiki.openstreetmap.org/index.php/Image:Grangefarm-wierd.jpg but 
> haven't uploaded the gpx file because it doesn't look right!
> 
> Any thoughts on what might have happened. I've been collecting traces 
> for a month & this is the first time I've seen anything like this. Have 
> I done something stupid?
> 
> Cheers. Baz.
> 
> 
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