The Points with red circles around it are marks where i've never been.
It's creating this false data sometimes when I am walking or standing still.
My question is: what is the cause of this false marks?
I thought that it had to do with either speed or the coverage of satellites.
The GPS receiver is using the NMEA protocol (see: http://home.mira.net/~gnb/gps/nmea.html#gpgsa) to log it's data.
I tried to figure out the problem by comparing good with bad data, but the amount of satellites on a bad mark were the same when being on a good mark.
In order to find the answer to the question I compared the bad marks with the good ones, but I cannot find an explanation.
It's nothing to do with the speed but geometry (is that what you meant by cover?) of the satellites that the GPS uses to calculate position.
With an open sky, the unit will use signals from satellites that are spread out, but there will be situations where the unit may have a restricted view and this affects the accuracy of the position calculated that the track log records. For example, the terrain may form corridors and limit the view to overhead satellites or if the unit is not sensitive enough, forest cover may block signals and limit the view to the horizon in one direction.
When comparing the NMEA outputs you should look at the Dilution of Precision (DOP) figures. The good marks have Horizontal DOP of 3.1, 1.2 and 2.6 which the references I have found rate as excellent. The bad marks have an HDOP of 13.8, 13.4 and 12.5 which only rate as fair and indicate the position is a very rough estimate and should be discarded.
Joined: Jan 14, 2005 Posts: 19636 Location: Blackpool , Lancs
Posted: Sun Dec 14, 2008 6:08 pm Post subject:
If you want to understand what the data you have recorded actually means I find one of the older Garmin manuals quite handy which can be downloaded as a pdf from This page. The explanation of the NMEA sentences can be found from page 18 (Para 4,2) onwards.
This manual is for a different device to the one you are using but the NMEA standard doesn't deviate between devices - you should be able to work out where Frabble got the HDOP figures from within the data you posted. Did you go under heavy tree cover at the points where the device gave you an AWOL position? - Mike
Joined: Dec 28, 2005 Posts: 2003 Location: Antrobus, Cheshire
Posted: Tue Dec 16, 2008 11:58 am Post subject:
On the first walk with my Colorado 300 (in the Derbyshire Peak District) - I was peturbed to find that one of the track log positions (and only one) placed me just off the coast of the Shetlands! The device had been left on in the rucsac whilst we enjoyed a light refreshment at The Cat and Fiddle (not really near any sea ) I guess it was caused by some form of electrical interference. My phone and GPS data logger were also on and in the same pocket and I have since observed some sort of interaction between GPS devices. I now walk with the Colorado, in it's case, clipped to my rucsac shoulder strap and the logger stashed away in one of the outside cargo nets. Don't seem to have had a similar experience since.
'Minor' deviations like you have can be caused by a wide range of conditions including atmospherics - you have to use a little bit of common sense when interpreting tracklogs and real-time fix data. My guess would be the same as Frabble - in this case trees and a poor sat geometry are the culprits. Rain, causing wet leaves) has a detrimental affect as well. _________________ Phil
I've used a Garmin Geko and a HP IPAQ with a SIRFIII chip. Both of them have recorded logs with 'rogue' points - especially in woodland or deep valleys (the Avagas gorge in Cyprus caused my IPAQ to reboot!). When I'm uploading logs I always edit them in GPSU to eliminate these.
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