[OSM-talk] Filtering out errors from GPX file

[Simon Wood]

Hi all,
I was wondering how to filter out 'obviously wrong' points from a GPX track log, which was recorded in heavy tree coverage. The track log was mostly good, but had a few outlying (individual) points, which would have been hard to find by hand given 1s data for 1 1/2 hours.

As it took me a while to work out I thought I would document it here, as it might be of use to someone else. I did this by using GPSBabel to compute the speed for each point and then used awk to filter out (drop) points which exceeded a threshold.

$ gpsbabel -t -i gpx -f track.gpx -x track,speed -o unicsv -F temp.unicsv
$ head -n 1 temp.unicsv > new.unicsv
$ head -n -1 temp.unicsv | awk -F ',' '{if ($5<20) print $0;}' >> new.unicsv 
$ gpsbabel -t -i unicsv -f new.unicsv -x transform,trk=wpt -o gpx -F new.gpx

The awk script could be made more complicated (ie. calculate acceleration over several samples) to give an even better result.

The difference is clear:
$ ./wherewasi.py track.gpx --summary


		WhereWasI Output
		=================

	Filename=track.gpx
	Number of Track Segments	= 1
	---------------------------------------------
	Start Segment / Point 	= 0/0
	End Segment / Point 	= 0/5365
	Start Time            	= 07/03/2009 17:35:20
	End Time              	= 07/03/2009 19:04:51
	Number of Track Points	= 5366
	---------------------------------------------
	Total Time            	= 01:29
	Total Distance        	= 12.03 km
	Total Climb           	=  4761 m
	---------------------------------------------
	Average Speed         	=  8.06 km/hr
	Maximum Speed         	= 1239.35 km/hr
	---------------------------------------------


$ ./wherewasi.py new.gpx --summary


		WhereWasI Output
		=================

	Filename=new.gpx
	Number of Track Segments	= 1
	---------------------------------------------
	Start Segment / Point 	= 0/0
	End Segment / Point 	= 0/5316
	Start Time            	= 07/03/2009 17:35:20
	End Time              	= 07/03/2009 19:04:50
	Number of Track Points	= 5317
	---------------------------------------------
	Total Time            	= 01:29
	Total Distance        	=  9.86 km
	Total Climb           	=  3469 m
	---------------------------------------------
	Average Speed         	=  6.61 km/hr
	Maximum Speed         	= 66.82 km/hr
	---------------------------------------------

Cheers,
Mungewell.

PS. Wherewasi can be found here:
http://code.google.com/p/wherewasi/