While going through the rest of my GPS traces from the HOT exploratory mission in Senegal [some write-ups on the HOT blog], I had found one GPS trace that has 2 problems that I’ve occassionally experienced while collecting gps traces for OpenStreetMap.
– The GPS signal is temporarily lost and the distance between 2 points of your trace is abnormally lengthy,
and cuts across roads and features.
An example: Did I time-travel across that area ? Nope.
– And what I call the ‘scribble effect’ (is there another name for it?), where I accidentally forget to turn off the gps after I arrive at my destination and end up with multiple points near each other, looking like a child scribbled in a coloring book.
If you’re planning to make any visualizations with your traces, these two symptoms allow viewers to misinterpret where you’ve been and make your visualizations inaccurate.
The scribble effect also negatively affects OpenStreetMap because GPS traces that are uploaded in the future in the same area as your scribbled GPS trace, other users will be able to not be discern the newer gps trace from your scribbled gps trace. There’s also still a few places on earth (including some regions of Senegal) that don’t have any satellite imagery to (legally) use for drawing ways for OSM.
So, after you complete a trace, how do you eliminate these symptoms described above?
I’ve tried a combination of different filters with gpsbabel – (explained more here)
but have yet to find a single command that would solve both issues.
gpsbabel -t -i gpx -f gpx_louga_03062012-withmay.gpx -x track,pack,sdistance=0.6k -o gpx -F test7.gpx
– this had fixed the temporary signal loss … but it didn’t fix the scribble effect.
gpsbabel -t -i gpx -f gpx_louga_03062012-withway.gpx -x discard,fixnone,fixunknown -o gpx -F test.gpx
(resulted in a blank GPX File.
gpsbabel -t -i gpx -f gpx_louga_03062012-withway.gpx -x discard,fixnone -o gpx -F test.gpx
gpsbabel -t -i gpx -f gpx_louga_03062012-withway.gpx -x position,distance=3m -o gpx -F test3.gpx
(this removed the long points between the 2 segments, but didn’t solve the scribble effect)
gpsbabel -t -i gpx -f gpx_louga_03062012-withway.gpx -x discard,hdop=5,hdop -o gpx -F test6.gpx
gpsbabel -t -i gpx -f gpx_louga_03062012-withway.gpx -x position,distance=10m -o gpx -F test8.gpx
– removed 1100 of the trace’s 2400 points and the , but still left me with the scribble effect.
I’ll continue to explore cleaning up the GPS traces with GPSbabel filters and hope to find a way but if you know of a specific filter in gpsbabel or any other way to do this, besides manually deleting the offending points in gpsprune, I’d appreciate it.
Update: Dec. 1, 2012. I also shared this in an OpenStreetMap Diary Entry in Sept. Other OSM users have also experienced this problem and hadn’t found a solution besides what I’ve described above. So, I’ve found that the test3 example above and manually removing the points in gpsprune (from the scribble effect) is the easiest way to clean up your GPS points.