A GPX tracklog consists of a lot of sections of code that look something this - a trackpoint.
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<trkpt lat="-43.502053000" lon="172.576317000">
<ele>16.480000</ele>
<time>2008-05-06T08:37:46Z</time>
</trkpt>
A trackpoint contains two key pieces of information - the time (in UTC - the Z after the time refers to this), and the location in latitude and longitude. A whole pile of these trackpoints are then added together to produce a tracklog. This of course presents a privacy risk as anyone that has access to the tracklog might be able to assume that the person that uploaded the tracklog was at that location at the time specified. And with GPS, this can be recorded to a high level of accuracy.
So, what we need to do is look at ways to protect some of this information. I'll write here about two techniques that I have used to protect information in tracklogs by editing them before uploading them to public websites. For most public websites, the most important information is location, and time is less important. So we need to take a two-pronged approach to tracklog privacy protection.
1. Delete track points that we might have privacy concerns with.
2. Remove timestamps that we don't want people knowing the time we were there.
Deleting Trackpoints
1. Protecting it manually. I have been using the free GPSTrackMaker to load and edit tracklogs before uploading them to OSM. This is a manual and sometime laborious process. I use this to remove any trackpoints around the final locations of puzzles/multi-caches that I have visited, and also to remove trackpoints close to home/home/friends etc. I also use it to touch up the tracklogs such as those areas that spray trackpoints around a wide area that don't mean anything - such as in urban canyons in Wellington. This can result in quite a 'rich' tracklog, especially if you delete those areas where the trackpoints are not that accurate due to GPS signal error.
2. Automating deletion of trackpoints. There are also a number of locations that one may always want to remove from a tracklog before making it publicly available. Locations such as home and work spring to mind. I was looking for a way to automate the removal of these locations using GPSBabel. Using nothing more than co-ordinates near your home and a radius, you can easily set up a filter to remove all points that fall with the circle using the following GPSBabel command. Note that the following command is needlessly complex as a little workaround is required to use the radius filter on trackpoints (you have to convert tracks to waypoints, do the radius filter on waypoints, and then convert the waypoints back to tracks - ugly but it works).
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gpsbabel -t -i gpx -f in.gpx -x transform,wpt=trk -x nuketypes,tracks -x radius,distance=0.3K,lat=-43.0,lon=172.5,exclude,nosort -x transform,trk=wpt -x nuketypes,waypoints -x track,pack,split=30m,title="LOG %Y%m%d" -o gpx -F out.gpx
It is possible to build a batch file that removes multiple locations such as home, work and friends, that requires very little input. Note that this process does not destroy the original tracklog that you keep, rather it creates a new tracklog with the sensitive data removed.
Removing Timestamps
For whatever reason, it makes some sense to also remove timestamp information from tracklogs - I won't go into the reasons here. Here is a little unix script that I use to change the timestamp information. Usually I don't mind people knowing what day I was somewhere, but I'm not that keen on them always knowing the time. So, I will remove either minutes/seconds, or minutes/seconds/hours as have every timestamp appear as midnight.
If you want to set it so that all times are set to the start of the hour e.g. hh:00:00, use this.
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#!/bin/sh
for f in *.gpx; do
sed 's/:[0-9][0-9]:[0-9][0-9]Z/:00:00Z/g' < $f > ${f%.gpx}-clean.gpx
done
If you want to set it so that all times are set to midnight e.g. 00:00:00, use this.
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#!/bin/sh
for f in *.gpx; do
sed 's/T[0-9][0-9]:[0-9][0-9]:[0-9][0-9]Z/T00:00:00Z/g' < $f > ${f%.gpx}-clean.gpx
done
Naturally, this isn't the easiest to do, but it is getting easier. It would be great if someone was able to write a tool/webpage that was able to do this sort of cleaning of tracklog data before uploading it to public websites.
Cheers Gav
