Translating track files between mapping formats (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Sun, 14 Aug 2016

Translating track files between mapping formats

I use map tracks quite a bit. On my Android phone, I use OsmAnd, an excellent open-source mapping tool that can download map data generated from free OpenStreetMap, then display the maps offline, so I can use them in places where there's no cellphone signal (like nearly any hiking trail). At my computer, I never found a decent open-source mapping program, so I wrote my own, PyTopo, which downloads tiles from OpenStreetMap.

In OsmAnd, I record tracks from all my hikes, upload the GPX files, and view them in PyTopo. But it's nice to go the other way, too, and take tracks or waypoints from other people or from the web and view them in my own mapping programs, or use them to find them when hiking.

Translating between KML, KMZ and GPX

Both OsmAnd and PyTopo can show Garmin track files in the GPX format. PyTopo can also show KML and KMZ files, Google's more complicated mapping format, but OsmAnd can't. A lot of track files are distributed in Google formats, and I find I have to translate them fairly often -- for instance, lists of trails or lists of waypoints on a new hike I plan to do may be distributed as KML or KMZ.

The command-line gpsbabel program does a fine job translating KML to GPX. But I find its syntax hard to remember, so I wrote a shell alias:

kml2gpx () {
        gpsbabel -i kml -f $1 -o gpx -F $1:t:r.gpx
}
so I can just type kml2gpx file.kml and it will create a file.gpx for me.

More often, people distribute KMZ files, because they're smaller. They're just gzipped KML files, so the shell alias is only a little bit longer:

kmz2gpx () {
        kmlfile=/tmp/$1:t:r.kml 
        gunzip -c $1 > $kmlfile
        gpsbabel -i kml -f $kmlfile -o gpx -F $kmlfile:t:r.gpx
}

Of course, if you ever have a need to go from GPX to KML, you can reverse the gpsbabel arguments appropriately; and if you need KMZ, run gzip afterward.

UTM coordinates

A couple of people I know use a different format, called UTM, which stands for Universal Transverse Mercator, for waypoints, and there are some secret lists of interesting local features passed around in that format.

It's a strange system. Instead of using latitude and longitude like most world mapping coordinate systems, UTM breaks the world into 60 longitudinal zones. UTM coordinates don't usually specify their zone (at least, none of the ones I've been given ever have), so if someone gives you a UTM coordinate, you need to know what zone you're in before you can translate it to a latitude and longitude. Then a pair of UTM coordinates specifies easting, and northing which tells you where you are inside the zone. Wikipedia has a map of UTM zones.

Note that UTM isn't a file format: it's just a way of specifying two (really three, if you count the zone) coordinates. So if you're given a list of UTM coordinate pairs, gpsbabel doesn't have a ready-made way to translate them into a GPX file. Fortunately, it allows a "universal CSV" (comma separated values) format, where the first line specifies which field goes where. So you can define a UTM UniCSV format that looks like this:

name,utm_z,utm_e,utm_n,comment
Trailhead,13,0395145,3966291,Trailhead on Buckman Rd
Sierra Club TH,13,0396210,3966597,Alternate trailhead in the arroyo
then translate it like this:
gpsbabel -i unicsv -f filename.csv -o gpx -F filename.gpx
I (and all the UTM coordinates I've had to deal with) are in zone 13, so that's what I used for that example and I hardwired that into my alias, but if you're near a zone boundary, you'll need to figure out which zone to use for each coordinate.

I also know someone who tends to send me single UTM coordinate pairs, because that's what she has her Garmin configured to show her. For instance, "We'll be using the trailhead at 0395145 3966291". This happened often enough, and I got tired of looking up the UTM UniCSV format every time, that I made another shell function just for that.

utm2gpx () {
        unicsv=`mktemp /tmp/point-XXXXX.csv` 
        gpxfile=$unicsv:r.gpx 
        echo "name,utm_z,utm_e,utm_n,comment" >> $unicsv
        printf "Point,13,%s,%s,point" $1 $2 >> $unicsv
        gpsbabel -i unicsv -f $unicsv -o gpx -F $gpxfile
        echo Created $gpxfile
}
So I can say utm2gpx 0395145 3966291, pasting the two coordinates from her email, and get a nice GPX file that I can push to my phone.

What if all you have is a printed map, or a scan of an old map from the pre-digital days? That's part 2, which I'll post in a few days.

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[ 10:29 Aug 14, 2016    More mapping | permalink to this entry | comments ]
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