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January 10, 2007

Uploading trip data to your GPS

Filed under: GPS,Howtos / tips / tricks — mtbguru @ 10:51 pm

In a previous article we talked about some tools to download data from your GPS unit and save into GPX files. Now we’d like to outline what you can do with GPX files you’ll find on MTBGuru (which aren’t necessarily yours).

For each public trip on MTBGuru that has GPS data, you can download the associated GPX file (using the ‘Download GPX file’ link in the sidebar) as well as a Google Earth (KML) file. By browsing around the map, using the search box or by clicking the ‘Recently Added Trips‘ link you may find a trip or ride that catches your interest. Say you want to do this ride yourself: the trip summary gives you an idea of what to expect in terms of distance and elevation gain and if there’s timing info you’ll know even better what you’re up for. You can now use the GPX file to send the data to your GPS unit, so you’ll be able to retrace the given route.

Some units allow a straight upload of the entire GPX file with its waypoints and tracks. In some cases though, there are complications:

  • For instance, on some Garmin units (e.g. the Etrex family) the size of tracks you can upload is limited to 500 points (even though they can *record* tracks of much larger size) – larger tracks will be truncated
  • Some units don’t allow upload of tracks, only waypoints and/or routes

A quick GPS terminology 101 may be in place here:
A ‘route’ is a sequence of waypoints that indicates a certain route. A ‘waypoint’ is basically a set of three numbers (latitude, longitude, elevation) indicating a point in space, usually on the surface of the planet ;) . A track is a sequence of (recorded) waypoints but generally also contains time information.

GPX files you download from public trips on MTBGuru contain tracks. The time info in this file is stripped but all spatial information (long,lat,ele) from the trip is available. Only trip owners have access to the time info in the GPX – using the ‘My GPS Files’ link in the sidebar they can always retrieve the original uploaded data.

As mentioned before, uploading these files directly to your unit can sometimes result in truncated data and other problems. However, you generally don’t need that many points to navigate and retrace a given trip. You can use a host of software tools (e.g. Garmin’s MapSource on PC, Routebuddy on Mac OS X) to create routes with a limited number of points from tracks and upload these instead.

GPSBabel filter

Our favorite tool, GPSBabel, has a very neat and simple way to achieve the same: by using its ‘filters’ you can automatically reduce the amount of points in a GPX file. Select your GPX file as input file and set up your filter as shown in the screenshot (using distance between the points as filter); the output file will be a GPX file with a reduced number of points. You can change the amount of points by changing the merge distance between points.

3 Responses to “Uploading trip data to your GPS”

  1. ragetty Says:

    i don’t like this filtering method by GPSBabel – simply filtering over distance can mean rounding of or cutting corners. an ideal filter would retain sufficient information in (even gentle) curves and chicanes and lose detail in the straights appropriately.

    erasing information indiscriminantly might mean losing the ability to recognise the proper direction at forks in the trail, etc.. it can also mean losing vertical detail in the route.

    TrailRunner for the Mac does an excellent job here, retaining both vertical detail and differentiating between cureves, corners and straights.

  2. mtbguru Says:

    Thanks for the tip, I’ll need to check out Trailrunner… and it even seems to be freeware.
    I would think though, that retracing/navigating a given route after doing the GPSBabel filtering on it (reducing the track to say 500 equidistant points) should be fairly easy, unless the route is extremely long and the distance between the points very large.
    For logging the track or if you’re interested in elevation profiles that filtering is probably not a good practice as you say (more adaptive filtering would be in place then).

  3. ragetty Says:

    TrailRunner is donationware, and is worth every penny. it looks like an app for runners, but actually it’s main function is for networking your gps routes with anybody else’s you can get hold of, so works great for bikers, etc. too. functions include route repair, assorted map downloads, new route creation, blah blah blah …

    the learning curve is a little steep at first, principally because it’s scope is not immediately obvious, and there are a few tricks to getting the best (geekiest) results, but it’s well worth the effort.

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