Off-line GPS tracking


In the last years we could see successful applications of GPS technology in orienteering. The two main fields where efforts were made to use GPS are mapping and tracking of runners. The first application has shown some benefits but has limitations as well. Herein I would not like to deal with GPS aided mapping as it is a professional surveying task. But the availability of better-and-better affordable handheld GPS receivers and the cancellation of SA makes tracking feasible for a wider orienteering community. In this article I try to summarize my experiences of off-line track analysis of orienteers with low cost GPS receivers.

Off-line tracking

Off-line track analysis means that a GPS receiver is used just to log positions during an orienteering activity and after the session the recorded track can be analised with the help of a computer software in conjunction with map, other runners or time. Todays consumer products make such an analysis affordable for any individual or club. Even on-line tracking can be accomplished with off-the-shelf products such as Benefon ESC, Garmin Rino or Sony IPS-8000. But returning to our topic, first some words about the technology and than about what it can be good for.

Challenges for the receivers

After selective availability was disabled the accuracy of a consumer handheld GPS receiver in good reception conditions can reach 7-15 m without additional differential correction. It is acceptable for our purposes but the emphases should be put on "good reception conditions". Orienteering is mostly done in forests and terrains with different relief. Dense tree canopy cover (leaves) obstructs significantly the view of the sky (satellites). My experiments show that decidous forest is more unfavourable than coniferous forest. Rain can further worsen the situation. In continental terrains with moderate to high elevation differences the relief can cause difficulties in reception measurable to those of tree cover.
These facts rise the question: what kind of receivers function better in such heavy conditions? I have tested a unit with patch antenna (Garmin eTrex Venture) and one with quadrifilar helix antenna (Garmin GPS76).
Venture GPS76
eTrex Venture GPS76

I have mixed feelings. While the patch antenna unit provided more accurate and stable positions in open areas it lost reception more easily under tree cover. The quad helix antenna seems to be more sensitive (quick acquisition times, reception even in poor signal conditions) but the unit produced more scattered positions. Other difficulty was caused by the orientation sensitivity of the antennas. In practice the unit must be carried in a way which disturbs the runner as little as possible. A conceivable solution is using a battery carrier harness. The patch antenna should be oriented face-up to the sky which position can be achieved easily by holding the unit as a plate compass (planned normal handheld operation) but not a favourable orientation when planning to pick-a-back the unit. As the Venture is a relatively small unit I could carry it face-up on one shoulder (taped to the harness). This solution is not the most comfortable. The GPS76 works best in vertical position. With this orientation it is possible to fix the unit in the battery holder strap of the harness. That way provided maximal comfortability for carrying the unit and nearly optimal signal reception. If you experience very weak reception you can try attaching an external amplified antenna carried on headlamp band on your head.
GPS76 carried in battery harness

Logging a track

Handheld GPS units support track log recording in different modes. You can choose automatic, time or distance interval based method. I found out that the time interval method is most suitable for our purposes. This method provides enough data points when the interval is set properly. To have a detailed track I used 10 second interval. The initial position fix (locking satellites) takes for a while and I can recommend to do it in an open place while staying still. Track recording can be started before the beginning of the course at the start place. Most units log the recording times along with the positions. It can be used to identify points of the track (a kind of time coding). You can use a watch with splittime memory to mark interresting moments e.g. taking controls (or use any electronic punching). If you can start the course whenever you want than the easiest way is to synchronize your watch with the GPS provided time and start at an integral time moment. Later at the track analysis you can easily match your splittimes to the track points.

Distortion of maps

Before coming to the point of analysis an other issue must be mentioned. The Hungarian (and maybe in other countries) mapping practices lead to maps which are not easily insertable into any orthogonal coordinate system. This result is caused by analogous photocopying, disassembling and reassembling of map segments, etc. These distortions are problematic when we want to display the tracks on map.

Putting the track onto map

Orienteering maps rarely have grid info on them so the viable solution for georeferencing a map image is a calibration process based on a transformation. Consumer GPS mapping softwares offer affine or polynomial transformations. You have to specify corresponding point pairs of image points and real coordinates. In my experiments by reason of the aforementioned distortion I was unable to calibrate maps acceptably by the transformations offered by low-end softwares. Satisfactory solution could be gained by using professional CAD softwares with a so called rubber-sheeting transformation. This fact made me to develop my own track display and analysis software.

Analysing the track

To eliminate the calibration difficulties caused by map distortion with a simple solution, a section-wise similarity transformation can be applied to the track in my software. Any number of track points can be reinstated on the map and the trackpoints between two such points are transformed by similarity transformation. This method gives relatively well fitted tracks but the transformation parametres must be determined for every new track even on the same map. Track points for the calibration can be identified by time codes. After having the track got on the map let us see what kind of intriguity can be found in it.
Track adjusted at control points

First the recorded track on the map can cast light upon the real way of a runner. It has not much sense in case of a skilled orienteer but for beginners and youth it can be used to compare the real and the perceived route. A more interesting feature is having time stamps for the positions. This information is the real value of GPS tracking. Imagine it as having a lot of splittimes (much more than from electronic punching). Using the time data it is possible to analyse the variation of the runner's speed along the course. You can see the effect of the vegetation, climbing, underlay and orienteering task. You can compare bee-line speeds and pure running speeds in different conditions. These speed analyses can involve more runners to compare, and can be applied not just for orienteering but long distance running as well. From the timing data you can determine more accurately the time cost of mistakes.
A big mistake I made (in leg 1-2) at a training running without compass. After not spotting the path I made a big bearing mistake. The situation became clear after a reorientation at the boundary of the field. From the GPS track you can see the exact trace and the set-back in speed for the rest of the leg. The time cost of the mistake turns out to be 3:10!

To make such analysis to happen I completed the track display program with speed analysis functionality. The program can draw tracks color coded according to speed. In one display mode the track is drawn in gray scale. The other mode uses green and red shades. The parts with speeds lover than the average speed over the whole course colored with shades of red and parts above the average speed colored green. The different shades (brightnesses) as with the gray scale mode show speed values. The track can be explored interactively. Clicking on it you can read km times, selecting two track points you get the distance (bee-line and on-track) and time difference between them.
If the track drawing is animated in a suitable time scale than we get a good tool for the visualization of more runners' achievements in a single display (see OOrg - SI analyser uploaded with the tracking data from WOC 2001).

Final thoughts

You can try GPS off-line tracking at orienteering or long running trainings. You can get intresting information of your spatio-temporal activity out there. You can combine GPS track recording for example with heart rate monitoring/recording (e.g. with a Polar HR monitor) if you are interested in physiology.
Track from a terrain running training (approx. 7 km) with heart rate data.
This information can be useful in evaluating youth or beginners. For me who is not training at the higest level any more, GPS tracking is a motivating factor which colors the trainings. Maybe GPS tracking can be offered as a hightech gadgetry application in VIP classes to dazzle sponsors at big competitions.