Comparing Positions with Time Lag
As blogged earlier I spent a week at the race office of the Patrouille des Glaciers (PdG). During the race, each PdG team of three athletes was equipped with a GPS device that kept broadcasting its position to the race office. Furthermore this device was equipped with an emergency button able to send a distress message along with its current position to the race office.
We wanted to know whether this security feature could be used for replaying the tracks of the individual teams. Replaying race tracks becomes interesting when you can compare teams. A team might ask: Where did we lose or win with regard to our peers? The start of the PdG, however, was staggered, i.e., there are several starts with a time lag between the starts of half an hour to an hour. Thus, comparing teams that competed in different time slots necessitates that we include a time shift compensation.
Initially, we analyzed the data from the first race (Tuesday/Wednesday). The teams follow a well signaled track (red line in figure below). As you can see the team positions reveal some astonishing errors. We suppose these positional errors are due to limited „visibility“ of GPS satellites in the mountaineous terrain as well as multipath GPS signals that introduce an error in time-of-flight measurement.
The GPS device is supposed to send its position every 2 minutes. However, this was clearly not always the case: In the example below we observed a gap of 40 minutes between positions – visible in below map where the points are located far apart.
Data Preparation for Track Replay
For displaying race tracks, we decided to show only points that are within a buffer of 1,000 meters rather than projecting the points onto the track from a too great distance and hence pretending a better accuracy.
GPS cannot measure elevation as precisely as position. Therefore we substituted the GPS elevations by elevations from a terrain model.
We did not interpolate intermediate points where there are time gaps. Therefore, not all the teams will move smoothly.
Track Replay Application
At EBP we then developed a proof-of-concept application Track Replay for replaying the 2016 PdG tracks. In real-time mode you can replay the Tuesday/Wednesday race the way it took place, i.e. including offset race starts for different teams. In compare mode all teams start at the same time in Zermatt and Arolla, respectively. In this mode, you can compare selected teams by ticking their respective check boxes on the left. Putting the cursor on a dot on the elevation profile identifies the team and highlights its position on the map.
The team list on the left is ordered by (current) rank. In theory, at the end of the replay the list order should correspond to the official ranking list Z1 and A1 of race result. However, this is not quite the case because our ranking is based on the distance on the track at a given time and the distance is derived from the GPS position projected onto the track. Since the quality of these positions is often questionable, the projected positions are also affected.
Thus, our proof of concept shows the idea of a track replay supporting a comparative mode. However, the capture of the positions with the tracking devices used in PdG 2016 is not yet quite suitable for this application. The great news is: An exciting and promising technology by race result that combines timing and tracking using active transponders will be available soon!
Please note that Track Replay is in the prototype phase. For best results (e.g., in order to display the dots on the elevation profile) we recommend to use the Firefox web browser. Track Replay will be online for a couple of days only. For more details concerning the Patrouille des Glaciers please have a look at the official PdG web site.
Are you interested in getting to know more? Feel free to contact me.