GPS Settings

The speed is calculated from the distance between successive locations in the track. This distance is calculated along the great-circle on the WGS-84 ellipsoid connecting the two points, that means it takes a straight line between points. This is not a problem for the Lelystad-Enkhuizen-Lelystad race, but if there are many twists and turns in the route, you must ensure that there are enough points in the track-log.

Another reason to keep enough points in the track is that the speed is shown for the duration between two locations. If this is too long, you’ll miss the acceleration, and a long time with the same speed displayed gives an impression that nothing is really happening.

Settings for Garmin GPS devices

These are the settings I use on my Garmin Venture Cx with an up-to-date firmware version. The current equivalent would be the Legend HCx. Other Garmin models probably can use a similar setting, by may have the menu items in slightly different locations.

Go to the ‘Tracks’ page and choose ‘Setup’.
On the ‘Record Method’ popup menu select the ‘Time’ option.
Choose an appropriate interval, I would suggest a value of 1 to 10 seconds, depending on the expected duration of the race, and the number of twists and turns in the track. The maximum number of track-points is 10,000 on most GPS devices, so a 1 second interval can record a two and a half hour race.
A way around this problem, and at the same time a clean way to obtain a suitable GPX file is to go to the ‘Data Card Setup’ from the previous ‘Setup’ menu. Here you can turn on the option to “Log Track to Data Card”. This will record the track log to the µ-SD card you have in the device, and will possibly allow you to record beyond the 10,000 point limit.
Do not forget to turn the track log on on the main ‘Tracks’ page.

Other recording methods can be used as well, for instance a displacement of at most 25 metres, or a “smart” method with the shortest possible interval. Whatever recording method you choose, make sure that your GPS device is recording for some time before starting the video recording. It takes some time to achieve maximum performance of the GPS.

Getting the data off de GPS device

Again, this applied to my Garmin Venture Cx, but will be similar on other devices.

Go to the main menu by pressing the menu button twice.
Go to the ‘Setup’ menu.
Go to the ‘Interface’ page.
Make sure you are connected to the computer with a suitable USB cable. If you experience any problems with the connection, make sure the GPS device is connected directly to your compter, and not through a USB hub.
Select ‘USB Mass Storage’.
The µ-SD card in your GPS device should now mount on the desktop, allowing you to copy the GPX file off the device.
Once you unmount the device, the GPS will restart, and resume normal operation.

The code for the distance between two points was originally written by Chris Veness, based on work by T. Vincenty.