Garmin Foot Pod Calibration

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This is a small Windows utility that analyzes a run and calculates the appropriate recalibration for your Garmin Foot Pod. This software was tested with a Garmin 310XT and 210, but should work with any Garmin that works with the footpod.

Note, this is open source and released under GPL.

1 How does it work?

The Garmin devices record your position along with other data such as heart rate every few seconds. If the footpod is active, then the speed according to the footpod is also recorded. By looking at each position that is recorded, you can work out how far you've traveled and how long it took. That should be the same as the foodpod speed if the GPS is accurate and footpod is calibrated.

1.1 Is this different to a Garmin calibration run?

This is using the same principle as the Garmin calibration run, where you tell the Garmin to calibrate and then run about 1 Kilometer. The calibration run is comparing the GPS distance covered with the footpod reported speed. The advantage of this utility is that you can use any run to check the calibration and you can run much further.

1.2 Flaws in the approach

The biggest flaw in the approach is that it relies on the GPS signal, and GPS is not that accurate itself. If you do not have a GPS signal, such as a treadmill run, then there is no calibration possible and this utility will show that none is needed. That is because the Garmin recorded data is based on the footpod, so it's calibrating the footpod against itself.

1.3 Getting the best data

The best run to use for calibration is a run that has the most accurate GPS data. That will be a run that is reasonably straight, as twists and turns tend to reduce the GPS accuracy. A good view of a clear sky can also help a little.

1.4 Is there a better way?

A better approach would be to run a known distance, either on a track or using a known course and calibrate against that known value. This utility does not do that yet, but it is a planned upgrade. It will require moving to a nicer GUI as well.

2 Installation

There is no further installation ;}

3 Gathering the data

This utility does not require a special calibration run to gather data. Any run with your Garmin 310XT and Foot Pod enabled will provide the data needed. Running at a steady pace will provide good data. If you take walking breaks, this will tend to produce a lot of variability.

4 Garmin Training Center

  • Transfer the record of the run to Garmin Training Center.
  • Export the record of the run from Garmin Training Center as a 'TCX' file.
  • Run the executable ParseTCX.exe passing the full path to the TCX file as a parameter.
    • Open a windows explorer window and navigate to the folder with the TCX file.
    • Right click on the TCX file and select "Open With…" then "Choose Program…"
    • Click on "Browse…" and navigate to where you downloaded ParseTCX.exe from.
    • I would recommend checking the box “Always use the selected program to open this kind of file" so that you can just double click on the TCX file in the future.
    • Click OK
  • A command window should open and print out a lot of data. At the end you should have a line that says "Multiply your current calibration factor by N" where N is the scaling factor.
  • On your Garmin, multiply your current calibration factor by this new scale. So if your scale is currently 990 and the calibration factor is 0.94255403, the new scale will be 933.

5 Garmin Ant Agent

  • Transfer the record of the run using the Ant Agent.
  • Run the executable ParseTCX.exe with no parameters. The utility will try to find the store of TCX files that the Ant Agent uses.
    • On XP this is under C:\Documents and Settings\<username>\Application Data\GARMIN\Devices\<device number>\History".
    • I have not tested this on Windows 7 - let me know if it works okay ;}
  • A command window should open and print out a scaling factor for each file it finds.
  • On your Garmin, multiply your current calibration factor by one of the new scaling factors. So if your scale is currently 990 and the calibration factor is 0.94255403, the new scale will be 933.

6 Consistency

The utility looks at how consist the footpod and the GPS are (standard deviation). This is to give you a sense of how useful the scaling factor change would be. This consistency rating is provisional, and is based on the following scale.

  • stddev < 0.02 = "very good"
  • stddev < 0.05 = "good"
  • stddev < 0.075 = "okay"
  • stddev < 0.1 = "poor"
  • stddev < 0.2 = "very poor"
  • stddev >= 0.2 = "appalling"

Let me know if you have a better suggestion for the scale.

7 Example Output

7.1 All Ant Agent Files

2011-01-13-083545.TCX calibration factor adjustment 0.9998, Consistency is appalling (stddev 0.2378)
2011-01-14-085610.TCX calibration factor adjustment 0.9909, Consistency is very poor (stddev 0.1322)
2011-01-15-090651.TCX calibration factor adjustment 0.7886, Consistency is appalling (stddev 0.2636)
2011-01-16-072027.TCX calibration factor adjustment 0.9816, Consistency is okay (stddev 0.0671)
2011-01-17-083519.TCX calibration factor adjustment 0.9642, Consistency is poor (stddev 0.0992)
2011-01-18-080330.TCX No usable data was found in
2011-01-19-075748.TCX calibration factor adjustment 0.8927, Consistency is appalling (stddev 0.2764)
2011-01-20-083432.TCX No usable data was found in
Press return to contine

7.2 Single File

Per Lap Scaling
===============
lap average scale 0.9977
lap average scale 0.9954
lap average scale 0.9853
lap average scale 0.9836
lap average scale 0.9892

Details
=======
Average scale              0.9909, 99.0857%
Consistency                very poor
  Standard Deviation scale 0.1322, 13.2209%
  Scale+stddev             1.1231, 112.31%
  Scale-stddev             0.8587, 85.87%
Average scale (per lap)  0.9902, 99.0234%

Calibration Instructions
========================

Multiply your current calibration factor by 0.9909

Press return to contine

8 Support

As always Contact me if you have a problem and I will try to help. Emailing me the TCX file helps debug any issues. You can also download the source code from http://fellrnr.com//Utilities/CalibrateFootPod/ParseTCX.zip

9 Change log

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