Audio Mapping: Calibration

Unless you are lucky, your voice recorder's clock is not as accurate as the atomic clocks used by the Global Positioning System. Therefore you need to be sure your recorder is accurate enough, and if it isn't to measure the difference and tell JOSM what that difference is. This process is called 'calibration'.

An error of 5 seconds per hour could mean you are 100m or more out after four hours surveying on a bike, and more in a car, if you are relying on the clock in the audio device to indicate position on a GPS track.

To calibrate, you need to enter a number close to but not exactly 1.0 in the calibration setting of JOSM's Audio Preferences. This number is the ratio of the sound recorder's notion of how long a recording is to the accurate length. For example, if the audio recording of a 3 hour interval turns out to be 3 hours and 15 seconds long, your calibration number is (3 x 60 x 60 + 15)/(3 x 60 x 60), which works out at 1.00139 (five decimal places is ample). If your recorder's clock runs fast, the number will be slightly less than 1.0. If it is very much different from 1.0, throw away your voice recorder!


  1. Make a recording of a precise interval, of the same order of length as a surveying session, say one to four hours: say "NOW!" or some such at a known time, or in the UK you could record the Greenwich Time Signal (the "pips"; use an FM, not DAB, radio) and again at the end.
  1. Measure how long your recorder thinks this is. You can't do this by playing back the recording on the recorder and timing it, because that will be using the same clock for playback as it used to record. You need to take the digital file and determine this according by comparing the purported sampling rate to the actual sampling rate. You can do this in JOSM or more accurately using a sound editor:
  • Using a sound editor such as Audacity, open the clip and delete the portions before and after your two marks. The pips are easy to edit visually. Read the length from the duration display (in Audacity this runs along the top of the soundtrack waveform).
  • In JOSM,
    1. open a GPX file (any will do providing it is long enough),
    2. Import Audio on the GPX layer's context menu to make an audio marker layer,
    3. play the audio until you hear your starting cue,
    4. Make an Audio Marker at the play head
    5. repeat for the end point.
    6. look at the labels for the two points, which are in GPS time offset from the start of the track; the difference is the measured sound track duration.
  1. Divide the measured time by the real time and enter the result in Audio Preferences.
  1. Repeat for each mapping session until and unless you are sure the discrepancy is constant enough. Also, consider that the discrepancy may vary according to the sound quality settings on your recorder.

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Last modified 14 years ago Last modified on 2009-12-30T15:58:47+01:00