Audio Mapping: Continuous audio with vocally-identified points of interest

This method of audio mapping uses the time into a continuous audio recording to find the corresponding time along the GPS track to identify the position of a feature. The content of the recording provides details about the feature. Synchronization and calibration of the sound track are critical because the clock in the voice recorder is being used indirectly as a measure of distance.

Procedure:

Before you start

1. Calibrate your voice recorder. Though a recorder may not be very accurate, it is unlikely to vary much, so you will probably only need to do this once.

While surveying

1. Start your voice recorder and obtain a fix with your GPS.
   • It is just a little easier to synchronize if you start the voice recorder first.

2. One you have a fix, start moving.
   • You need a synchronization point that is unambiguous in time, which you can't achieve while standing in one place.

3. As you pass a location that will be recognizable from the shape of the GPS track (e.g. an obvious junction) add an audio cue to the sound track that you can synchronize the sound track with later, for example "synchronization cue.... NOW!".

4. Do your surveying. Whenever you want to identify a location give a short cue ("MARK!") just before you dictate notes.
   • Street names don't really need the cue as they don't refer to a single location.
   • It will be easier to find your voice notes if you are consistent about where you record street names: just as you turn in to a street, for example.

On the computer

1. Extract your tracks from the GPS as a GPX file, and your sound track from the recorder as a WAV file.
   • JOSM (actually, Java's built-in audio facilities) doesn't recognise every variety of WAV file encodings. If you need to (JOSM will tell you), convert your recording to a suitable format using e.g. ​Audacity. 8,000 16-bit samples per second is a reasonable format.

2. Open your GPX file. This will create a GPX layer showing the track

3. Make Sampled Audio Layer using the context menu (right mouse button) for the GPX layer. This then asks for your WAV file. It should produce Marker Layer containing a series of markers at convenient intervals along the track, labelled according to the time into the track.
   • if you don't see labels with the Markers, check that Show/Hide Text/Icons on the context menu for the Marker Layer is not off.
   • if the markers are too dense or too far apart, you can vary the sampling rate using the two 'Min audio marker sample rate' options Audio Preferences. By default they are at least 75 metres and 15 seconds apart.

4. Locate your synchronization point near the beginning of the track. Unless you are very lucky, there won't be a Marker exactly at that point, so you need to add one...

5. Play the audio by clicking on the marker immediately before the synchronization point, ignoring what it is saying. When the orange play head arrow reaches the synchronization location, pause the Audio.
   • if you don't see an orange play head arrow moving along the track while you play audio, make sure the 'Display live audio trace' option in Audio Preferences is on.

6. Make Audio Marker At Play Head using the context menu (right mouse button) of the Marker Layer to add the extra point.

7. Start synchronizing your sound track to the GPS data by playing the audio from the synchronization point by clicking the icon you just added.

8. When you hear your synchronization cue ("NOW!"), pause the play back.
   • if you started the recorder after getting a GPS fix, you will need to jump backwards in the sound track. Don't click on an earlier marker to do this, as that would synchronize to the wrong marker.

9. Select Synchronize Audio on the context menu for the Marker Layer.
   • The play head should jump back to the marker, and when you play the marker you should hear your "NOW!" cue. Clicking on other markers should play the audio for that point along the track.

10. You may find it convenient to use the automatically generated markers to jump in to your sound track, or instead to use the jump forward, fast forward and slow forward audio controls to work your way through the sound track. Either way, when you hear your "MARK!" cues, pause the recording, and the play head will indicate the corresponding location and therefore where to add your feature to the map.
    • If you start finding play back isn't happening where you expect:
      1. you may not have calibrated correctly
      2. you may have been speaking too soon or too late while mapping: you can insert a lead-in time to compensate for this using 'Lead-in time' in Audio Preferences.
      3. you may need to re-synchronize (as per 7ff above) at a later point (perhaps you paused while making the recording). Re-synchronization only affects later markers, so earlier ones will not change.

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