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.
It is also possible to include some explicit waypoints as well if you want. You could, for example, use an explicit waypoint for synchronization, but then identify points of interest vocally.
Before you start
- 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.
- Start your voice recorder and obtain a fix with your GPS.
- 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.
- 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!".
- 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
- 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.
- Open your GPX file. This will create a GPX layer showing the track.
- Import Audio 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 single audio marker labelled 'start'.
- 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 you had some explicit waypoints in your GPX file, you may see these as well or instead of 'start'. Choose how audio markers are created using 'When importing audio, make markers from...' in Audio Preferences.
- Synchronize your sound track to the GPS data. Though there are two ways of doing this, for this kind of audio mapping finding your synchronization cue on the sound recording first will be the easiest method.
- Make the map using your commentary. You will want to drag the orange play head arrow around track, 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 aren't using the markers much, and they are in the way, you can turn them off (once you have synchronized) using Show/Hide Text/Icons on the context menu (right mouse button) on the Marker Layer; the play head will remain visible.
- If you start finding play back isn't happening where you expect:
- you may not have calibrated correctly
- 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.
- you may need to re-synchronize at a later point (perhaps you paused while making the recording). Re-synchronization only affects later markers, so the earlier portion will not change.
- You may find it convenient to add additional audio markers along the track to give you a 'play' button directly at that location. To do this Make Audio Marker At Play Head on the Marker Layer's context (right mouse button) menu.