Bad Elf / MotionX technical notes

I got a Bad Elf GPS for my iPhone. Some notes on using it with MotionX to record tracks:

  • It seems to work better in airplane mode. First time I tried it, I never got < 50m accuracy, maybe it was still using wifi or 3G positioning? Airplane mode guarantees the only location fix is the Bad Elf GPS.
  • MotionX seems to be the best GPS tracking software. The UI is awfully non-Apple, but it seems to track well.
  • I consumed about 60% of my iPhone 3GS battery creating a 2.5 hour long track. GPS had a good view of half the sky (on the glareshield of the plane), airplane mode, doing nothing but running MotionX. I put it manually in Flying mode.
  • The 2.5 hour track was 260kb of GPX, 132kb of KML.
  • The MotionX overview of the track says it’s 191.8nm, 2:29:13 elapsed time, avg speed 77kts, max speed 152kts. My (known buggy) calculation on the AMOD log was 196.9nm, 2.3h elapsed, 1.9h in motion.
  • The Bad Elf + MotionX track looks right. I overlapped it with the AMOD log track and they were mostly on top of each other. Tracks diverged by as much as 16m. Spot check on one specific divergence and the AMOD said it has 10 satellites and HDOP of 1.1, a very accurate fix. Hmm.
  • GPX file contains a list of fixes: (lon, lat, ele, time). Once in motion it’s one fix every 2 or 3 seconds. The timing discrepancy doesn’t seem to just be a rounding error; I’ve got a sequence of 20:07:13 20:07:15 20:07:18 20:07:20 20:07:22 20:07:24 20:07:27.
  • KMZ archive contains a “doc.kml” whose main contents are a LineString with (lon, lat, ele). No timestamps. The KML is rendered without altitude by default in Google Earth but you can change the properties to show an extruded track, elevation profile, etc. It appears the elevation is clamped to 0′. It’s not DEM corrected, just a hard-coded 0?
  • KMZ archive also contains an extra file “raw.xml” which seems to contain raw data, not relevant to Google Earth. But lots of extra data from MotionX! It contains two sections. The XML has “locations”: a list of (time, activeTime, distance, stopped, lat, lon, alt). Time looks accurate to 1ms. activeTime and delta are just deltas from previous locations (it has both 2 and 3s activeTimes)  Stopped must be MotionX’s guess of whether we’re moving? The XML also has “recordingSnapshots”, which look to be 1 minute rollups of distance and altitude.
  • The email also contained a Google Maps link, which incorporates the same KMZ archive as was mailed to me (including raw.xml) served for 6 months from share.gps.motionxlive.com.
  • MotionX does not seem to record any measure of GPS fix accuracy. Too bad, that data is available in the Apple API. It may be smart enough to automatically filter out fixes with bad data.

I’ve been meaning to measure Bad Elf + GPS when the phone isn’t moving. My first attempt recorded no track, I think because it was clever enough to not record when not moving. Need to investigate Settings / Global: Accelerometrics Assisted GPS, Accuracy Filter.