Track precision bug

[AmateurCartographer]

I am an avid contributor to OpenStreetMap.  In doing so, I often record traces when I’m driving, even in areas where the roads are already mapped.  Uploading traces can help in fine tuning road configurations and adjusting satellite overlays for advance mapping.

This has lead me to notice a quirk of the way Pocket Earth, and another iOS app, Galileo, lay down traces in some areas.  Over some roads, seems like it’s only on small local roads, the track points are laid down is an almost perfectly straight line, straighter than one would expect with the variability of GPS.  And then, on subsequent passes over the same road, the points are again laid down in the same perfectly straight line.  It almost seems like the points are snapping to the road.  This is a problem when I’m trying to fine tune the location of a road, and I need to make many passes over the same stretch of road to take an average of those tracks.  Large local roads, highways, don’t seem to have this problem though, it seems like it’s confined to local roads when I’m driving. Walking down the same roads doesn’t have this problem. 

There is another iOS app which doesn’t seem to have this problem, myTracks.  It always lays down an independent track when it is recording.  The points never look like they are snapping to some pre-defined line.

I’m baffled as to what might be causing this bug, and wonder if it might be rooted in how location data is supplied to the app from iOS.  Though I hope you might be able to solve it.  I like the way Pocket Earth handles multi-recording track files and would love to be able to use PE for all my tracking.

I’ve attached a photo of a track I recently recorded using PE v2.0.4 on an iPhone 5 running iOS 7.0.4.  It is a portion of recording where I made several passes over the same stretch of road.  On the right, there is the normal variability of GPS tracks, however, on the left, the tracks all seem to be snapped to the road.  I can send you the PocketEarth archive or gpx of this track if it would help in tracking down the bug.  Unfortunately the forum will not allow me to attach these file types.

Thanks for the great app.

[GeoMagik]

Pocket Earth doesn’t snap to roads nor any other modification of the GPS fixes. It may filter out samples though as determined by settings in Settings > GPS & Compass > GPS Track Recording. 

Perhaps you could try changing the GPS Accuracy setting to Highest or double checking the filtering settings. Often these movements are due to non-GPS Cellular fixes which Apple mixes in with the GPS fixes, and you can try to filter them out by lowering the minimum accuracy setting.

If that doesn’t help, are you using an external GPS unit?   Does this happen only when Pocket Earth is in the background, or also when its open on the screen?

Also you mention 2.0.4, I’d recommend updating to the latest 2.1 update as more than 50 bugs were corrected since 2.0.4 including one where PE could show a straight line during BG recording if you stop moving for 15 minutes then resume again without re-opening the app (a battery saving feature we didn’t realize Apple had added :/  The new update may *suggest* that you also download the latest maps data, but you will not be forced to.

[AmateurCartographer]

Thanks for the suggestions. I have done a bit more experimenting.

In my first post, I should have mentioned the settings I used:
GPS Accuracy: Highest
Required Accuracy: 10 m
Minimum Distance: 1m
Minimum Time: 1s
Using the built in GPS receiver (no external unit). I was recording the track with PE running on screen. Also experimented with wifi both on and off; made no difference.

I updated to the latest version of PE (2.1.1) and the latest iOS (7.1.2) and recorded some more tracks. I found the same tendancy for the track “snap” to a line on some streets when recording a track with two iPhone 5 at the same time. They independanly laid almost the exact same line. It is very repeatable, where it does occur, it does almost every time I record a track, day after day.

However, an iPad Air, also recording at the same time, recorded the track perfectly. The track showed the variation one would expect with independent overlapping GPS tracks, with none exactly overlapping another. I have found that the Air tends to record very smooth tracks compared to the iPhone 5. Perhaps the M7 chip provides some location assistance using accelerometers to smooth out some of the GPS readings.

[Author]

Posts