Moving GPX Waypoints

September 12, 2015 1:25 AM

I'm currently working with a charity in the Caucasus. Yesterday I spent ten hours surveying and hiking in the mountains, recording waypoints using Viewranger on my phone. Today, I see I didn't change a setting (? I think?) and all of my waypoints are about 80km out. Is there an easy, free way to fix this?

All of the way points are correct in relation to each other, so the easiest would be if there was an app that allowed me to select them all, and drag them to where they should be, which I can work out from landmarks I waypointed. Or, calculate exactly how far away they are and in what direction, and correctly alter the co-ordinates (which are in degrees and minutes).

For the life of me, I cannot find software that will help me to do either of these things! Can you?

posted by sarahdal to Technology (7 answers total)

GPS Prune should do it, but it's a desktop Java app. It might be a datum shift, but I can't see any way that that sort of error could be added by your app.
posted by scruss at 4:30 AM on September 12, 2015

Response by poster: I literally have no idea how this has happened - I was in forest in between two small mobile phone antennae. Right now my app is working and displaying my position correctly, but for it to be wrong to an identical degree over the course of ten hours is baffling!
posted by sarahdal at 4:58 AM on September 12, 2015

Do you see the 80km shift in the app on your phone, or when you import the points to a computer?
Is the direction of the shift East West, North South, or a combination?
posted by DelusionsofGrandeur at 5:24 AM on September 12, 2015

Response by poster: The shift is a combination - I ended up manually editing the gpx file to add the difference between the actual points and what is in my GPS (0.31444 and 0.79383 to Long and Lat) and that has worked! still cannot work out how this has happened.
posted by sarahdal at 5:29 AM on September 12, 2015

If your phone had to rely on cell triangulation, it can get confused sometimes and report huge errors. Phone GPS units don't have room for a really sensitive antenna, so you might get no GPS lock in a forest. At best, you'll get 10-100 metres position uncertainty from a phone.

They're big, clunky and still quite pricey, but if you need locations other people will be relying on, handheld GPS units with a helical antenna will give you solid results. I have a Garmin 60CSx that's a decade old but still outperforms any phone when there's tree cover or urban canyon.
posted by scruss at 9:18 AM on September 12, 2015

My guess is the problem was the wrong coordinate system That's the most likely explanation for all the points being correct relative to each other but globally wrong. It's the only explanation if your phone was truly using GPS for location. OTOH most mobile apps are only using WGS84 for coordinates, so I'm a bit baffled how it could be wrong. Another possibility is if the phone was using wifi it's possible that the database has the wrong location for that wifi node.

Editing the GPX file is a good solution; I'd do the same. One tool to do that editing for you is ogr2ogr, particularly if you can identify a coordinate system mismatch as the source of the problem. But if you've not used it before I wouldn't call using it "easy", certainly not as easy as just editing the GPX.
posted by Nelson at 9:46 AM on September 12, 2015 [1 favorite]

Seconding Nelson that you might have your coordinate systems crossed. On a local scale, a linear shift is usually good enough as the errors will be too small to matter (unless you're building a high rise, launching a space ship, etc.)
posted by klanawa at 11:09 AM on September 12, 2015