As a side project I've been hacking together a simple tracking system for my wayward dog using a cheap GPS / GPRS tracker.

I've successfully got it transmitting data to my server over GPRS but while the tracker display shows the correct LAT / LNG the data sent via. GPRS is out (by 8° LAT and -0.37° LNG) so I'm guessing I need to do some post processing?

Raw data


which has 4 fixes at 20s intervals which I've parsed out as;

[lat] => -4.026244 [lng] => 55.564382 [date] => 2013-10-15 16:53:18

[lat] => -4.026248 [lng] => 55.564386 [date] => 2013-10-15 16:53:38

[lat] => -4.026259 [lng] => 55.564371 [date] => 2013-10-15 16:53:58

[lat] => -4.026266 [lng] => 55.564364 [date] => 2013-10-15 16:54:18

  • GPS Visualizer will aid you gpsvisualizer.com/convert_input – Mapperz Oct 15 '13 at 13:40
  • You know these are DDMM.mmmm? And you have the lat/lon mixed up (I assume). Are you near Broadwood Loch? Or somewhere in Tanzania? The GPS values should be within a few hundred meters of true location in WGS84. – mkennedy Oct 15 '13 at 17:16
  • @mkennedy yes right on all counts I figured it out a few hours ago but had an 8 hour lock on answering my own question as a newbie. Definitely not Tanzania outside the window last time I checked (although we have a comparable rainfall at the moment) sloppy copy+pasting on my part. – chris Oct 15 '13 at 22:24

So the answer was staring me in the face, the coordinates weren't returned in decimal notation they're degrees + minutes so;

55.564382° is actually 55° 56.4382" which in decimal is 55 + (56.4382 / 60) = 55.94063667°

  • Hi Chris, can you please let me know how were you able to send data to the server from GPS tracker? Which tracker did you use? I am also looking for something similar. – applefreak Jun 9 '15 at 20:24

Fun with trilateration! You'll need to solve for X, Y, Z, and T. T is for the time it takes for the GPS signal to travel from the satellite to you. This page has a formula at the bottom: http://geospatialrevolution.psu.edu/educators/trilateration.html

  • thanks for the link @Mintx but I think the trilateration has already happened in device and its the (unadjusted?) Lat and Lng being returned – chris Oct 15 '13 at 9:25
  • I think you're right, as those coordinates are usually Cartesian XYZ. I know part of the post processing error correction involves taking into account the speed of light as it travels through the atmosphere, but I've never had to deal with that. – Mintx Oct 15 '13 at 14:07

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