I'm decoding some AIS messages and outputting KML files showing routes. Looking at some ships in the Adriatic Sea there are some weird Latitude jumps that I don't understand.

  • Several ships show similar anomalies correlating to Latitude information.

    Some ship's position jumps are:

    45.73537 , 13.63126
    -45.141627 , 13.631353
    45.734942 , 13.63136

    45.641197 , 13.745698
    -34.749633 , 13.745702
    45.641188 , 13.745702

    45.637602 , 13.75268
    -20.772212 , 13.752528
    45.637602 , 13.75268

    45.612017, 13.7784
    -93.531987, 59.310132
    45.61205, 13.7784

So I'm wondering if this is a GPS error, or an AIS error ? or what are some other possibilities ?

  • 1
    Are the jumps present in the raw AIS data? Are they randomly or evenly spaced - both in terms of real time between 'errors' and number of observations. – Mikkel Lydholm Rasmussen Jun 18 '15 at 13:11
  • 1
    this sounds more like a software/hardware question than GIS. The GIS part of this question is only pos/neg latitudes are going to be in northern and southern hemispheres. what is causing it seems to have very little if anything to do with GIS. Can you make your question have to do with GIS? – Brad Nesom Jun 18 '15 at 13:22
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    @Brad Nesom I figured the question involved GIS with the inclusion of KML mapping and the question involving geographic information. If there's a more relevant stack site let me know. – cat_bug Jun 18 '15 at 13:26
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    I have worked on AIS in the past. If memory serves me correctly it uses two's complement to encode the position in the message, so it would only require a one bit change to go from +ve to -ve. We used to get quite a few odd readings, as there is no error correction mechanism built into the message, so we just used to discard anything that was a fair distance away from the receiver in question (there range only being about 20km or so). If you have mostly sensible readings showing progress along a route, I would just mark it down to occasional noise and ignore. – John Powell Jun 18 '15 at 13:50
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    There is no reason that GPS yields those jumps (affecting altitude instead of latitude would not create jumps). You could have GPS errors, but not of this large range. One GPS could have a bug, but if this occurs on different ships this is very unlikely. I've never worked with AIS, but it look like a sign error in most cases. Also, latitude -93 does not exist. – radouxju Jun 18 '15 at 14:34

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