YES. You will find loads of errors. Here are a few reasons :
- AIS transponder not setup correctly.
- Instruments on board gives errors or are not hooked up correctly with transponder.
- Users change or modifies the data sent out. For example offset positions, to avoid show correct position or send out fake positions or other information (they manipulate the data sent out by the AIS transponder).
- Error in in transmission.
- Error in storage or decoding.
- The very same message has been received by several base stations and got timestamped with different times. See illustration below.
- Data holes and or variable coverage: The AIS system has been down / out of order, no data coverage. Moved base stations, weather, mountains etc.
When the databases are populated, duplicate messages i.e. messages containing the very same: mmsi, time and position, has been removed. In the past few years, that is 30-40 % of the original data. Nevertheless, there is still a problem with base stations with overlapping coverage with non-synchronized time (time offsets). This is very unfortunate, and can give severe faults and cause very wrong interpretations. Vessels can seem to sail back-and-forth along their track. This gives implications for passage time calculations, distance sailed by the vessel, density plots and so on. The figure below exemplifies.
To the left, only points are plotted. Everything looks ok - But the points are actually duplicates. Two (or more!) points are actually on top of each other - but with different timestamp. When the data is sorted by time, and a track is created between the points closest in time, it appears as the vessel has sailed back-and-forth along the track. One base station has in this case logged the very same points but with ~ 5 min delay.