The Nautical Institute’s AIS error reports webpage tells us:
I have observed too many vessels with wrong data transmitted by their AIS. Mostly headings are not correct, but the main problem – is offset correction given by GPS. If GPS has offset correction (set by navigator on bridge) then AIS transmits wrong vessel’s position (equal to offset).
You can watch now on ECDIS some vessel in the harbour is not alongside the berth but e.g. stays in the middle of harbour instead. The greater offset – the greater error. Some of the vessels have offset from 0.02 up to 0.3 nautical mile, and if you meet such vessel in Singapore Strait – then AIS information maybe very dangerous, as you can mistake such target with others.
I like AIS information very much, but when I watch such a picture on ECDIS – I would like to suggest to all navigators not to use on their GPS any offset. Nowadays GPS is reliable information and better to plot position on sea chart manually using satellite-derived calculation instead of inserted offset into GPS unit.
Lesson Learned: check the GPS offsets on your ship and assure you are sending accurate data… or you could confuse surrounding ships. Also keep an eye on NI’s AIS error reports webpage for continuous updates on the reliability of AIS.