At ABPmer, a recent article in Hydro International made us wonder whether there may be value in using these AIS data to supplement or even replace more traditional ocean monitoring and prediction techniques.
Numerical modelling and measurement with buoys or drifters are two ways to ascertain the tidal currents in a particular area. However, both are time consuming and relatively expensive.
Freely available AIS data provide speed and direction over ground, as well as heading, for each vessel broadcasting. Recent studies, like the one in Hydro International, have suggested that AIS can be used with relatively complicated techniques to calculate tidal constituents in busy shipping lanes.
ABPmer’s Modelling Team consider that AIS could be used to:
- deduce the residual current in a tidal strait by calculating the average difference in speed between vessels travelling in opposite directions.
- determine existence of a cross-current along a route by comparing course over ground and the heading.
- use vessel broadcasts whilst at anchor to deduce direction and time of turn of tide (through the heading of the vessel, assuming there are not strong effects of windage on the vessel).
However, there are a few limitations that would need to be overcome in order for this technique to be viable as a reliable predictor of tidal currents:
- The area being studied would need to have a consistently high volume of shipping traffic in order to effectively average the data
- The technique would not work in an area subject to a shipping speed restriction
- Areas with stronger tidal currents would likely provide a more obvious correlation between the AIS data and the tidal current.
A study in the Straits of Dover presents an ideal candidate site for a trial study to understand reliability and cost comparison.
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