Creative use of GPS in tidal stream deduction
A final source of tidal stream information is your electronic fixing system. The difference between a dead reckoning position corrected for leeway (Chapter 17) and a GPS fix is the set and drift experienced since the last plotted position. Alternatively, you can compare the GPS 'course and speed over ground' (COG and SOG) readouts with the compass heading and logged speed. The onboard data which the latter pair supply can only be measured through the moving water, while the GPS information is equally surely rooted to the seabed. This time, the difference is your set and drift at the moment you take the readings. A fully interfaced system (Chapter 19) will read out set and drift on demand.
If your onboard information is sound, these methods can give a highly accurate indication of what the set and drift are, or have recently been. Sadly, they cannot peek into the future which, in the end, is more generally what is required. However, comparing the known set and drift with the predictions can indicate whether a particular tide is behaving itself. Sometimes, a tide is larger than predicted and produces faster streams. Noting any regular trend may therefore prove helpful if you have time to handle the rather cumbersome process of deduction.
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