Interpolation, Extrapolation for Tidal Streams


 The range of a tide indicates the likely tidal stream for that day. This may be between the mean values, so calculate an in between value (interpolation) , or perhaps outside the mean values. requiring an extrapolation.

Assuming a uniform change between springs and neaps (may as well!), identify similar triangles and take the ratio of the corresponding sides. This gives a formula for the tidal stream, if you know the mean values for neaps and springs, and today's range of tide.

A typical yachtmaster question would be:

Use diamond R to find the tidal stream on Saturday 31st August 1715 - 1815 DST.

From the PNT HW Plymouth is 2045DST; tidal hour is HW-3; diamond R gives streams of 2.8sp and 1.2np. The similar triangles formula would solve the problem thus:

The same answer could have been obtained by plotting the Plymouth values onto the computation of rates graph. It is no sweat for the computer to use the same formula to work out all currents for all the diamonds for all the times for one tide. As supplied on the previous page.