Guide

How secondary ports work

286 places on this site have no predictions of their own. Here is exactly what is being done to produce their times, and how far to trust it.

Standard versus secondary

LINZ computes full predictions for 88 standard ports from long records of observed water level. For hundreds of other places it publishes something smaller: a time difference from a nearby standard port, and a ratio for the heights.

What this site does with them

Exactly what the table says to do, and nothing more: it takes the standard port's high and low waters, shifts each by the published mean difference, and scales the heights by the published ratio where LINZ gives one. No modelling, no interpolation, no invention. Where LINZ publishes no difference, no times are shown for that place at all — it gets a page with its tidal levels and a plain statement that predictions cannot be derived.

How far to trust it

The published difference is a mean. The real difference moves through the month, and LINZ prints the range it moves across — often twenty or thirty minutes either side. A derived time is a good guide for planning a walk or a fish. It is not a substitute for the official tables, and it is not for navigation.