Secondary-port tidal calculations: the correct sequence

Most ports are listed as secondary ports, with differences applied to a nearby standard port. The almanac provides time differences (e.g., HW is 20 minutes later) and height differences or ratios to convert standard port predictions to the secondary port.

Use the publication's own layout and instructions. First identify the named standard port and the correct date, then obtain the bracketing standard-port HW and LW predictions. Apply the printed secondary-port time corrections separately to HW and LW, and apply the stated height corrections or ratios to the corresponding heights. Only after establishing corrected secondary-port HW and LW times and heights should you calculate range and use the curve or interpolation method specified by the publication.

Some correction tables vary by the standard-port HW time, predicted height, spring/neap range or another printed argument. Interpolate between the correct columns or rows when instructed; do not transfer a correction from a different tide or assume every secondary port uses the same format. Keep signs explicit, show whether a correction is added or subtracted, and perform a reasonableness check on the resulting sequence, range and time zone.