Secondary port

How to think it through

1The almanac gives tides for standard ports only; for the others it gives differences.
2Differences change with the time and height of HW: interpolate between the two table values.
3Standard port time and height plus today's differences = secondary port time and height.

Every step says why you do it, which formula you use, the working and the answer.

Secondary port: time and height of HW

Today at Victoria (standard port) HW is at 03:30, 3.8 m. Using the almanac differences, find the time and height of HW at Port Fraser.

What is today's time difference, in minutes?
Why: The difference depends on the time of HW at the standard port: interpolate between the two the almanac gives.
$D = D_{1} + \frac{T - T _{1}}{T _{2} - T _{1}} \times (D_{2} - D_{1})$
Working: -20 + 0.58 × (5 − -20) = -5 min
Answer: -5 min
What time is HW at Port Fraser?
Why: Standard port time plus the time difference.
$T_{secondary} = T_{standard} + D$
Working: 03:30 − 5 min = 03:25
Answer: 03:25
What is today's height difference?
Why: The height difference is interpolated too: it depends on whether today's tide is nearer springs (MHWS) or neaps (MHWN).
$Δ h = Δ h_{N} + \frac{h - MHWN}{MHWS - MHWN} \times (Δ h_{S} - Δ h_{N})$
Working: 0.4 + 0.13 × (-0.3 − 0.4) = 0.31 m
Answer: 0.31 m
What is the height of HW at Port Fraser?
Why: Standard port height plus the height difference.
$h_{secondary} = h_{standard} + Δ h$
Working: 3.8 + 0.31 = 4.1 m
Answer: 4.1 m