A line connecting points with the same tidal phase, so high water arrives along it simultaneously. Cotidal lines are labelled in hours or degrees of phase and fan out from amphidromic points, showing how the tide wave travels around a basin. Hydrographers use cotidal charts to build tide zoning for surveys far from gauges, assigning time corrections between the lines.
Cotidal lines bunch up in the narrows, so the time correction changes fast once you're past the point.
Also heard as
- co-tidal line
Do not mix it up with
- Corange line: Cotidal lines join points of equal timing; corange lines join points of equal range.
Related terms
Corange line
A line joining places that share the same tidal range or the same constituent amplitude.
Amphidromic point
A point in the sea where a tidal constituent has zero range and around which its high water rotates.
Phase lag
How far a constituent's local high water trails the corresponding astronomical forcing, given in degrees.
Kelvin wave
A long wave trapped against a coast by Earth's rotation, the form in which tides travel around ocean basins.