Cotidal Line

A chart line connecting places with the same tidal hour or phase under a stated tidal reference.

Version

v1 · 2026-09-28 · History

Domain-specific #

8753

Domain group

Natural Sciences

Origin domain

Marine Science & Oceanography

Subdomains

Oceanography, Tidal Analysis → Marine Science & Oceanography

Aliases

Co-tidal line

Core Idea

A cotidal line is an equal-phase contour on a tidal chart. It joins locations assigned the same tidal hour relative to a named reference; in a simple high-water reading, the marked sites experience high tide at the same tidal time. NOAA's cartographic definition is more precise than mere coincident high water. The line represents a tidal timing field, not a physical boundary or a path that water follows.

A tidal system may organize phase lines around an amphidromic point where range is very small. Equal phase must not be mistaken for equal water-level range, and a line's exact shape depends on observation, interpolation, constituent, and reference convention. NOAA's Gulf of Mexico map displays co-time and co-range contours separately, making the timing-versus-amplitude distinction visible in actual charting.

How would you explain it like I'm…

Same-Tide-Time Lines

On a map of the sea, a cotidal line joins all the places where high tide comes at the same time. It is like drawing a line through every house where the doorbell rings at the same moment. The line is not a wall and not a path the water swims along; it only shows timing. Places on the same line can still have very different high-water heights.

Tide-Timing Map Line

The sea rises and falls with the tides, but high tide doesn't arrive everywhere at once. A cotidal line on a tide chart connects places that reach the same stage of the tide at the same tidal time. It is a line about when, not how much: places on one line can have big or small tides. It is also not a real boundary or a route that water flows along. In some seas these lines fan out around a point where the tide barely rises and falls at all.

Equal-Tidal-Phase Contour

A cotidal line is a contour on a tidal chart connecting locations that share the same tidal phase, measured as a tidal hour relative to a named reference. In the simplest reading, all points on the line have high water at the same tidal time, though official charting definitions are more precise than 'simultaneous high tide'. It maps a timing field, so it is neither a physical boundary nor a path that water travels. Timing must not be confused with amplitude: lines of equal tidal range (co-range lines) are separate contours, and charts may show both. In many tidal systems cotidal lines radiate around an amphidromic point, where the tidal range is very small.

 

A cotidal (co-phase, or co-time) line is an equal-phase contour of the tidal field: it joins locations assigned the same tidal hour relative to a specified reference. Under a simple high-water interpretation, sites on the line reach high water at the same tidal time, but cartographic definitions such as NOAA's specify phase more exactly than coincident high water. The line depicts a scalar timing field; it is not a physical front, boundary or trajectory of water. Phase and amplitude are distinct properties of the tide, so equal phase does not imply equal range, and charts can display co-time and co-range contours separately, as NOAA's Gulf of Mexico map does. Phase lines often organize around an amphidromic point, where the tidal range is near zero. The exact geometry of a cotidal line depends on the observations, the interpolation method, the tidal constituent considered and the reference convention.

Scope of Application

These uses concern equal tidal timing on a map, not equal water-level range.

• Tidal chart reading. Interpret the spatial progression of phase around a basin.
• Amphidromic analysis. Distinguish phase pattern from very low range near a nodal center.
• Co-time/co-range comparison. Read NOAA charts without exchanging timing and amplitude contours.
• Map provenance. State which tidal-hour convention and observations underlie a line.

Clarity

A cotidal line connects charted places assigned the same tidal hour under a stated phase and reference convention. Its inclusion test is equal timing, not equal height. A co-range line can sit on the same chart but joins locations of equal tidal range instead; it is the closest near-miss. Neither contour is a physical wall or the route taken by a water parcel. Interpolation and tidal constituent choice can change the line's shape without changing its kind.

Manages Complexity

The line compresses many spatial timing observations into a readable equal-value contour. That helps compare regions and see rotation around an amphidromic center, but it hides interpolation and constituent choices. Its simplicity is safe only when time reference and amplitude caveat remain visible.

Abstract Reasoning

1. Identify the mapped tidal field and reference convention.
2. Read the hour or phase assigned to the contour.
3. Verify that connected places share that timing value.
4. Separate phase contours from range or current contours.
5. Qualify interpretation near low-amplitude centers or sparse measurements.

Knowledge Transfer

Equal-value contour reasoning transfers to isotherms and other maps as a representational relation, but tidal hour, amphidromic phase behavior, and a particular chart's reference do not transfer outside tide analysis. Even between tidal constituents, a cotidal line must be recomputed rather than reused unchanged.

Relationships to Other Abstractions

Current abstraction Cotidal Line Domain-specific

Parents (1) — more general patterns this builds on

• Cotidal Line is a kind of Representation Prime

  A cotidal line maps equal tidal-hour relations among locations into a chart contour with a stated timing convention.

Hierarchy path (1) — routes to 1 parentless root

• Cotidal Line → Representation → Abstraction

Neighborhood in Abstraction Space

Cotidal Line sits in a moderately populated region (43^rd percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.

Family — Geographic Mapping & Positioning (14 abstractions)

Nearest neighbors

• Tidal atlas — 0.90
• Theory of Tides — 0.88
• Differential GNSS — 0.87
• Reverse geocoding — 0.86
• Map analysis — 0.86

Computed from structural-signature embeddings · 2026-10-08