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.

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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.

Structural Signature¶

Sig role-phrases:

mapped coastal locations — Supply the geographic points whose tide timing is compared. It is constitutive. Counterfactual: A time series at one station alone is not a spatial line.
specified tidal hour — Gives a common time or phase reference for those locations. It is constitutive. Counterfactual: Equal water height is a different contour criterion.
equality grouping — Connects places that share the selected timing value. It is constitutive. Counterfactual: A route through stations with different phases is not cotidal.
chart line — Encodes the equal-hour set as a visible geographic contour. It is constitutive. Counterfactual: An uncharted coincidence of high water is not itself the representational line.
reference and uncertainty — Records the tidal constituent or reference and sparse observations or model limits. It is boundary. Counterfactual: A cotidal chart is not an exact prediction of every high water at every point.

What It Is Not¶

A co-range line. Equal amplitude or tidal range is not equal timing.
A physical wavefront barrier. Chart ink connects equal phase estimates; it is not a material object.
One station's tide time. A cotidal line is a spatial equal-value set, not a single timestamp.
Exact universal forecast. Its meaning depends on constituent, reference, and chart data.
Closest near-miss. At an amphidromic center tidal amplitude can approach zero, so the phase-line geometry is meaningful as a tidal-system representation even where 'the moment of high tide' is a weak local description.

Scope of Application¶

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¶

Check the legend for tidal hour and its reference, then follow the line through places with that same value. A nearby co-range contour answers a different question: how large is the tide? At an amphidromic point, where amplitude is tiny, do not treat a phase-line convergence as an ordinary station high-water schedule.

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¶

Identify the mapped tidal field and reference convention.
Read the hour or phase assigned to the contour.
Verify that connected places share that timing value.
Separate phase contours from range or current contours.
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.

Examples¶

Canonical¶

NOAA's glossary defines the line on a chart through points sharing a tidal hour; in an amphidromic system such equal-hour contours may radiate around the low-amplitude center. The line represents timing, not a wall of water moving along its inked path.

Mapped back: mapped coastal locations → chart positions around a tidal system; specified tidal hour → one common reference hour; equality grouping → same phase at selected positions; chart line → drawn cotidal contour; reference and uncertainty → glossary's specified tidal-hour convention.

Applied / In Practice¶

A NOAA Gulf of Mexico co-tidal chart in technical memorandum NOS CO-OPS 0048 draws co-time lines separately from blue co-range lines. The chart is real tidal-analysis practice and makes the distinction between equal timing and equal range visible.

Mapped back: mapped coastal locations → Gulf of Mexico chart locations; specified tidal hour → tropic lower-low-water interval hours; equality grouping → same co-time value; chart line → green time contours; reference and uncertainty → specified Gulf chart/interval.

Structural Tensions¶

T1 — Spatial Compression versus Local Variation. A contour summarizes a continuous tidal timing field while station density and model assumptions bound precision.

Diagnostic: What data or model supports the line between measured places?

T2 — Phase Depiction versus Amplitude Distinction. The same chart can show timing and range, but near amphidromic low amplitude the timing language needs care.

Diagnostic: Is the reader confusing co-time with co-range?

Structural–Framed Character¶

The approved DAG parent is Representation: a chart line maps equal tidal phase among locations into a visible contour while omitting amplitude. A tidal-hour reference and constituent convention are necessary for reading it.

Evaluative weight: Low; the line describes phase, not hazard or water height. Human-practice-bound: Moderate, because chart conventions and reference epoch are chosen while tides are physical. Institutional origin: Hydrographic practice publishes maps; one chart's phase cannot be reused blindly. Vocabulary travels: Equal-value contour reasoning applies elsewhere, but isotherms have another variable. Import versus recognize: Recognize a cotidal line by shared tidal hour under one reference; treating a height contour as one imports the wrong property.

Its character: A tidal representation with portable contour mapping and phase-specific interpretation.

Structural Core vs. Domain Accent¶

Skeletal core. A medium preserves a selected equal-value relation among spatial targets.

Domain-bound accent. The value is tidal hour or phase for a declared reference, not amplitude or height.

Why not prime. Representation is broad; other contours do not carry the tidal timing relation.

This entry is a kind of Representation.

Strict parent — representation. The chart line maps spatial equal-phase relations into visible geometry under a reading convention and explicit fidelity limits.
Related — boundary. A contour partitions phase values on a map but is not a physical territorial boundary in the sea.

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

Not to Be Confused With¶

Co-range line. Tell: Does the contour join equal range rather than equal hour?
Amphidromic point. Tell: Is this a central low-amplitude point rather than the entire equal-hour line?
Tidal-current line. Tell: Is the quantity water-level phase or current timing?
High-water prediction. Tell: Is the claim about one station's time rather than a charted spatial set?

References¶

NOAA Tides & Currents Glossary: https://tidesandcurrents.noaa.gov/glossary.html
NOAA Technical Memorandum NOS CO-OPS 0048, Gulf of Mexico cotidal chart: https://tidesandcurrents.noaa.gov/publications/Technical_Memorandum_NOS_COOPS_0048_Updt.pdf
Frozen Wikipedia discovery revision: https://en.wikipedia.org/wiki/Cotidal_line (revision 1292611140).
Preserved source candidate: https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095642258
Preserved source candidate: https://core.ac.uk/download/pdf/31067737.pdf

The frozen Wikipedia revision is discovery provenance. The retained source set was reviewed for identity, formal or operational relation, and scope. The encyclopedia's structural synthesis is bounded to those claims; a thin authority surface is recorded as a nonblocking source-strengthening repair rather than concealed.