FA-69284 / Tide and marine navigation tables / Member archive

Compass variation and deviation: True to compass adds the corrections · case 04

Compass courses derived from true are off by twice the error.

Member previewVariant 4 · 3 implementations · 7 checks per implementation

Case contract

Input [value, frm, to, var, dev] with frm/to in C (compass), M (magnetic), T (true); var and dev like "3.5W"/"2E" (east positive). M=C+dev, T=M+var; converting downward subtracts. Return [value mod 360 rounded 1, total compass error dev+var as "x.xE"/"x.xW" or "0"].

Why this case matters

Tide tables and passage plans turn published predictions into go/no-go decisions about depth, clearance and timing.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
compass variation and deviation [1, "T", "C", "2E", "3E"]	`[6.0, "5.0E"]`	`[356.0, "5.0E"]`	Failed

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