FA-70705 / GIS polygon topology / Member archive

WKT polygon and multipolygon ring structure: ordinate truncation · case 05

Rings from 3D sources carry elevations and fail closure when only z differs.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input: a WKT string. The keyword (case-insensitive) must be POLYGON or MULTIPOLYGON, else None; "<keyword> EMPTY" returns []. Otherwise parse the parenthesised body into polygons -> rings -> positions, keeping only x and y of every position (Z/M ordinates are dropped). Every ring must have at least 4 positions and be closed in x/y, else None. Return a list of polygons (a POLYGON gives a one-element list).

Why this case matters

WKT is the lingua franca between databases and GIS tools; losing a dimension or nesting level corrupts rings.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression #3	`[[[[0.0, 0.0, 5.0, 1.0], [4.0, 0.0, 5.0, 2.0], [4.0, 4.0, 6.0, 3.0], [0.0, 0.0, 5.0, 1.0]]]]`	`[[[[0.0, 0.0], [4.0, 0.0], [4.0, 4.0], [0.0, 0.0]]]]`	Failed

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