@turf/polygonize fails on certain line arrangements

[OlivierLeal]

Hi,

I found an unexpected behavior when using turf.polygonize.

I have two sets of LineStrings:

"Wrong" set – four lines that form a nearly square shape, but turf.polygonize fails to produce a polygon.

"Correct" set – four lines that form a similar square (slightly rotated), and turf.polygonize works as expected.

Example code:

`import * as turf from "@turf/turf";

// WRONG
const wrongLineA = turf.lineString([[-38.487131111537174, -12.969499981792461], [-38.48702388505574, -12.961422253524237]]);
const wrongLineB = turf.lineString([[-38.48916841468447, -12.963048521826027], [-38.47908912542943, -12.962708971301478]]);
const wrongLineC = turf.lineString([[-38.48103707317553, -12.961743932968547], [-38.48071539373122, -12.969928887718208]]);
const wrongLineD = turf.lineString([[-38.488310602832975, -12.968534943459533], [-38.47887467246656, -12.96830261941642]]);
const wrongLines = turf.featureCollection([wrongLineA, wrongLineB, wrongLineC, wrongLineD]);
const wrongPolygons = turf.polygonize(wrongLines);
console.log("Polygons wrong:", wrongPolygons);

// CORRECT
const correctLineE = turf.lineString([[-38.474339679399634, -12.961745441517174], [-38.47405825002598, -12.968280856971925]]);
const correctLineF = turf.lineString([[-38.474339679399634, -12.961745441517174], [-38.46767918422326, -12.961213852700281]]);
const correctLineG = turf.lineString([[-38.46767918422326, -12.961213852700281], [-38.46724140519758, -12.967499108711786]]);
const correctLineH = turf.lineString([[-38.47405825002598, -12.968280856971925], [-38.46724140519758, -12.967499108711786]]);
const correctLines = turf.featureCollection([correctLineE, correctLineF, correctLineG, correctLineH]);
const correctPolygons = turf.polygonize(correctLines);
console.log("Polygons correct:", correctPolygons);
`

Observed behavior:

The "wrong" set does not produce a polygon.

The "correct" set produces the polygon as expected.

It seems turf.polygonize requires exact matches between line endpoints to form polygons. In the "wrong" set, the lines are close but endpoints do not coincide exactly, causing the function to fail. Precision issues or slight misalignments prevent proper polygonization.

Question / Request:

Is this expected behavior?

Any suggestions on implementations I can do to make the wrong example work?

[smallsaucepan]

Hi @OlivierLeal. I reckon you are seeing the expected behaviour. According to the docs

Edges must be correctly noded, i.e., they must only meet at their endpoints

Depending on how flexible the solution needs to be you could solve it this way:

1. Find all the split points with lineIntersect - lineIntersect(a, b), lineIntersect(b, c), etc. For N lines that will give you N points.
2. Split each of your N lines using all the split points. Will give you N three segment lines.
3. Calling polygonize on those should then do the trick. They now share vertices at each of the five split points. Polygonize then drops the "dangles".

Here's an example of five intersecting lines, with the polygonize polygon in red:

Hope that's of help!