Turfjs / Turfjs/turf

Union can't join touching polygons on anti-meridian

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@turf/union
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TypeScript
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Description

Turf 7.1.0

Union can't join touching polygons on anti-meridian unless coordinates are adjusted by 360.

image

// CORRECT - produces {"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[-183,67.35710304434852],[-180,64.8381209933682],[-173,67.35710304434852],[-180,69.40628708036891],[-183,67.35710304434852]]]}}

union(featureCollection([polygon([
    [
      [-180, 64.8381209933682],
      [-185, 67.35710304434852],
      [-180, 69.40628708036891],
      [-180, 64.8381209933682],
    ],
  ]), polygon([
    [
      [-180, 64.8381209933682],
      [-173, 67.35710304434852],
      [-180, 69.40628708036891],
      [-180, 64.8381209933682],
    ]
  ])]));
// INCORRECT - produces {"type":"Feature","properties":{},"geometry":{"type":"MultiPolygon","coordinates":[[[[-180,64.8381209933682],[-173,67.35710304434852],[-180,69.40628708036891],[-180,64.8381209933682]]],[[[173,67.35710304434852],[180,64.8381209933682],[180,69.40628708036891],[173,67.35710304434852]]]]}}

union(featureCollection([polygon([
    [
      [180, 64.8381209933682],
      [175, 67.35710304434852],
      [180, 69.40628708036891],
      [180, 64.8381209933682]
    ]
  ]), polygon([
    [
      [-180, 64.8381209933682],
      [-173, 67.35710304434852],
      [-180, 69.40628708036891],
      [-180, 64.8381209933682],
    ]
  ])]));

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the provided union(featureCollection(...)) reproduction in Turf 7.1.0 and compare the incorrect anti-meridian result with the expected single Polygon. Done means touching polygons on opposite sides of the anti-meridian union correctly without requiring coordinates to be adjusted by 360.

Written by the indexing model from the issue text.

Assessment

Tech stack
javascript, typescript
Domain
data
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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