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Fixing a self-intersecting polygon

Self-intersection is one of the most common and most confusing EUDR polygon rejections. The plot looks fine on a map, yet TRACES refuses it. Here is what is actually wrong and how to repair it safely.

A polygon self-intersects when its boundary crosses over itself, forming a figure-eight or a spike where two edges overlap. It often comes from GPS noise, a stray vertex, or a boundary digitised slightly out of order. To a person the shape reads as a field. To a geometry checker it is invalid, and TRACES rejects it under an invalid-polygon rule.

Why it happens

  • GPS drift nudges one vertex across a neighbouring edge.
  • Out-of-order points when a boundary was walked or drawn make the ring cross itself.
  • A duplicated or near-duplicated vertex collapses an edge into a spike.

Why TRACES refuses it

An invalid polygon has an ambiguous interior: it is not clear which side is inside the plot and which is outside, so its area is undefined. EUDR needs an unambiguous plot boundary, so a self-intersecting ring is a hard error, not a warning.

How to repair it safely

You do not need to redraw the plot. A topology safe repair resolves the crossing into a valid polygon while preserving the intended shape as closely as possible. The important safeguard: any repair that moves the plot area by more than a whisker is flagged, so you can confirm the corrected boundary still matches the real field.

The fixer repairs the self-intersection, corrects winding, closes the ring, and flags the plot if its area changed during the repair, so nothing shifts silently.

Do not eyeball it

The reason self-intersection is frustrating is that the naked eye cannot see it on most maps. A deterministic geometry repair catches and resolves it reliably, which is far faster than hunting for the crossing vertex by hand across dozens of plots.

Common questions

My polygon looks fine on the map. Why is it invalid?

Self-intersections are usually invisible at map zoom. Two edges cross by a tiny amount, which makes the area undefined and the polygon invalid even though it looks like a normal field.

Will repairing it change my plot boundary?

A topology safe repair preserves the shape as closely as possible. If the area moves by more than a whisker, the plot is flagged so you can review it.

Repair a self-intersecting plot now

Paste the polygon and the fixer resolves the self-intersection safely, flagging any change in area.

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