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Geometry repair and topology checks

Outcome: identify and document geometry problems before analysis.

Geometry validity​

Use Check validity or relevant Processing tools to identify self-intersections, ring problems and other geometry errors.

Run repair tools on a working copy, not automatically on the authoritative source download.

What repair means​

Fix geometries creates GIS-valid geometry for processing. It does not make a boundary legally or historically correct.

For cadastral, Māori land, heritage or research data, preserve the source version and document any repair.

Topology questions​

Depending on the dataset, useful checks include:

  • polygons must not overlap
  • polygons must not have gaps
  • lines must connect at endpoints
  • points must fall within an area
  • duplicate geometry/IDs

Only impose a rule if the data model actually requires it. Overlapping rohe or research areas can be entirely legitimate.

Duplicate records​

Check both geometry and identifiers. Two identical points may represent duplicate entry, two observations at the same place, or two different source references.

Exercise​

Create a copy of a synthetic polygon layer and deliberately introduce:

  • one overlap
  • one self-intersection
  • one duplicate ID

Run checks, fix the working copy, and record what changed.

Check​

  • source remains untouched
  • repair action is recorded
  • topology rule matches the dataset's intended model
  • overlapping cultural/research representations are not treated as errors merely because polygons overlap

Next: Web services and field preparation.

Official reference: QGIS vector geometry checks.

Last reviewed: 26 August 2026

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