Validate Geometry

Invalid geometries are the second great source of silent GIS errors. Before spatial operations, check validity and repair deliberately.

What to check

Before an overlay, join, buffer, or dissolve, verify the geometries are valid and clean:

• Validity — self-intersections ("bowties"), unclosed rings, wrong ring orientation. Use ST_IsValid (PostGIS), .is_valid (Shapely/GeoPandas), Check Geometry (ArcPy), or ogrinfo (GDAL).
• Empties and nulls — null or empty geometries that will break joins or produce surprising results.
• Geometry-type consistency — mixed single/multi, or polygons and lines living in one layer.
• Duplicate vertices and slivers that throw off topology.

Repair deliberately, not blindly

• Use the right repair tool: ST_MakeValid (PostGIS), make_valid() (Shapely — and know the older buffer(0) trick can silently drop slivers), Repair Geometry (ArcPy), ogr2ogr -makevalid (GDAL).
• Understand what the repair changed. Repair can split a feature, drop a sliver, or turn a polygon into a multipolygon. Report it — "repaired 14 self-intersecting polygons; 2 became multipolygons" — instead of silently mutating the data.
• Repair a copy, or clearly flag the change. Don't quietly overwrite the source.

Why this matters

An invalid polygon doesn't announce itself. It makes an intersection return empty, a dissolve leak across boundaries, or an area come back negative — output that looks valid and passes review on its way to the map. Checking validity before the operation turns a silent wrong answer into either a clean result or an explicit "here's what I had to fix."