Geometry & topology

This is the topic where the shapes themselves live: clipping, buffering, dissolving, overlaying, simplifying, and the question that sits under all of them — whether a geometry is valid.

Validity is not a formality. A self-intersecting polygon has no well-defined inside, so every operation that depends on inside and outside is free to return anything. GEOS will often give you an answer rather than an error, which is why an overlay can produce a plausible-looking layer with the wrong area in it. Repair is cheap; noticing that you needed it is the hard part, and make_valid can hand back a different geometry type while it is at it.

Work down the page: the concepts settle what the operations actually compute and what valid means, the how-to guides are the operations themselves, and the fixes are what the operations do when a geometry, a precision or a CRS was not what the function assumed.

Learning paths run through this topic too — see the end of this page.