Shapely : cascaded_union seems to return too many polygons

They Don’t Actually Touch: This one’s pretty straightforward. If your polygons are just hanging out, not overlapping at all, then the union is, well, just a collection of those separate polygons. Shapely’s just doing its job here.

Something’s Fishy with the Shapes: Shapely’s pretty smart, but it can get tripped up by “invalid” geometries. Think of it like trying to assemble a puzzle with bent pieces. These invalid shapes can have self-intersections or weird ring orientations. cascaded_union might throw its hands up and give you a fragmented result.

The Tolerance Tango: Under the hood, Shapely uses something called GEOS, and GEOS works with a certain level of precision, or “tolerance.” Imagine trying to fit two pieces of wood together, but your saw isn’t perfectly accurate. If your polygon coordinates are almost the same, but not quite, cascaded_union might not see them as overlapping.

Old News: cascaded_union is a bit of a relic these days. While it might still work, the cool kids are using unary_union. It’s generally faster and more reliable.

Shape Up Your Shapes: First things first, make sure your polygons are valid! Use the is_valid attribute to check. If you find a bad one, use make_valid from shapely.validation to try and fix it. It’s like a geometry doctor!

Double-Check the Connection: Are your polygons actually overlapping? Use the intersects method to be sure. No overlap, no union, simple as that.

Upgrade to unary_union: Seriously, just do it. It’s the modern way.

Give Them a Trim: Sometimes, simplifying your polygons can help. The simplify method lets you reduce the number of points, which can smooth out those tiny coordinate differences. Just be careful not to overdo it and change the shape too much!

Round ‘Em Up: If you’re dealing with super-precise coordinates, try rounding them off a bit. This can help avoid those tolerance issues.

Dissolve, Dissolve, Dissolve: Even after all this, unary_union might still give you a MultiPolygon. In that case, try running unary_union again on the individual polygons within the MultiPolygon. It’s like giving it a second chance to merge everything together.