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faq.rst

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Frequently asked questions and answers

Why does intersects() return False for a point on a line?

Determining if a line and a point intersect is complicated by the inherent inexactness of floating point values. There is no solution for this. Comparing the distance between a line and point to a tolerance value may be a useful approach for some applications. Line vs line intersection, on the other hand, is robust.

See also: https://libgeos.org/usage/faq/#robustness.

I installed shapely in a conda environment using pip. Why doesn't it work?

Shapely versions < 2.0 load a GEOS shared library using ctypes. It's not uncommon for users to have multiple copies of GEOS libs on their system. Loading the correct one is complicated and shapely has a number of platform-dependent GEOS library loading bugs. The project has particularly poor support for finding the correct GEOS library for a shapely package installed from PyPI into a conda environment. We recommend that conda users always get shapely from conda-forge.

Are there references for the algorithms used by shapely?

Generally speaking, shapely's predicates and operations are derived from methods of the same name from GEOS and the JTS Topology Suite. See the JTS FAQ for references describing the JTS algorithms.

I used .buffer() on a geometry with Z coordinates. Where did the Z coordinates go?

The buffer algorithm in GEOS is purely two-dimensional and discards any Z coordinates. This is generally the case for the GEOS algorithms.