@turf/kinks returns false self-intersections in MultiLineString features with branches

[ovdleondle]

turf 7.1.0

When a MultiLineString feature has branches (e.g. a river with tributaries, or a street with side streets), kinks module returns self-intersection at the branching point.
For this geojson feature:
{"type":"FeatureCollection","features":[{"type":"Feature","properties":{},"geometry":{"type":"MultiLineString","coordinates":[[[0,0],[0,1],[0,2]],[[0,1],[1,1]]]}}]}

kinks would return a self-intersection at 0,1 coordinates, which is false.

[smallsaucepan]

Thanks for reporting this @ovdleondle.

Could it possibly be related to #2722? Would you mind checking to see if the behaviour is correct for v6.5.0?

[pm0u]

this appears to be unrelated to #2722:

• this module actually uses its own line-intersect algorithm
• this module, per its description in jsdoc, is intended to report only self intersections

i am not sure that this qualifies as a bug per the documentation within turf but i could be missing some context for what would be expected of a module by this name. this case seems to be behaving as expected just by appearance

[smallsaucepan]

Thanks for ruling that out @pm0u. Will take a closer look at this one in due course then.

[ovdleondle]

@smallsaucepan I confirm that this behavior is not related to #2722, as the same issue occurs in version 6.5.0.

After reviewing the kinks documentation, I also agree with @pm0u that the module is technically functioning as intended.

Our goal was to use the kinks module as a geometry validator on the client side. However, geometry validity is a nuanced concept, as described in the PostGIS introduction:

Validity is most important for polygons, which define bounded areas and require a good deal of structure. Lines are very simple and cannot be invalid, nor can points.
Some of the rules of polygon validity feel obvious, and others feel arbitrary (and in fact, are arbitrary).

Additionally, geometry validation in PostGIS evaluates whether a geometry is simple and whether it is valid:

A simple geometry is one that has no anomalous geometric points, such as self intersection or self tangency.

A MULTILINESTRING is simple only if all of its elements are simple and the only intersection between any two elements occurs at points that are on the boundaries of both elements.

Geometry validity primarily applies to 2-dimensional geometries (POLYGONs and MULTIPOLYGONs) .

For linear geometry the only validity rule is that LINESTRINGs must have at least two points and have non-zero length (or equivalently, have at least two distinct points.) Note that non-simple (self-intersecting) lines are valid.

To summarize, we plan to use the kinks module exclusively for polygon and multipolygon features. For lines, we will try using lineIntersect with ignoreSelfIntersections: false.