Turf::intersection does not observe edge cases

[westnordost]

I think this function does not observe the following edge cases:

• both lines are colinear (denominator == 0): the line segments could still intersect - the intersection would be another line segment
• what about the 180th meridian? what about line segments crossing the north and south pole?
• the current algorithm does not seem to count the start end end point as part of the line segment. This is a matter of definition, but usually at least the start point is counted as part of the line (i.e. the line segment is then closed-open)

[1ec5]

turf-swift/Sources/Turf/Turf.swift

Lines 15 to 16 in 19d1841

	public func intersection(_ line1: LineSegment, _ line2: LineSegment) -> CLLocationCoordinate2D? {
	// Ported from https://github.com/Turfjs/turf/blob/142e137ce0c758e2825a260ab32b24db0aa19439/packages/turf-point-on-line/index.js, in turn adapted from http://jsfiddle.net/justin_c_rounds/Gd2S2/light/

turf-point-on-line has since been replaced by turf-nearest-point-on-line, which calls out to turf-line-intersect, which at a glance seems to handle two of these issues.

what about the 180th meridian? what about line segments crossing the north and south pole?

This is a good point; geometries that straddle the antimeridian probably aren’t handled well by other methods in this library either. GeoJSON technically represents such geometries as multi-geometries, but many clients including the Mapbox Maps SDK work around this limitation by allowing longitudes to extend beyond ±180°.