point-on-line results are distorted

[mourner]

The point-on-line module doesn't return correct results. Take a look at the official example with a connection line added:

As you can see, the line isn't perpendicular, so the snapped point isn't actually the closest one. This happens because the lineIntersects routine that's used to calculate the intersection point between a line segment and a line perpendicular to it from the point does the calculation on latitude/longitude values as if they were cartesian coordinates, while they are not — at a local scale, the ratio between latitude and longitude deltas is approximately cos(latitude), not 1.

On higher latitudes, the distortion will be more pronounced. Here's how the same example looks on latitude ~78:

cc @morganherlocker @tmcw @tcql

[DenisCarriere]

@mourner Would refactoring @turf/point-on-line lineIntersects() with @turf/line-intersect solve this issue?

[rowanwins]

Gday @DenisCarriere & @mourner

It looks like replacing the lineIntersects function with @turf/line-intersect will address the issue. The screenshot below is taken at 80degs latitude.

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Will tidy up the rejigged code and will put in a pull request, a couple of tests to fix up, but other than that it was a pretty straightforward replacement.

[DenisCarriere]

👍 Good catch, @turf/line-intersect does sound like it would fix this issue (with some "rejigging").

[DenisCarriere]

👍 Looks like this got fixed in https://github.com/Turfjs/turf/issues/689.

Added a test case for it: test/in/line-northern-latitude-#344.geojson