lineIntersect returns the same point several times

[grundid]

I'm trying to find out the point at which two lines intersect. The two lines have the intersecting point in common like:

  const line1 = turf.lineString([[7, 50], [8, 50], [9, 50]]);
  const line2 = turf.lineString([[8, 49], [8, 50], [8, 51]]);

  var features = turf.lineIntersect(line1, line2);

In this case lineIntersect returns the point [8, 50] 4 times. By looking at the implementation the problem is obvious since the given lines are split into 2-vertex lines and then the intersections are calculated.

I'm not sure if you want to fix this, but it would be nice if the documentation would explain this scenario.

[rowanwins]

Gday @grundid

Thanks for the report, that does seem to be an unexpected output! I think we can mark this as a bug, it should be fairly easy to remove duplicate pairs of output coordinates.

[DenisCarriere]

Agreed, this is a bug, I can definitely have a look at this.

[grundid]

Thanks for looking into it. Unfortunately the problem is also visible in other functions like lineSplit.

When I split a line like this:

  const point = turf.point([8, 50]);
  const line1 = turf.lineString([[7, 50], [8, 50], [9, 50]]);
  const splitLines = turf.lineSplit(line1, point);

I receive two lines, which is correct, but the second line contains the splitter point twice.

line[0]: { type: 'LineString', coordinates: [ [ 7, 50 ], [ 8, 50 ] ] }
line[1]: { type: 'LineString', coordinates: [ [ 8, 50 ], [ 8, 50 ], [ 9, 50 ] ] }

Wherever listSegment is used to split the line into 2-vertex lines one would have to rethink the algorithm, I guess.

[DenisCarriere]

Once @turf/line-intersect is fixed this shouldn't happen on @turf/line-split.

[grundid]

I'm sorry, but the splitLineWithPoint function in the line-split module doesn't use lineIntersect.

I guess, a simple fix for the splitLineWithPoint function might be to check if the initialValue (or getCoords(segments.features[0])[0]) in line 143 equals to the coords in line 148 and then don't add it to the returning array in line 151. I haven't tested it though ;)

[DenisCarriere]

👍 Oh that's true, I assumed it was since the require('@turf/line-intersect) (I should know, I built the module 🤦‍♂️ ).

Feel free to send a PR with a fix, we can hash out any details in the PR.

[DenisCarriere]

Fixed in next minor release.