turf.nearest for linestrings

[peterqliu]

For example, when I want to find the closest gas station to the stretch of freeway where I'll likely be low on gas.

This is better than doing turf.nearest on a single point, for two reasons:

• Turf.nearest on a linestring is likely to return nearer results than on a point, because it's a larger search area.
• when looking for gas stations, I want to find one that's not too far out of my way, and I'm willing to go a couple more exits before turning off if that one is more convenient. I don't really care where the station is, so I don't have a specific point in mind to search around.

[nevi-me]

The challenge with nearest for linestrings is that a linestring could have only 2 points, making it difficult to run nearest through each sets of coordinates in that point. I solved this problem by taking linestring and 'complexifying' it by adding points on the linestring within a certain distance of each other. Then I can search for nearest in that list of coordinates on the linestring.

I did this in rwt-to/GeoJSON-Tools (MovingGauteng/GeoJSON-Tools@35abcf0), but I'm not promoting that you use the tool. I just saw Turfjs today, and if someone would like to port the 'complexify' method from GeoJSON-Tools, you're welcome to.
We use GeoJSON-Tools internally, and there's lots of work to do, so I haven't committed anything to it in some time now.

[rowanwins]

Gday @peterqliu

I've had a shot at creating this module, basically you input a collection of points and a linestring, and it returns the nearest point as well as the distance that point is from the line, this is included as a property on the feature. The user can also specify the distance units as a parameter (as per the existing turf distance module) which defaults to kilometers.

It was actually quite simple to do as it builds on a number of existing turf modules, namely point-on-line and distance.

cc @tcql & @morganherlocker . I still need to write some tests but if you're interested in transferring ownership at some stage let me know.

Cheers
Rowan

[peterqliu]

@rowanwins thank you so much for jumping in here. hoping to see this in the not-too-distance turf future.

[stebogit]

@rowanwins @DenisCarriere it seems to me that @turf/point-on-line is exactly what this issue refers to, isn't it?
However, wouldn't be nice, for next major release, to improve @turf/nearest so that it could consistently handle all features? We could then drop @turf/point-on-line, which would be redundant.

Thoughts?

[rowanwins]

Mmm not quite @stebogit . point-on-line only takes a point and a line and works out the nearest point on the line. The idea of this was to pass in a collection of points and work out which point was closest to the line. My first pass module was basically a wrapper to do the iterating through the collection.

It would be neat to bulk out nearest to support other scenarios, even if we did this point-on-line might still have it's place.

[stebogit]

@rowanwins 👍 of course! Now I got the difference 😄
I'll work on that then.

[morganherlocker]

How would the case of a parallel line be handled? There is no single closest point.

[rowanwins]

If in doubt @morganherlocker the answer is c - we'll just place something artibitralily 😜!

[stebogit]

Implemented with #939