along should support negative offset

[evolkmann]

@turfjs/along should allow a negative offset.

Example

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [
            9.309196472167969,
            52.23736673260212
          ],
          [
            9.33314323425293,
            52.24073057268207
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "properties": {
        "marker-color": "#7e7e7e",
        "marker-size": "medium",
        "marker-symbol": "",
        "info": "1 mile offset"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          9.332232811468383,
          52.24060278205829
        ]
      }
    },
    {
      "type": "Feature",
      "properties": {
        "marker-color": "#7e7e7e",
        "marker-size": "medium",
        "marker-symbol": "",
        "info": "I would expect a point around here for `-1 mile`"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          9.286966323852539,
          52.23434431509133
        ]
      }
    }
  ]
}

When you use along with 1 mile offset, the point is correctly returned. If I call the function with -1 mile, an error is thrown.

I briefly tried to implement this inside the along source without a quick success, so I raise this issue as a reminder and maybe someone else also has a need for this requirement.

My current workaround is to flip the original line (swap coordinates) and apply a positive offset.

Version: 6.5.0

[twelch]

Sounds like a nice addition.

[JamesLMilner]

Thanks for raising this as an idea, appreciate the input. Unfortunately I'm not sure this makes sense for one primary reason which is that this seems to work on the assumption that a LineString Feature is straight. For example -1 mile along a non straight line would lead to the question of -1 mile in which direction? As such I am not convinced this should be implemented. I'll leave this open for a bit if someone can describe how this might work.

[twelch]

I interpreted the use case as - I want the point 1 mile along the line, starting from the end of the Linestring, instead of the beginning.

Instead of flipping the order of the Linestring coordinates and using a positive distance to make it work as @evolkmann mentioned for a workaround, the along method could accept a negative distance which would indicate to start from the end Position of the Linestring and work backwards 1 mile.