buffer units other than degrees

[SirenHound]

Am I missing something or does the buffer function just change all units to degrees at the equator and then buffer the geoJSON by degrees regardless of its latitude?

[stevevance]

The same happens to me.
It looks like this in Chicago (which is 41 degrees north):

[SirenHound]

I'm glad it's not just me.
I wrote a patch, if people are happy with it I can try a pull request, using the GeoJSON as a proxy does seem a little kludgy to me, but it saves rewriting functions down the rabbit hole.
This should solve the latitude issue for locally sized shapes (buffers for points far from the offset longitude become skewed).
Uses a sphere, better use would be an oblate spheroid but that's a little more complicated, this at least moves in the right direction.

Constructive criticism appreciated.
Starting from line 2528 in turf.js:

module.exports = function(feature, radius, units){
  var buffered;
  var unitsPerDegree;

  switch(units){
    case 'miles':
      //radius = radius / 69.047;
      unitsPerDegree = 7917.5 * Math.PI / 180;
      break
    case 'feet':
      //radius = radius / 364568.0;
      unitsPerDegree = 41804000 * Math.PI / 180;
      break
    case 'kilometers':
    case 'kilometres':
      //radius = radius / 111.12;
      unitsPerDegree = 6371 * Math.PI / 180;
      break
    case 'meters':
    case 'metres':
      //radius = radius / 111120.0;
      unitsPerDegree = 6371000 * Math.PI / 180;
      break
    case 'degrees':
      unitsPerDegree = 1;
      break
  }


  if(feature.type === 'FeatureCollection'){
    var feature = combine(feature);
    feature.properties = {};
  }

  //create fake GeoJSON object with projected 'lnglats' to use with bufferOp
  var newLnglats = [];
  var lnglats = feature.geometry.coordinates[0];
  var lngOffset = lnglats[0][0]; //offset projection to minimize skewed buffers (will not work for global scale polygons)
  console.log(lngOffset, "offset");
  for (var i = 0; i < lnglats.length; i++){
    var x = (lnglats[i][0] - lngOffset) * Math.cos(lnglats[i][1]*Math.PI/180) * unitsPerDegree;
    var y = lnglats[i][1] * unitsPerDegree;
    newLnglats.push([x,y]);
  }
  var fakeGeoJSON = {type:"Feature", geometry: {type: "Polygon", coordinates:[newLnglats]}};

  buffered = bufferOp(fakeGeoJSON, radius);

  //put object back on sphere
  newBufferLnglats = [];
  bufferLnglats = buffered.features[0].geometry.coordinates[0];

  for (i = 0; i < bufferLnglats.length; i++){
    var lat = bufferLnglats[i][1] / unitsPerDegree;
    var lng = lngOffset + bufferLnglats[i][0] / Math.cos(lat*Math.PI/180) / unitsPerDegree;
    console.log("lng",lng);
    newBufferLnglats.push([lng, lat]);
  }
  buffered.features[0].geometry.coordinates[0] = newBufferLnglats;
    console.log(buffered.features[0].geometry.coordinates[0]);

  return buffered;

};

[SirenHound]

Actually, it just occurred to me that the above will only work for polygons (my use case), it may need additional checks for points etc

[SirenHound]

For a single point you'd change this:

  //create fake GeoJSON object with projected 'lnglats' to use with bufferOp
  var newLnglats = [];
  var lnglats = feature.geometry.coordinates[0];

to this:

  //create fake GeoJSON object with projected 'lnglats' to use with bufferOp
  var newLnglats = [];
  var lnglats = [feature.geometry.coordinates];

You could set up a switch from feature.geometry.type if you like.

[DenisCarriere]

This is still occurring in the current @turf/buffer module.

This could be fixed by using @turf/circle + @turf/dissolve (not implemented yet).

test/out/north-latitude-points.geojson

[DenisCarriere]

Duplicate issue, tracking issue here: #660