line offset is misaligned

I used https://cdn.jsdelivr.net/npm/@turf/turf@5/turf.min.js

      var line = {'type': 'LineString', 'coordinates': [
        [
          19.925,
          50.0555
        ],
        [
          19.924,
          50.055
        ]
      ]}
      var a = turf.lineOffset(line, 50/1000, {units: 'kilometers'})
      var b = turf.lineOffset(line, -50/1000, {units: 'kilometers'})
      console.warn({'type': 'MultiLineString', 'coordinates': [line.coordinates, a.geometry.coordinates, b.geometry.coordinates]})

produces

while I would expect what is here marked with a blue lines to be produced as offsets

Not sure whether it is a bug or I misunderstood documentation http://turfjs.org/docs/#lineOffset

[Uolc]

Here’s my code to get correct parallel line with more than 2 points :

const getLineOffset = (line, distance, { units = 'kilometers' } = {}) => {
    const lineCoords = line.geometry.coordinates;
    const transformAngle = distance < 0 ? -90 : 90;
    if (distance < 0) distance = -distance;

    const offsetLines = [];
    for (let i = 0; i < lineCoords.length - 1; i++) { // Translating each segment of the line to correct position
        const angle = bearing(lineCoords[i], lineCoords[i + 1]) + transformAngle;
        const firstPoint = transformTranslate(point(lineCoords[i]), distance, angle, { units })?.geometry.coordinates;
        const secondPoint = transformTranslate(point(lineCoords[i + 1]), distance, angle, { units })?.geometry.coordinates;
        offsetLines.push([firstPoint, secondPoint]);
    }

    const offsetCoords = [offsetLines[0][0]]; // First point inserted
    for (let i = 0; i < offsetLines.length; i++) { // For each translated segment of the initial line
        if (offsetLines[i + 1]) { // If there's another segment after this one
            const firstLine = transformScale(lineString(offsetLines[i]), 2); // transformScale is useful in case the two segment don't have an intersection point
            const secondLine = transformScale(lineString(offsetLines[i + 1]), 2); // Which happen when the resulting offset line is bigger than the initial one
            // We're calculating the intersection point between the two translated & scaled segments
            offsetCoords.push(lineIntersect(firstLine, secondLine).features[0].geometry.coordinates);
        } else offsetCoords.push(offsetLines[i][1]); // If there's no other segment after this one, we simply push the last point of the line
    }

    return lineString(offsetCoords);
};

[nzjony]

I had the same issue, I even tried the code above, i.e. the getLineOffset function and when compared to the lineOffset function, there is actually very little difference, see the attached screenshots. Note, this is using a mercator projection.

I then graphed the output of the above function getLineOffset in desmos and I get the following:

So the problem I’m having in my case is that I’m displacing the units in geographic coordinates and that shift doesn’t work in Mercator. So I will need to shift in mercator space. Unless I’ve missed something.