Encoded polyline

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The encoded polyline algorithm stores a line as a short string. The routing services return their routes in this format: Google Maps, OSRM and Valhalla. EncodedPolyline.Decode returns the positions, and EncodedPolyline.Encode encodes them:

// The geometry of a route returned by OSRM or by the Google Maps APIs
var positions = EncodedPolyline.Decode("_p~iF~ps|U_ulLnnqC_mqNvxq`@");

Assert.Equal(new GeoPosition(-120.2, 38.5), positions[0]);
var route = new GeoLineString(positions);
Assert.Equal(3, route.Positions.Count);

// Valhalla, and OSRM with geometries=polyline6, use 6 decimal digits
var encoded = EncodedPolyline.Encode(route.Positions, precision: 6);
Assert.Equal("_izlhA~rlgdF_{geC~ywl@_kwzCn`{nI", encoded);

The precision is the number of decimal digits of the coordinates: 5 for Google Maps and OSRM, 6 for Valhalla and the polyline6 geometries of OSRM. Decoding with the wrong precision gives positions 10 times too large or too small.
The format has no altitude: the altitudes are ignored when encoding.
Build a line with new GeoLineString(positions) to compute its length or write it to another format.