GeoJSON mapping discrepancies between d3 and Leaflet

Question

My objective is to use d3 to generate svg paths for each feature in a given GeoJSON feature collection.

When I map the paths using leaflet, all the features look perfect.

d3.json("ct_counties.geo.json", function(data) {
    var leaflet_paths = leaflet_map.addLayer(new L.GeoJSON(data));
});

But when I map the paths using d3, some of the features look wrong.

d3.json("ct_counties.geo.json", function(collection) {
    var bounds = d3.geo.bounds(collection);

    var path = d3.geo.path().projection(project);

    var feature = g.selectAll("path")
        .data(collection.features)
      .enter().append("path")
        .attr('class','county');

    d3_map.on("viewreset", reset);

    reset();

    function project(x) {
      var point = d3_map.latLngToLayerPoint(new L.LatLng(x[1], x[0]));
      return [point.x, point.y];
    }

    function reset() {
      var bottomLeft = project(bounds[0]); 
      var topRight = project(bounds[1]);
      svg.attr("width", topRight[0] - bottomLeft[0])
        .attr("height", bottomLeft[1] - topRight[1])
        .style("margin-left", bottomLeft[0] + "px")
        .style("margin-top", topRight[1] + "px");
      g.attr("transform", "translate(" + -bottomLeft[0] + "," + -topRight[1] + ")");
      feature.attr("d", path);
    }
});

View the map differences here.

And refer to the full code here.

Since both maps use the same feature collection, why is the d3 version wrong?

Answer 1

D3 is not wrong, the data is incorrect and Leaflet is more lenient.

Taking Litchfield (top left county) as an example:

{
    "type" : "Feature",
    "properties" : {
        "kind" : "county",
        "name" : "Litchfield",
        "state" : "CT"
    },
    "geometry" : {
        "type" : "MultiPolygon",
        "coordinates" : [ [ [ [ -73.0535, 42.0390 ], [ -73.0097, 42.0390 ],
                [ -73.0316, 41.9678 ], [ -72.8892, 41.9733 ],
                [ -72.9385, 41.8966 ], [ -72.9495, 41.8090 ],
                [ -73.0152, 41.7981 ], [ -72.9823, 41.6392 ],
                [ -73.1631, 41.5571 ], [ -73.1576, 41.5133 ],
                [ -73.3219, 41.5078 ], [ -73.3109, 41.4694 ],
                [ -73.3876, 41.5133 ], [ -73.4424, 41.4914 ],
                [ -73.4862, 41.6447 ], [ -73.5191, 41.6666 ],
                [ -73.4862, 42.0500 ] ] ] ]
    }
}

The multipolygon is not closed, its end does not equal the beginning. I've plotted the coordinates, marking the first coordinate red, and the last one green:

As you can see, the last coordinate gets discarded by d3.

The GeoJSON specification says

A LinearRing is closed LineString with 4 or more positions. The first and last positions are equivalent (they represent equivalent points).

So d3 has a point (no pun intended) and the MultiPolygon should be closed by adding the start coordinate at the end:

...[ -73.4862, 42.0500 ], [ -73.0535, 42.0390 ] ] ] ]