gistfile1.txt

var wgs2merc = function(lon,lat) {
    lon = parseFloat(lon);
    lat = parseFloat(lat);
    var x = lon * 20037508.34 / 180;
    var y = Math.log(Math.tan((90 + lat) * Math.PI / 360)) / (Math.PI / 180);
    y = y * 20037508.34 / 180;
    return [x,y];
}

I am running into some issues with this.

console.log(
     [doc.lon, doc.lat],  // the original wgs points
    wgs2merc(doc.lon, doc.lat),  // the merc points with this function
    proj.forward([doc.lon, doc.lat]) // the merc points from mapnik
);

These are some of the results.

[ '-104.9923', '39.74001' ] [ -11687689.371587677, -6491423.869377617 ] [ -11687689.373214617, 4828232.857062782 ]
[ '-103.1974', '40.63698' ] [ -11487882.017590644, -6336684.282449202 ] [ -11487882.01918977, 4958942.857859804 ]
[ '-103.5433', '37.97146' ] [ -11526387.429450678, -6805287.497430911 ] [ -11526387.431055164, 4575394.852208842 ]
[ '-105.0303', '39.7877' ] [ -11691919.512237232, -6483123.962506243 ] [ -11691919.513864761, 4835139.212464451 ]
[ '-104.5809', '38.30399' ] [ -11641892.5330817, -6745345.698014055 ] [ -11641892.534702264, 4622458.858854637 ]
[ '-122.0542', '37.97832' ] [ -13587011.391289044, -6804046.4251634665 ] [ -13587011.393180372, 4576363.608526837 ]
[ '-104.5137', '37.17283' ] [ -11634411.863301434, -6951092.813871358 ] [ -11634411.864920955, 4463224.517928745 ]
[ '-104.8895', '39.01105' ] [ -11676245.727935722, -6619339.337465406 ] [ -11676245.729561068, 4723254.513887428 ]
[ '-105.2677', '40.00346' ] [ -11718346.759347878, -6445676.65063133 ] [ -11718346.760979084, 4866445.090013317 ]
[ '-104.9594', '39.67668' ] [ -11684026.960341088, -6502458.5960280355 ] [ -11684026.961967519, 4819068.9359239 ]
[ '-106.9178', '38.54679' ] [ -11902035.051080288, -6701856.2786959605 ] [ -11902035.052737065, 4656959.487782303 ]
[ '-72.92149', '41.33078' ] [ -8117583.133556815, -6218920.194063545 ] [ -8117583.134686791, 5061254.750090784 ]
[ '115.8815', '-31.93685' ] [ 12899869.570565056, NaN ] [ 12899869.57236073, -3755024.0598698533 ]
value larger than maximum allowed value

i resolved it with parseFloat, but maybe that should be part of the function itself ?

sure, parseFloat sounds good.

also, heads up that mapnik's proj.forward may not be fully reliable. Ideally you should use mapnik's ProjTransform.forward not Projection.forward. Only the latter however is currently exposed in node-mapnik. I need to expose the ProjTransform. The problem with Projection.forward is that is makes assumptions about the source projection that may not be as robust as setting up the from->to relationship that is done in ProjTransform.

I'll knock out getting this exposed now: mapnik/node-mapnik#57