JavaScript Leaflet – Reproject WKT Polygon

coordinate systemjavascriptleafletwell-known-text

I have a polygon in WKT in EPSG:4326:

polygonWKT = 'POLYGON((13.215179443359373 52.588032137196755,13.42803955078125 52.592620721000365,13.454132080078125 52.46897854656702,13.347015380859375 52.45893824522763,13.215179443359373 52.588032137196755))';

How can I reproject this to EPSG:3857 with Leaflet or any other Javascript Library?

Best Answer

Terraformer can parse WKT, and it can project from Geographic Latlong to WebMercator, but it can't write a WKT.

However we can build the WKT string from projected polygon.

First refer to the terraformer-core and teraformer wkt parser libraries in your code, like this:

<script src="terraformer-1.0.3.min.js"></script>
<script src="terraformer-wkt-parser.min.js"></script>

Then you can use the following code to project. 

    function project(geoPolygonWKT) {
    var geoPoly = Terraformer.WKT.parse(geoPolygonWKT); //Create Polygon from WKT
    if (geoPoly.type != "Polygon") {
        throw "Not a Polygon";
        return "";
    }
    geoPoly.toMercator(); //project Polygon to Mercator
    var coord = geoPoly.coordinates;

    var projectedWKT = 'POLYGON(';

    var ringCount = coord.length;
    for (var i = 0; i < ringCount; i++) {
        var ring = coord[i];
        //ring starts; add opening bracket
        projectedWKT = projectedWKT + "(";
        var ptCount = ring.length;
        var coordList = "";
        for (var j = 0; j < ptCount; j++) {
            var pt = ring[j];
            //write the coordinates
            coordList = coordList + String(pt[0]) + " " + String(pt[1]) + ", ";
        }
        //remove the last comma (indicating end of coordinate)
        coordList = coordList.substring(0, coordList.lastIndexOf(','));
        //add to the WKT String
        projectedWKT = projectedWKT + coordList + "), ";
    }
    //remove the last comma (indicating end of ring)
    projectedWKT = projectedWKT.substring(0, projectedWKT.lastIndexOf(','));

    //closing bracket
    projectedWKT = projectedWKT + ")";
    console.log(projectedWKT); //<-- this will be your projected WKT
    return projectedWKT;
}
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