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<html lang="en">
<head>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/proj4js/2.3.3/proj4.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/openlayers.js"></script>
<script type="text/javascript">
//Snippet comes from https://osgeo-org.1560.x6.nabble.com/OL-2-13-1-latest-Proj4js-td5081636.html
window.Proj4js = {
Proj: function(code) {
return proj4(Proj4js.defs[code]);
},
defs: proj4.defs,
transform: proj4
};
// Definition comes from https://epsg.io/31466/
proj4.defs["EPSG:31466"] = "+proj=tmerc +lat_0=0 +lon_0=6 +k=1 +x_0=2500000 +y_0=0 +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +units=m +no_defs";
console.log(Proj4js.defs["EPSG:31466"]);
var proj_wgs84 = new OpenLayers.Projection("EPSG:4326");
var proj_31466 = new OpenLayers.Projection("EPSG:31466");
// Center for projection EPSG:31466
var point = new OpenLayers.LonLat(2769212.70, 5678724.61);
var new_point= point.transform(proj_31466, proj_wgs84);
console.log(new_point.lat, new_point.lon);
</script>
</head>
<body>
</body>
</html>
Updated missing url //cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js to //cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/openlayers.js
https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.3.3/proj4.js
https://cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js