diff --git a/externs/proj4js.js b/externs/proj4js.js
new file mode 100644
index 0000000000..a6fa18768a
--- /dev/null
+++ b/externs/proj4js.js
@@ -0,0 +1,73 @@
+/**
+ * @externs
+ * @see http://trac.osgeo.org/proj4js/
+ */
+
+
+/**
+ * @type {Object}
+ */
+var Proj4js = {};
+
+
+/**
+ * @type {Object.<string, string>}
+ */
+Proj4js.defs;
+
+
+/**
+ * @type {function(string)}
+ */
+Proj4js.reportError;
+
+
+
+/**
+ * @constructor
+ * @param {number} x X.
+ * @param {number} y Y.
+ */
+Proj4js.Point = function(x, y) {};
+
+
+/**
+ * @type {number}
+ */
+Proj4js.Point.prototype.x;
+
+
+/**
+ * @type {number}
+ */
+Proj4js.Point.prototype.y;
+
+
+
+/**
+ * @constructor
+ * @param {string} srsCode SRS code.
+ * @param {Function=} callback Callback.
+ */
+Proj4js.Proj = function(srsCode, callback) {};
+
+
+/**
+ * @type {string}
+ */
+Proj4js.Proj.prototype.title;
+
+
+/**
+ * @type {string}
+ */
+Proj4js.Proj.prototype.units;
+
+
+/**
+ * @param {Proj4js.Proj} source Source projection.
+ * @param {Proj4js.Proj} dest Destination projection.
+ * @param {Proj4js.Point|{x:number, y:number}} point Point.
+ * @return {Proj4js.Point} Point.
+ */
+Proj4js.transform = function(source, dest, point) {};