diff --git a/externs/topojson.js b/externs/topojson.js
new file mode 100644
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+++ b/externs/topojson.js
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+
+/**
+ * @fileoverview Externs for TopoJSON.
+ * @see https://github.com/mbostock/topojson/wiki/Specification
+ * @externs
+ */
+
+
+
+/**
+ * @constructor
+ */
+var TopoJSONTopology = function() {};
+
+
+/**
+ * @type {string}
+ */
+TopoJSONTopology.prototype.type;
+
+
+/**
+ * @type {TopoJSONTransform}
+ */
+TopoJSONTopology.prototype.transform;
+
+
+/**
+ * @type {Object.<string, TopoJSONGeometry>}
+ */
+TopoJSONTopology.prototype.objects;
+
+
+/**
+ * @type {!Array.<Array.<Array.<number>>>}
+ */
+TopoJSONTopology.prototype.arcs;
+
+
+
+/**
+ * @constructor
+ */
+var TopoJSONTransform = function() {};
+
+
+/**
+ * @type {!Array.<number>}
+ */
+TopoJSONTransform.prototype.scale;
+
+
+/**
+ * @type {!Array.<number>}
+ */
+TopoJSONTransform.prototype.translate;
+
+
+
+/**
+ * @constructor
+ */
+var TopoJSONGeometry = function() {};
+
+
+/**
+ * @type {string}
+ */
+TopoJSONGeometry.prototype.type;
+
+
+/**
+ * @type {string|number|undefined}
+ */
+TopoJSONGeometry.prototype.id;
+
+
+
+/**
+ * @constructor
+ * @extends {TopoJSONGeometry}
+ */
+var TopoJSONPoint = function() {};
+
+
+/**
+ * @type {!Array.<number>}
+ */
+TopoJSONPoint.prototype.coordinates;
+
+
+
+/**
+ * @constructor
+ * @extends {TopoJSONGeometry}
+ */
+var TopoJSONLineString = function() {};
+
+
+/**
+ * @type {!Array.<number>}
+ */
+TopoJSONLineString.prototype.arcs;
+
+
+
+/**
+ * @constructor
+ * @extends {TopoJSONGeometry}
+ */
+var TopoJSONPolygon = function() {};
+
+
+/**
+ * @type {!Array.<Array.<number>>}
+ */
+TopoJSONPolygon.prototype.arcs;
+
+
+
+/**
+ * @constructor
+ * @extends {TopoJSONGeometry}
+ */
+var TopoJSONMultiPoint = function() {};
+
+
+/**
+ * @type {!Array.<Array.<number>>}
+ */
+TopoJSONMultiPoint.prototype.coordinates;
+
+
+
+/**
+ * @constructor
+ * @extends {TopoJSONGeometry}
+ */
+var TopoJSONMultiLineString = function() {};
+
+
+/**
+ * @type {!Array.<Array.<number>>}
+ */
+TopoJSONMultiLineString.prototype.arcs;
+
+
+
+/**
+ * @constructor
+ * @extends {TopoJSONGeometry}
+ */
+var TopoJSONMultiPolygon = function() {};
+
+
+/**
+ * @type {!Array.<Array.<Array.<number>>>}
+ */
+TopoJSONMultiPolygon.prototype.arcs;