Try harder to get the projection from GeoTIFF headers

src/ol/proj/Units.js

@@ -8,6 +8,11 @@
  * @enum {string}
  */
 const Units = {
+  /**
+   * Radians
+   * @api
+   */
+  RADIANS: 'radians',
   /**
    * Degrees
    * @api
@@ -40,6 +45,26 @@ const Units = {
   USFEET: 'us-ft',
 };
 
+/**
+ * See http://duff.ess.washington.edu/data/raster/drg/docs/geotiff.txt
+ * @type {Object<number, Units>}
+ */
+const unitByCode = {
+  '9001': Units.METERS,
+  '9002': Units.FEET,
+  '9003': Units.USFEET,
+  '9101': Units.RADIANS,
+  '9102': Units.DEGREES,
+};
+
+/**
+ * @param {number} code Unit code.
+ * @return {Units} Units.
+ */
+export function fromCode(code) {
+  return unitByCode[code];
+}
+
 /**
  * Meters per unit lookup table.
  * @const
@@ -48,6 +73,7 @@ const Units = {
  */
 export const METERS_PER_UNIT = {};
 // use the radius of the Normal sphere
+METERS_PER_UNIT[Units.RADIANS] = 6370997 / (2 * Math.PI);
 METERS_PER_UNIT[Units.DEGREES] = (2 * Math.PI * 6370997) / 360;
 METERS_PER_UNIT[Units.FEET] = 0.3048;
 METERS_PER_UNIT[Units.METERS] = 1;