+#ifndef _MAGVAR_HXX
+#define _MAGVAR_HXX
+
+
+#ifndef __cplusplus
+# error This library requires C++
+#endif
+
+
+// forward decls
+class SGGeod;
+
+/**
+ * Magnetic variation wrapper class.
+ *
+ * The SGMagVar class calculates the magnetic variation and dip for
+ * any position, altitude, and time. It is a complete
+ * re-implimentation of the NIMA WMM 2000 (not derived from their demo
+ * code.)
+ *
+ * The SGMagVar class is really a simple wrapper around the core Ed
+ * Williams code which does all the hard work. This class allows you
+ * to crunch the math once and then do multiple polls of the
+ * data. However, if your position, altitude, or time has changed
+ * significantly, you should call the update() method to recrunch new
+ * numbers.
+ */
+class SGMagVar {
+
+private:
+
+ double magvar;
+ double magdip;
+
+public:
+
+ /**
+ * This creates an instance of the SGMagVar object.
+ * You must call the update() method before any queries will be valid.
+ */
+ SGMagVar();
+
+ /** Destructor */
+ ~SGMagVar();
+
+ /** Recalculate the magnetic offset and dip.
+ * The update() method requires you to pass in your position and
+ * the julian date. Lon and lat are specified in radians, altitude
+ * is specified in meters. Julian date can be conveniently
+ * calculated by using an instance of the SGTime class.
+ * @param lon longitude in radians
+ * @param lat latitude in radians
+ * @param alt_m altitude above sea level in meters
+ * @param jd julian date
+ */
+ void update( double lon, double lat, double alt_m, double jd );