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diff --git a/ao-tools/altosui/AltosGreatCircle.java b/ao-tools/altosui/AltosGreatCircle.java
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+/*
+ * Copyright © 2010 Keith Packard <keithp@keithp.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+package altosui;
+
+import java.lang.Math;
+
+public class AltosGreatCircle {
+	double	distance;
+	double	bearing;
+
+	double sqr(double a) { return a * a; }
+
+	static final double rad = Math.PI / 180;
+	static final double earth_radius = 6371.2 * 1000;	/* in meters */
+
+	AltosGreatCircle (double start_lat, double start_lon,
+			  double end_lat, double end_lon)
+	{
+		double lat1 = rad * start_lat;
+		double lon1 = rad * -start_lon;
+		double lat2 = rad * end_lat;
+		double lon2 = rad * -end_lon;
+
+		double d_lon = lon2 - lon1;
+
+		/* From http://en.wikipedia.org/wiki/Great-circle_distance */
+		double vdn = Math.sqrt(sqr(Math.cos(lat2) * Math.sin(d_lon)) +
+				       sqr(Math.cos(lat1) * Math.sin(lat2) -
+					   Math.sin(lat1) * Math.cos(lat2) * Math.cos(d_lon)));
+		double vdd = Math.sin(lat1) * Math.sin(lat2) + Math.cos(lat1) * Math.cos(lat2) * Math.cos(d_lon);
+		double d = Math.atan2(vdn,vdd);
+		double course;
+
+		if (Math.cos(lat1) < 1e-20) {
+			if (lat1 > 0)
+				course = Math.PI;
+			else
+				course = -Math.PI;
+		} else {
+			if (d < 1e-10)
+				course = 0;
+			else
+				course = Math.acos((Math.sin(lat2)-Math.sin(lat1)*Math.cos(d)) /
+						   (Math.sin(d)*Math.cos(lat1)));
+			if (Math.sin(lon2-lon1) > 0)
+				course = 2 * Math.PI-course;
+		}
+		distance = d * earth_radius;
+		bearing = course * 180/Math.PI;
+	}
+}