Usage
To use the map select the origin by positioning the cursor and holding the mouse button down, drag to destination and release. If the map appears cropped you may need to enlarge your browser window, or get a new browser......

To use the co-ordinate entry mode just enter the Latitude and longitude in degrees, minutes and seconds of both points and click . Empty, or out of range fields will be silently zeroed.

The distance is by default shown in Kilometres, that being what most of the world uses. Limeys and Yanks can chose miles from the pull down. Navigators may also like to display the distance in Nautical miles.

Geographic locations can be looked up at the excellent Getty Information reference or the Astro Dienst, online atlas.

Not got a true East/West bearing as expected ?
Perhaps you are wondering why the bearing between two locations at the same latitude do not come out as a compass heading of true West (090°) or East (270°). Well, the Earth is a sphere, and whilst it is true that the shortest distance between two points is a straight line, one would probably not want to have to bore a substantial tunnel to achieve this. The shortest distance across the surface of the earth is an arc of a great circle and this arc generally tends towards either the North or South pole.

If you still don't believe me, mark the poles and your two locations on an Orange and cut it exactly in half so that both locations are on the edge of the cut.....

Course and Bearing

Course from origin is the bearing one must travel when leaving the origin to begin the Great Circle route. On a Great Circle route the course normally* constantly changes throughout the journey. Traditionally ocean navigators recalculated their course every 12 hours whilst the auto-pilots on modern aircraft do so continously. This calculator recomputes the course every 200km (approximately)
I was contacted by muslim gentleman who had plotted the route to Mecca and was concerned that this was the direction he should face when praying - not so.
*The course will not vary along the route when both points are on either the equator (east-west), or the same line of longitude (north-south)

The course is displayed as both a True (geographic north) and Magnetic heading. The code to calculate the magnetic declination at origin is experimental and I would welcome comments regarding the accuracy of this. For the technically minded the calculator computes the current declination at the earth's surface using the IGRF2000 model.

Magnetic declination is a measure of the difference between geographic and magnetic north, and varies over both time and location. Details about magnetic declination and an online calculator, can be found at The Canadian Geological Survey. My thanks go to Larry Newitt there for providing some very useful pointers.

Please don't ask me to supply the course/bearing formula for free - I can't.

Source Code Is not available, for free anyhow. The Java API is now available for purchase. I used to mail the source of earlier version out to people who asked for it. I no longer do so however, owing to it finding it's way onto other sites without any acknowledgement. The code segment for the distance calculation is available on the Formula page.

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Logging

The calculator logs activity and information such as browser and JVM versions. This allows me to get some limited feedback on what it's being used for and to identify bugs. Some of this information is available in summary form on the statistics page.

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Contact the author, Andrew Gray