# Great Circle Calculator.

### By Ed Williams

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### Compute true course and distance between points.

Enter lat/lon of points, select distance units and earth model and click "compute". Lat/lons may be entered in DD.DD, DD:MM.MM or DD:MM:SS.SS formats.

Note that if either point is very close to a pole, the course may be inaccurate, because of its extreme sensitivity to position and inevitable rounding error.

 Lat1 Lon1 NS WE Lat2 Lon2 NS WE

 Course 1-2 Course 2-1 Distance

Distance Units: Earth model:

### Compute lat/lon given radial and distance from a known point

Enter lat/lon of initial point, true course and distance. Select distance units and earth model and click "compute". Lat/lons may be entered in DD.DD, DD:MM.MM or DD:MM:SS.SS formats.

Note that the starting point cannot be a pole.

 Lat1 Lon1 NS WE Course 1-2 Distance 1-2

 Lat2 Lon2

Distance Units: Earth model:

### Spheroid

Enter "user defined" spheroid parameters here:

Major radius a (km): Inverse flattening 1/f:

Get lat/lon information on: from Paulo Santos' Airnav site.

### History

6/8/03 Added option to get distance in feet

3/23/00 Added warning notes about not starting at poles. Changed default ellipsoid to WGS84

7/13/98 Removed a line of debugging code, giving irritating Javascript alerts, left over from the 5/16/98 changes

5/16/98 Changed the great circle radial and distance algorithm to one valid for unlimited distances. Previous was only valid for ranges up to one quarter of the earth's circumference in longitude. Note that the Vicenty algorithm for (ellipsoidal) course and distance may fail to converge correctly, or at all, if the final point is within 37' of arc of the antipodes of the first point.

5/12/98 Eccentricity of NAD27 (Clarke1866) ellipsoid obtained from my reference "Coordinate Systems and Map Projections" by D. H. Maling was in error. 1/f changed from 298.97866982 to 294.9786982138

11/9/97 Fixed bug where if the output lat/lon was DD 6.MMMM (ie 6 minutes), the result was incorrectly rounded up to the next degree.

3/4/97 Fixed bug where deg/minute entries 08 or 09 would be (mis)read in octal!

Ed Williams
72347.1516@CompuServe.COM