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New procedure to plot great circles on a map

A few days ago, I wrote:

>I seem to recall there is a way to draw great circles on a map. However
>the best I can come up with is
>map_set, /orth, /isot, /cont, /grid
>plots, [-45,-45], [45,-45]
>I really want a line between the two points that takes the shortest
>distance (in this case it would follow the longitude grid line).
>Any hints?

After no responses from the newsgroup (everyone must be outside enjoying
spring or fall?), I contacted RSI technical support, and the response
was that in IDL 4.0.1, you could provide just endpoints and the line
would conform to the map projection (I *knew* I remembered it from
somewhere!).  However, because of several requests to change it, a line
just drawn between two endpoints is a straight line in IDL 5.0 and
higher (who made this request?).

Anyway, I like a challenge, so I put together a procedure named
great_circle.pro that plots a great circle curve between a pair of
latitude/longitude points on an existing map projection. Great circles
crossing the international date line are handled correctly. You can find
it at


It requires the neat ESRG routine compass.pro which is available at



Liam E. Gumley
Space Science and Engineering Center, UW-Madison