New procedure to plot great circles on a map

[Liam Gumley <Liam.Gumley(at)ssec.wisc.edu>]

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

ftp://origin.ssec.wisc.edu/pub/gumley/great_circle.pro

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

ftp://origin.ssec.wisc.edu/pub/gumley/compass.pro

Cheers,
Liam.

---
Liam E. Gumley
Space Science and Engineering Center, UW-Madison
http://cimss.ssec.wisc.edu/~gumley