(User.programs)
Item: 377 by akcs.billb@hpcvbbs.cv.hp.com
Author: [William R. Brassfield]
  Subj: Great Circle Distance and Direction Find
  Keyw: latitude, longitude, distance, direction
  Date: Mon Mar 30 1992

To use this program, press the "GCIRC" menu option, and you will be
prompted for latitude and longitude data, for locations 1 and 2.  Enter
them in either degrees/decimal_degrees, or degrees/minutes/decimal_minutes,
or degrees/minutes/seconds (you must be consistent, however).  After this,
press the "GO" menu option, and you will be prompted for the format of your
latitude/longitude data.  Select and press the appropriate menu item.  The
great circle distance and INITIAL heading will then be displayed.  (Press
the VAR button to return to the normal stack display (or the ON button, but
the VAR button is preferred).  For two locations with the same latitude but
different longitudes, the direction will be either 90 degrees or 270
degrees, ONLY AT THE EQUATOR LATITUDE.  At higher (northern) latitudes, the
direction will be 90- or 270+; at lower (southern) latitudes, the direction
will be 90+ or 270-, depending on whether you want to travel east or west.
This is perfectly normal because only the equator latitude line represents
a "great circle" path.

This program can be quite useful for the pilot planning a cross-country
flight.  It saves a lot of time, especially when the departure and
destination locations are on different sectional charts.

It should be noted that this program does not account for the equatorial
"bulge" of the earth, which will cause some error (but probably not enough
to get worried about).  In this program, the earth is assumed to be a
perfect sphere.

Another note:  In a super-long-distance cross-country flight, a
great-circle path does not have a constant heading!  For example, if you
are on the 45 degree north latitude line, and you want to travel 2000 miles
east (destination also on 45 degree latitude line), then your initial
direction (no-wind heading) will be 79.67 degrees; at mid-point, your
direction will be 90.00 degrees, and as you arrive at your destination,
your direction will be 100.33 degrees.  This does not mean that you will be
turning the plane; in fact, you will be flying as straight a path as
possible for the given curvature of the earth.  Your reference to true
north, however, does change, and you will need to periodically check your
magnetic compass and magnetic variation on your sectional chart to keep
yourself on track.

Enough said about the program and how to use it.  Enjoy!

Oops!  Forgot to mention -- it's a directory called "GCIRCLE".  In it you
will find { GCIRCLE DEGS DMIN DMS LAT1 LON1 LAT2 LON2 HM-> MILES HEAD MESG
LA1 LO1 LA2 LO2 PN GO }

Northern latitudes are positive; southern ones are negative.  Western
longitudes are positive; eastern ones are negative.  All continental U.S.
latitudes and longitudes are, therefore, positive.