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Unprojected SAS map data sets have X and Y coordinates represented as
longitude and latitude in radians.  Mileage between two points in a map
data set may be calculated as follows:

Assume the longitude and latitude of point 1 are represented by X1 and
Y1, while point 2 is represented by X2 and Y2.  The distance is:

   DISTANCE = 4000*ARCOS(arc)
where:
   arc = SIN(Y2)*SIN(Y1) + COS(Y2)*COS(Y1)*COS(X2-X1)

  * The longitude for the western hemisphere should be negative, and the
    eastern should be positive.  The latitude in the northern
    hemisphere should be positive, and the southern should be negative.

  * If your coordinates are in degrees/minutes/seconds, first use this
    equation to calculate a real number from degrees/minutes/seconds
    and then convert to radians:

       VALUE = (degrees) + (minutes/60) + (seconds/3600);
       pi=4*atan(1);
       radians=value/(180/pi);

Another formula is the Great Circle Distance Formula which will
calculate the distance in miles between two pairs of latitude/longitude
values specified in radians:

   Dist = 3949.99 * arcos(sin(Y1) * sin(Y2) +
          cos(Y1) * cos(Y2) *
          cos(X1 - X2));

This formula more closely compensates for the curvature of the Earth.

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