VMS Help FORTRAN, Intrinsic Procedures, ATAN2  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  ATAN2 (real-number, real-number)

  A function that returns the arc tangent of the quotient of the two
  arguments in radians.  If both arguments are zero, the result is
  undefined.  If the first argument is positive, the result is
  positive.  If the first argument is negative, the result is
  negative.  If the first argument is zero, the result is zero.  If
  the second argument is zero, the absolute value of the result is
  pi/2.

  +------+---------+----------+------------+-------------+
  | Args | Generic | Specific |  Argument  | Result Type |
  +------+---------+----------+------------+-------------+
  |  2   |  ATAN2  | ATAN2    | REAL*4     | REAL*4      |
  |      |         | DATAN2   | REAL*8     | REAL*8      |
  |      |         | QATAN2   | REAL*16    | REAL*16     |
  +------+---------+----------+------------+-------------+
  The range of the result is -pi < result < pi.