I am not a doctor of math, but I notice that this description of IDL's ATAN function from IDL Online Help: "For real input, the range of ATAN is between -pi/2 and pi/2 for the single argument case, and between -pi and pi if two arguments are given." matches, in the two-argument case, the definition of 'atan2'. So, I believe that IDL's ATAN performs both the standard 'atan' function as well as the 'atan2' function. James Jones