This function returns ATN(y/x) and is specific only to SQL Server. We have included it for compatibility.
From SQL Server documentation:
Returns the angle, in radians, between the positive x-axis and the ray from the origin to the point (y, x), where x and y are the values of the two specified float expressions.
The function may be simulated by using the following formula:
Atn2 = ATN(y / x)
SELECT ATN2(35.175643,129.44) RETURNS: 0.265344532064832