ArcMap Field Calculator - Reverse Line Azimuth Angle

reverseAzimuth.py

# This script is used to calculate the segment angle for CE-QUAL-W2
# My segment streamline was drawn going downstream
# The CE-QUAL input wants radians east of north (CW), pointing upstream
# atan2 results are north of east (CCW), the sign change makes it CW 
# adding pi/2 gives the reverse azimuth, subtracting would be azimuth
# In ArcMap Field Calculator using Python Parser:
# Pre-Logic Script Code:
def reverseAzimuth(shp):
  a = -math.atan2(shp.firstPoint.Y - shp.lastPoint.Y, shp.firstPoint.X - shp.lastPoint.X) + math.pi/2
  if a < 0:
    return a + math.pi*2
  else:
    return a
  
# field = 
reverseAzimuth (!Shape!)