I'm trying to calculate the angle between two line segments in ArcMap 10.0. I have a point shapefile, so I'm using the xy coordinates of 3 successive points to calculate the angle. I'm trying to use Python in the Field Calculator, because I'm not good with Python and I'm not sure how to use the Python window. I finally got my code to work, but encountered an error when two successive points have the exact same coordinates. This creates a null value, and I get the error "The field is not nullable". I tried to put in an if statement to change nulls to 0s, but I still get the same error. I'm not sure if I'm identifying null values correctly, though, because I've found conflicting ways to represent them, and have tried "None" and ' ' and "Null" to no avail. It seems like there should be some way to just replace the angle value with 0 if the calculation returns a null value.
Does anyone have any ideas on how to get past the null values?
My code is below.
Record = 0 def turnangle(X3, Y3): import math global X1, Y1, X2, Y2, Record if Record == 0: #first two rows aren't calculated angle = 0 X1 = X3 Y1 = Y3 elif Record == 1: angle = 0 X2 = X3 Y2 = Y3 else: #calculating angle BAx = X1 - X2 BAy = Y1 - Y2 BCx = X3 - X2 BCy = Y3 - Y2 dotprod = BAx * BCx + BAy * BCy lenA = math.sqrt((X1-X2)**2+(Y1-Y2)**2) lenB = math.sqrt((X2-X3)**2+(Y2-Y3)**2) angle = math.acos((dotprod)/(lenA*lenB))*180/math.pi X1 = X2 #advancing to next coordinates Y1 = Y2 X2 = X3 Y2 = Y3 Record = Record + 1 if angle is not None: #Here is where I'm trying to substitute 0 for null values return angle else: return 0