Find angle between two lines features that share a point

Question

I'm trying to write a script that will find angles between polyline features. the polyline features always contain only two points (start and end).

A few sources indicates that the declination of a line can be calculated as so:

rads = math.atan2 (-dY, dX)

where dY is the difference in Y values and dX is the differece in X values of the start and end points. This seems to be where I'm failing.

Here's my code with comments:

import arcpy
import math

def GetAngle (p1, p2):
    x1, y1 = p1
    x2, y2 = p2
    dX = x2 = x1
    dY = y2 - y1
    rads = math.atan2 (-dY, dX) #wrong for finding angle/declination?
    return math.degrees (rads)

def LineToXYs (line): #return first and last coordinates
    firstX, firstY = (line.firstPoint.X, line.firstPoint.Y)
    lastX, lastY = (line.lastPoint.X, line.lastPoint.Y)
    return [(firstX, firstY), (lastX, lastY)]

def AngleFromLines (lines):
    #lines is a python list of line geometries that share a vertex
    for line1 in lines:
        for line2 in lines:
            if line1 == line2:
                continue
            line1StPnt, line1EndPnt = LineToXYs (line1) #get start and end xys for first line
            line2StPnt, line2EndPnt  = LineToXYs (line2) #get start and end xys for second line
            angle1 = GetAngle (line1StPnt, line1EndPnt) #calc angle - Doesn't work
            angle2 = GetAngle (line2StPnt, line2EndPnt) #calc angle - Doesn't work
            print "first line start and end coordinates:", line1StPnt, line1EndPnt
            print "second line start and end coordinates:", line2StPnt, line2EndPnt
            print "angle 1:", angle1
            print "angle 2:", angle2
            print "length 1:", line1.length
            print "length 2:", line2.length
            angle = abs (angle1 - angle2)
            print "angle between lines:", angle

AngleFromLines ([lineGeom1, lineGeom2]) #lineGeom* = arcpy polyline geometry object

For example, these two polyline geometries should return something around 270 degrees (or 90, also fine):

Instead I run the script and get:

first line start and end coordinates: (1974143.8040161133, 13562128.28540039) (1974120.358215332, 13562109.576782227)
second line start and end coordinates: (1974141.000427246, 13562131.794433594) (1974143.8040161133, 13562128.28540039)
angle 1: -0.000542988605953
angle 2: -0.000101843033066
length 1: 29.9952990964
length 2: 4.49148355856
angle between lines: 0.000441145572887

Any help would be great.

Answer 1

Your GetAngle function, as is, has a typo in computing delta X.

def GetAngle (p1, p2):
    x1, y1 = p1
    x2, y2 = p2
    dX = x2 = x1
    #...

It should be dX = x2 - x1. Is that the problem?