Calculating polyline azimuth - code error

[CamillaRootes]

Hi. I'm trying to run a Python script in the field calculator to output the azimuth values of a series of polylines. However, everytime I try to run the code it throws the following error:

"File "<string>", line 2, in GetAzimuthPolyline

AttributeError: 'str' object has no attribute 'x'

Failed to execute (CalculateField)."

The code block I am using is:

1. import math 
2. def GetAzimuthPolyline(shape): 
3. radian = math.atan((shape.lastpoint.x - shape.firstpoint.x)/(shape.lastpoint.y - shape.firstpoint.y)) 
4. degrees = radian * 180 / math.pi 
5. return degrees 

I sourced the code from this thread Calculate angle of each line in a polyline shapefile‌. It seems to be working for the people in the thread but not for me. I'm running Arc 10.1 with the basic license, by the way.

Can anyone see where this is going wrong?

[CamillaRootes]

Great, that's done it. Thanks to both of you for your help, much appreciated!

[DavidHarbor]

Thanks for the postings, but I just tried this on an NHD dataset looking at the azimuths of stream polylines.  It doesn't return the correct azimuth  The azimuth -90 to 90 works, but between 90 and 270 degrees are incorrect. The streams flow direction is reversed.  I am going to look at the math, and will post if I see an improved answer.

[DavidHarbor]

Here is some code that works for me, where the "answer" does not as shown in previous post.

def CalculaAzimuth(linea):

    if (hasattr(linea,'type') and linea.type == 'polyline'):

        xf = linea.firstPoint.X

        yf = linea.firstPoint.Y

        xl = linea.lastPoint.X

        yl = linea.lastPoint.Y

        dX = xl - xf

        dY = yl - yf

        PI = math.pi

        Azimuth = 0 #Default case, dX = 0 and dY >= 0

        if dX > 0:

            Azimuth = 90 - math.atan( dY / dX ) * 180 / PI

        elif dX < 0:

            Azimuth = 270 - math.atan( dY / dX )* 180 / PI

        elif dY < 0:

            Azimuth = 180

        return Azimuth

    else:

        return False

which comes from here

Syntax Errors when Calculating Azimuth with Python in Arcgis 10 - Stack Overflow

Wish I could make use of "atan2(y,x)" but I couldn't get it to return 0-360 correctly in all quadrants.

[BruceHarold]

Hi, try this:

	#Function to return the north azimuth bearing of a line segment in degrees
	def NorthAzimuth(x1,y1,x2,y2):
	degBearing = math.degrees(math.atan2((x2 - x1),(y2 - y1)))
	if (degBearing < 0):
	degBearing += 360.0
	return degBearing

[DavidHarbor]

Two questions Bruce

why reverse the y and x  in the atan function?

what is the operator +=  ?

(sorry, I'm a python noob)

[BruceHarold]

To get north azimuth angles; try it:

>>> for x1y1x2y2 in [(0,0,1,1),(0,0,1,-1),(0,0,-1,-1),(0,0,-1,1)]:

...     x1,y1,x2,y2 = x1y1x2y2[0],x1y1x2y2[1],x1y1x2y2[2],x1y1x2y2[3]

...     print str(math.degrees(math.atan2((x2 - x1),(y2 - y1))))

...   

45.0

135.0

-135.0

-45.0

The += is shorthand for degBearing = degBearing + 360.0

[DavidHarbor]

Sweet. Thanks.