topic Re: Calculating polyline azimuth - code error in Python Questions

Calculating polyline azimuth - code error

CamillaRootes — Tue, 05 Aug 2014 08:43:14 GMT

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:

import math
def GetAzimuthPolyline(shape):
radian = math.atan((shape.lastpoint.x - shape.firstpoint.x)/(shape.lastpoint.y - shape.firstpoint.y))
degrees = radian * 180 / math.pi
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?

Re: Calculating polyline azimuth - code error

DanPatterson_Retired — Tue, 05 Aug 2014 09:33:09 GMT

A screen grab of what you are using would help since the code block seems incomplete and improperly formatted. Also, you are running this with the shapefield as input?

Re: Calculating polyline azimuth - code error

CamillaRootes — Tue, 05 Aug 2014 09:44:27 GMT

Here are some screenshots of what I've been doing:

And this is the error that I am getting:

Re: Calculating polyline azimuth - code error

DanPatterson_Retired — Tue, 05 Aug 2014 10:25:15 GMT

See the attached image, you were missing the bottom part of the dialog I think. I am using the field calculator which is much quicker

Also ensure that your field is a Double field with the appropriate width and precision.

Re: Calculating polyline azimuth - code error

CamillaRootes — Tue, 05 Aug 2014 10:43:28 GMT

By the bottom part, do you mean the "test = test(!shape!)" bit? Should I put this below my code block or in the 'Expression' box?

I had been using a Float field, so have made a Double field called 'Angle' to see if this works better. But Arc keeps changing 'Angle' back to a Float, any idea why it won't let me use a Double field?

Re: Calculating polyline azimuth - code error

RiyasDeen — Tue, 05 Aug 2014 10:46:26 GMT

hi Camilla,

What i see from your screen shot is you are trying to calculate value for field Shape and you are passing your shape input as Shape field again.

What is the data type of Shape field? is it a number or geometry?

Re: Calculating polyline azimuth - code error

CamillaRootes — Tue, 05 Aug 2014 10:55:50 GMT

Hi, yeah I realised that I was doing that. Hence creating the new 'Angle' field because the 'Shape' field was geometry. But when I try running it to fill the 'Angle' field I now get a generic syntax error.

Re: Calculating polyline azimuth - code error

DanPatterson_Retired — Tue, 05 Aug 2014 11:01:54 GMT

Last kick. Add a double field, precision 16, scale 4

emulate the field calculator expression before heading to the tool in arctoolbox

Re: Calculating polyline azimuth - code error

CamillaRootes — Tue, 05 Aug 2014 11:02:29 GMT

Ah ha! It's just worked. This is the set up that worked:

The only problem is that it is giving me a negative output (-61 degrees in the case of the tester polyline that I used). How can I make the code output the azimuth in the familar True North 0-360 degree output?

Re: Calculating polyline azimuth - code error

RiyasDeen — Tue, 05 Aug 2014 11:31:06 GMT

def GetAzimuthPolyline(shape):

radian = math.atan((shape.lastPoint.X - shape.firstPoint.X)/(shape.lastPoint.Y - shape.firstPoint.Y))

degrees = math.degrees(radian)

if degrees < 0:

return degrees + 360

else:

return degrees

Re: Calculating polyline azimuth - code error

CamillaRootes — Tue, 05 Aug 2014 11:57:26 GMT

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

Re: Calculating polyline azimuth - code error

DavidHarbor — Wed, 18 Feb 2015 21:52:29 GMT

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.

Re: Calculating polyline azimuth - code error

DavidHarbor — Thu, 19 Feb 2015 20:37:17 GMT

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.

Re: Calculating polyline azimuth - code error

BruceHarold — Thu, 19 Feb 2015 20:47:59 GMT

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

Re: Calculating polyline azimuth - code error

DavidHarbor — Thu, 19 Feb 2015 21:04:36 GMT

Two questions Bruce

why reverse the y and x in the atan function?

what is the operator += ?

(sorry, I'm a python noob)

Re: Calculating polyline azimuth - code error

BruceHarold — Thu, 19 Feb 2015 21:35:10 GMT

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

Re: Calculating polyline azimuth - code error

DavidHarbor — Thu, 19 Feb 2015 21:50:22 GMT

Sweet. Thanks.