Calculating the inner angle

Question

I have some polygons and I want to calculate the inner angle of each polygon. Now, I have the geometry of every line of polygons in the same attribute table as shown in the following image.
I want to calculate the inner angle using the following equation:

inner_angle= the bearing angle of the previous line - the bearing angle of the following line +180 (in degrees).

How can I write this in the code block in the model builder and do this for each polygon respectively?

DanPatterson_Retired · Answer

hmmm, fun stuff ... modified hexagons, which followed the (n-2)*180 rule Curious as to what you are using this for? A convexity index? :---------------------------------------------------------------------- :Angle report.... .... .... snip .... .... (2) number of angles... (7) :array points... [[ 300025.98 5000035. ] [ 300034.96 5000033.03] [ 300043.3 5000035. ] [ 300043.3 5000025. ] [ 300034.64 5000020. ] [ 300025.98 5000025. ] [ 300025.98 5000035. ]] :interior angles [ 77.61 154.32 76.71 120. 120. 120. ] :sum interior... 668.641029246‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

DanPatterson_Retired · Answer

So.. if you want to give this a shot and you are working with single part shapes since I have experimented a whole 5 minutes. I pulled this from elsewhere and cobbled a framework around the base code. Actually it is more useful without the lines 30 and 31. It is designed to return all the angles of a poly* feature. If the feature is convex then it returns a boring 360 degrees for polygons. Concave closed-loop shapes should differ from 360 For polylines (or could be modified just to accept sequential point objects), you can get all the angles. The code can be modified ever so slightly to calculate the sequential differences in lengths and you can use those to derived 'weighted direction', but I will leave that out for this discussion. Yes... to call it use the expression angles_poly(!Shape!)‍ import numpy as np import arcpy def angles_poly(a, inside=True, in_deg=True): """Sequential angles from a poly* shape """ a = a.getPart() a =np.asarray([[i.X, i.Y] for j in a for i in j]) if len(a) < 2: return None elif len(a) == 2: # **** check ba = a[1] - a[0] return np.arctan2(*ba[::-1]) else: angles = [] if np.allclose(a[0], a[-1]): # closed loop a = a[:-1] r = (-1,) + tuple(range(len(a))) + (0,) else: r = tuple(range(len(a))) for i in range(len(r)-2): p0, p1, p2 = a[r[i]], a[r[i+1]], a[r[i+2]] ba = p1 - p0 bc = p1 - p2 cr = np.cross(ba, bc) dt = np.dot(ba, bc) ang = np.arctan2(np.linalg.norm(cr), dt) angles.append(ang) if in_deg: angles = np.degrees(angles) angle = np.sum(angles) return angle‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

XanderBakker · Answer

I would probably create a script tool that would be based on the following parameters: * the featureclass * the input polygon ID field * the input line bearing field * the "input" inner angle field (should exist or should be created by the tool) where the results will be written * the same input featureclass as output parameter defined so you can connect the output to other tools in your model The code would be something like this: def main(): import arcpy from collections import OrderedDict # read parameters fc = arcpy.GetParameterAsText(0) # r'C:\GeoNet\InnerAngle\data.gdb\angles' fld_pol_id = arcpy.GetParameterAsText(1) # 'polygonID' fld_bearing = arcpy.GetParameterAsText(2) # 'line_bearing' fld_inner = arcpy.GetParameterAsText(3) # 'inner_angle' # create a lst of the data fld_oid = 'OID@' flds = (fld_oid, fld_pol_id, fld_bearing) dct_data = OrderedDict((r[0], [r[1], r[2]]) for r in arcpy.da.SearchCursor(fc, flds)) # create a list with polygon IDs pol_ids = list(set([data[0] for oid, data in dct_data.items()])) # process the polygons and store results in dct dct_res = {} for pol_id in pol_ids: dct_bearing = OrderedDict((oid, data[1]) for oid, data in dct_data.items() if data[0] == pol_id) lst_bearings = dct_bearing.values() cnt = 0 for oid, bearing in dct_bearing.items(): if cnt == len(lst_bearings)-1: inner_angle = lst_bearings[cnt] - lst_bearings[0] + 180 else: inner_angle = lst_bearings[cnt] - lst_bearings[cnt+1] + 180 if inner_angle > 360: inner_angle -= 360 if inner_angle < 0: inner_angle += 360 dct_res[oid] = inner_angle cnt += 1 # update the fc with the results flds = (fld_oid, fld_inner) with arcpy.da.UpdateCursor(fc, flds) as curs: for row in curs: oid = row[0] if oid in dct_res: row[1] = dct_res[oid] curs.updateRow(row) # set output arcpy.SetParameterAsText(4, fc) if __name__ == '__main__': main() ‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍ This is what a test generated: The reason that this will not be possible using a simple feature iteration is that when writing the current feature the line bearing of the next feature must be known.