How to find a minimum angle in polygon?

Question

Is it possible to calculate the minimum angle in a polygon? I have several polygons that have sharp edges, as in the attached picture, and I would like to indicate them automatically. For example, can I create a column in the attribute table with minimum angle values in a polygon?

ArcMap.PNG

KK2 · Answer

Thank you for the answer. I have found an easy solution in QGIS: Vector > Geometry Tools >Check Geometries > Minimum angle between segments (deg). After its finished you can export a point shapefile indicating places where polygons have lower angle than a specified threshold, and the ID of the original polygons.

DanPatterson_Retired · Answer

PS, you picture is for polygons, so moving to Python since this isn't a Spatial Analyst thread. Feed polygon shapes to 'angles_poly'. Numpy required Polygons can be converted to numpy arrays using. This is the implementation for a whole featureclass converted to a numpy object array def poly2array(polys): """Convert polyline or polygon shapes to arrays for use in the Geo class. Parameters ---------- polys : tuple, list Polyline or polygons in a list/tuple """ def _p2p_(poly): """Convert a single ``poly`` shape to numpy arrays or object""" sub = [] for arr in poly: pnts = [[pt.X, pt.Y] if pt else null_pnt for pt in arr] sub.append(np.asarray(pnts)) return sub # ---- if not isinstance(polys, (list, tuple)): polys = [polys] out = [] for poly in polys: out.append(_p2p_(poly)) return out‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍ In a script, a search cursor can be used to get a polygon, and it can be converted using _p2p_ in the above.. def _p2p_(poly): """Convert a single ``poly`` shape to numpy arrays or object""" sub = [] for arr in poly: pnts = [[pt.X, pt.Y] if pt else null_pnt for pt in arr] sub.append(np.asarray(pnts)) return sub‍‍‍‍‍‍‍‍‍‍‍‍‍‍ def angles_poly(a=None, inside=True, in_deg=True): """Sequential 3 point angles from a poly* shape Parameters ---------- a : array an array of points, derived from a polygon/polyline geometry inside : boolean determine inside angles, outside if False in_deg : boolean convert to degrees from radians Sample data: the letter C >>> a = np.array([[ 0, 0], [ 0, 100], [100, 100], [100, 80], [ 20, 80], [ 20, 20], [100, 20], [100, 0], [ 0, 0]]) >>> angles_poly(a) # array([ 90., 90., 90., 270., 270., 90., 90.]) """ if len(a) < 2: return None if len(a) == 2: ba = a[1] - a[0] return np.arctan2(*ba[::-1]) dx, dy = a[0] - a[-1] if np.allclose(dx, dy): # closed loop a = a[:-1] a0 = np.roll(a, -1, 0) a1 = a a2 = np.roll(a, 1, 0) ba = a1 - a0 bc = a1 - a2 cr = np.cross(ba, bc) dt = np.einsum('ij,ij->i', ba, bc) ang = np.arctan2(cr, dt) two_pi = np.pi*2. if inside: ang = np.where(ang < 0, ang + two_pi, ang) else: ang = np.where(ang > 0, two_pi - ang, ang) if in_deg: angles = np.degrees(ang) return angles # ---- demo --- a polygon "C" import numpy as np a = np.array([[ 0, 0], [ 0, 100], [100, 100], [100, 80], [ 20, 80], [ 20, 20], [100, 20], [100, 0], [ 0, 0]]) angles = angles_poly(a, inside=True, in_deg=True) min(angles) 90.0 angles array([270., 270., 270., 270., 90., 90., 270., 270.]) ‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍ So the code would entail open a search cursor convert each shape to an array using _p2p_ call angles_poly take its minimum

JB1234510 · Answer

To find sharp edges you can convert the polygons to raster, boundary clean, reconvert to polygon, buffer with a appropriate distance and clip?