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When does Shapely's “polygonize_full” detect a dangle?

Finding Nearest Line Segments to Point using shapely?Equivalent function to shapely's envelope in ogr?Speed up row-wise point in polygon with GeopandasEnforce topology rule over a shapefileGDAL/OGR ogr.Layer.Intersection fails when producing mixed geometry resultsWhy is Shapely's snapping (GEO snaps) not working as expected?Make ST_Polygonize in Postgis return dangles, cuts and invalid ringsIntersection with Shapely and Fiona gives Key ErrorIdentifying road segments completely cut off by polygon using QGIS?Is this a bug in Shapely's ops.snap?

2

Shapely's polygonize_full operation, which merges line segments into polygons, returns four geometry collections: polygons, dangles, cuts and invalid ring lines. Dangles are defined in the documentation as "edges which have one or both ends which are not incident on another edge endpoint", while cuts are "connected at both ends but do not form part of polygon".

I assumed that the following code, which tries to polygonize a square with one edge missing, should find two dangles and one cut edge:

import shapely



lines = [

    ((0,0),(1,0)),

    ((1,0),(1,1)),

    ((1,1),(0,1))

]



polygons, dangles, cuts, invalids = shapely.ops.polygonize_full(lines)

However, the polygons, dangles and invalids collections are all empty, while the cuts collection contains the three original edges:

[str(cut) for cut in cuts]

# This returns ['LINESTRING (1 1, 0 1)', 'LINESTRING (1 0, 1 1)', 'LINESTRING (0 0, 1 0)']

When does polygonize_full detect a dangle? Is there a way to get the dangling edges in my test case (0 0, 1 0 and 1 1, 0 1) using Shapely?

python topology shapely geometry-conversion

share|improve this question

edited Jun 20 '17 at 20:34

Jake

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

add a comment | 

2

Shapely's polygonize_full operation, which merges line segments into polygons, returns four geometry collections: polygons, dangles, cuts and invalid ring lines. Dangles are defined in the documentation as "edges which have one or both ends which are not incident on another edge endpoint", while cuts are "connected at both ends but do not form part of polygon".

I assumed that the following code, which tries to polygonize a square with one edge missing, should find two dangles and one cut edge:

import shapely



lines = [

    ((0,0),(1,0)),

    ((1,0),(1,1)),

    ((1,1),(0,1))

]



polygons, dangles, cuts, invalids = shapely.ops.polygonize_full(lines)

However, the polygons, dangles and invalids collections are all empty, while the cuts collection contains the three original edges:

[str(cut) for cut in cuts]

# This returns ['LINESTRING (1 1, 0 1)', 'LINESTRING (1 0, 1 1)', 'LINESTRING (0 0, 1 0)']

When does polygonize_full detect a dangle? Is there a way to get the dangling edges in my test case (0 0, 1 0 and 1 1, 0 1) using Shapely?

python topology shapely geometry-conversion

share|improve this question

edited Jun 20 '17 at 20:34

Jake

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

add a comment | 

Shapely's polygonize_full operation, which merges line segments into polygons, returns four geometry collections: polygons, dangles, cuts and invalid ring lines. Dangles are defined in the documentation as "edges which have one or both ends which are not incident on another edge endpoint", while cuts are "connected at both ends but do not form part of polygon".

I assumed that the following code, which tries to polygonize a square with one edge missing, should find two dangles and one cut edge:

import shapely



lines = [

    ((0,0),(1,0)),

    ((1,0),(1,1)),

    ((1,1),(0,1))

]



polygons, dangles, cuts, invalids = shapely.ops.polygonize_full(lines)

However, the polygons, dangles and invalids collections are all empty, while the cuts collection contains the three original edges:

[str(cut) for cut in cuts]

# This returns ['LINESTRING (1 1, 0 1)', 'LINESTRING (1 0, 1 1)', 'LINESTRING (0 0, 1 0)']

When does polygonize_full detect a dangle? Is there a way to get the dangling edges in my test case (0 0, 1 0 and 1 1, 0 1) using Shapely?

python topology shapely geometry-conversion

share|improve this question

edited Jun 20 '17 at 20:34

Jake

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

import shapely



lines = [

    ((0,0),(1,0)),

    ((1,0),(1,1)),

    ((1,1),(0,1))

]



polygons, dangles, cuts, invalids = shapely.ops.polygonize_full(lines)

[str(cut) for cut in cuts]

# This returns ['LINESTRING (1 1, 0 1)', 'LINESTRING (1 0, 1 1)', 'LINESTRING (0 0, 1 0)']

share|improve this question

edited Jun 20 '17 at 20:34

Jake

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

share|improve this question

edited Jun 20 '17 at 20:34

Jake

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

asked Jun 20 '17 at 20:27

JakeJake

4,9202336

1 Answer
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This is an old question, but:
polygonize_full will not introduce (in my understanding) edges. Thus, it will not close a polygon. What it does is it generates polygons from line segments, introducing nodes where two segments intersect. Yet, they still need to be able to close the polygon with (sub)segments from the original input. In your input, the shape is a reversed "C", which is open, and forms no polygon.

share

answered 1 min ago

MartinTMartinT

38919

add a comment | 

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0

This is an old question, but:
polygonize_full will not introduce (in my understanding) edges. Thus, it will not close a polygon. What it does is it generates polygons from line segments, introducing nodes where two segments intersect. Yet, they still need to be able to close the polygon with (sub)segments from the original input. In your input, the shape is a reversed "C", which is open, and forms no polygon.

share

answered 1 min ago

MartinTMartinT

38919

add a comment | 

0

This is an old question, but:
polygonize_full will not introduce (in my understanding) edges. Thus, it will not close a polygon. What it does is it generates polygons from line segments, introducing nodes where two segments intersect. Yet, they still need to be able to close the polygon with (sub)segments from the original input. In your input, the shape is a reversed "C", which is open, and forms no polygon.

share

answered 1 min ago

MartinTMartinT

38919

add a comment | 

This is an old question, but:
polygonize_full will not introduce (in my understanding) edges. Thus, it will not close a polygon. What it does is it generates polygons from line segments, introducing nodes where two segments intersect. Yet, they still need to be able to close the polygon with (sub)segments from the original input. In your input, the shape is a reversed "C", which is open, and forms no polygon.

share

answered 1 min ago

MartinTMartinT

38919

share

answered 1 min ago

MartinTMartinT

38919

answered 1 min ago

MartinTMartinT

38919

answered 1 min ago

MartinTMartinT

38919