topic Re: Is a polygon with a hole a multipart polygon? in Python Questions

Is a polygon with a hole a multipart polygon?

EidurEidsson — Sat, 11 Dec 2021 15:31:53 GMT

Consider the following script, the polygon is a simple one with a hole inside:

import arcpy

poly1 = arcpy.FromWKT("POLYGON ((35 10, 45 45, 15 40, 10 20, 35 10), (20 30, 35 35, 30 20, 20 30))")

print(poly1.isMultipart)
print(poly1.partCount)‍‍‍‍‍‍‍‍‍‍‍‍

The result I get is

>>>
True
1

So is a polygon "multipart" if it has holes? ArcGIS Data Reviewer does not agree.

Thanks

ArcGIS Desktop 10.5.1

Re: Is a polygon with a hole a multipart polygon?

DanPatterson_Retired — Tue, 09 Jan 2018 19:43:52 GMT

it is a multipart and the count is 1. one point ring is clockwise, the other is clockwise, hence the hole

To experiment, repeat the experiment with both point rings oriented in a clockwise order

Re: Is a polygon with a hole a multipart polygon?

EidurEidsson — Wed, 10 Jan 2018 11:11:36 GMT

Outer–inner: CCW–CW, CCW–CCW, CW–CW, CW–CCW, all same result.

The WKT is first converted to an arcpy geometry object, which I don't know how is structured exactly. If they are printed as WKT they all become CCW–CW multipolgygons. What I can read from the documentation is that isMultipart should agree with partCount.

Re: Is a polygon with a hole a multipart polygon?

XanderBakker — Sat, 11 Dec 2021 15:31:56 GMT

I kinda agree that a polygon with a hole that contains only 1 part should not be a multipart. Maybe it is implemented this way since the polygon object does not have a "hasHoles" property (or something similar). To understand how ArcGIS stores the geometry you can run something like this:

import arcpy

poly1 = arcpy.FromWKT("POLYGON ((35 10, 45 45, 15 40, 10 20, 35 10), (20 30, 35 35, 30 20, 20 30))")

print(poly1.isMultipart)
print(poly1.partCount)

cnt_part = 0
for part in poly1:
    cnt_part += 1
    print("Part: {}".format(cnt_part))
    cnt_pnt = 0
    for pnt in part:
        cnt_pnt += 1
        print(" - Pnt {}: {}".format(cnt_pnt, pnt))‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

It will yield this:

True
1
Part: 1
 - Pnt 1: 35 10 NaN NaN
 - Pnt 2: 10 20 NaN NaN
 - Pnt 3: 15 40 NaN NaN
 - Pnt 4: 45 45 NaN NaN
 - Pnt 5: 35 10 NaN NaN
 - Pnt 6: None
 - Pnt 7: 20 30 NaN NaN
 - Pnt 8: 30 20 NaN NaN
 - Pnt 9: 35 35 NaN NaN
 - Pnt 10: 20 30 NaN NaN‍‍‍‍‍‍‍‍‍‍‍‍‍

As you can see Pnt 6 is None and indicates the start of a hole.

Re: Is a polygon with a hole a multipart polygon?

EidurEidsson — Wed, 10 Jan 2018 12:12:05 GMT

Then I suppose it is "multipart" if there exists a break in the sequence, no matter what comes after it. That might be useful for some tasks, but this meaning is quite different from what the description implies.

Thanks

Re: Is a polygon with a hole a multipart polygon?

VinceAngelo — Wed, 10 Jan 2018 13:59:00 GMT

In truth, a polygon with a hole is multi-ring, but not multi-part. If the proper WKT starts with "POLYGON" not "MULTIPOLYGON", it is single part, with one or more interior rings (which is why even a single-ring polygon has two left and right parens - "(( ... ))"). The Geometry object method is poorly documented on this score.

- V

Re: Is a polygon with a hole a multipart polygon?

VinceAngelo — Wed, 10 Jan 2018 15:08:50 GMT

All geometry objects are ingested from Well-Known Text using the SgShape geometry library, which has a default "Left Hand Rule" storage orientation*. No matter how you incorrectly wrap the polygons, if the import succeeds, the result polygon will be native left-hand rule (with the polygon shells corrected, if necessary, so that holes follow the outer rings to which they belong, with opposite ring wrapping direction).

- V

*Not to be confused with what Google calls "Right Hand Rule" which is the same orientation, but as described with the hand over the "upward" face of the shape with the thumb pointing out, and fingers curling in the direction of outer ring rotation.

Re: Is a polygon with a hole a multipart polygon?

JoshuaBixby — Wed, 10 Jan 2018 15:14:10 GMT

Both Dan Patterson‌ and myself have touched on this topic here and there on GeoNet in the past. You might find a blog post I a couple years back informative: /blogs/tilting/2016/02/20/the-single-multipart-polygons-with-interior-boundaries.

I spent some time trying the example above in different spatial systems, even other Esri software components outside of ArcPy, and I have come to the conclusion the real problem is in the labels themselves. Specifically, the problem lies in the use of "part" to talk about geometries. Take a look at the List of SQL functions for Esri's ST_Geometry data type, the OGC Methods on Geometry Instances for Microsoft's geometry data type, or the PostGIS Reference; you will not find a single function, method, or property with the word "part" in it. There are accessor functions for counting geometries, but they count "geometries" and not "parts." There are accessor functions for counting interior rings, but they count "interior rings" and not "parts." You see where this is going.

The problem with the ArcPy Geometry classes, at least one of them, is that Esri replaced multiple, specific geometry components with a single generic one, the "part." By abstracting geometries and rings with parts, not only did they deviate from geospatial standards and norms, they created the sideshow-worthy multi-part single-part polygon.

Re: Is a polygon with a hole a multipart polygon?

XanderBakker — Thu, 11 Jan 2018 00:52:36 GMT

Hi Dan Patterson‌, Vince Angelo‌ and Joshua Bixby‌,

I made a note at the help page with a link to this thread today and Jim Barry‌ contacted me directly after, asking what a good description could be or what should be changed at the help page (or if this is something that should be changed in the software)?

IMHO I think that when you talk about parts (in case of a polygon) this would refer to the number of individual disconnected areas in a single feature, and should not refer to "parts" of an inferior geometry dimension (like rings or lines). Maybe an addition property like "hasHoles" could be useful, however this could lead to unfortunate misspellings .

Since I consider that each one of you has more experience in this field, my question to you guys is, what suggestions do you have to obtain a clear explanation at the help page or is there something that should be changed in the behavior of the software?

Re: Is a polygon with a hole a multipart polygon?

DanPatterson_Retired — Thu, 11 Jan 2018 02:04:00 GMT

I agree, multipart should be separate, clockwise oriented geometry, whether they overlap or not (which is possible).

A counterclockwise ring within a clockwise ring should be 'hasholes' and that the number of 'holes' should be tracked at the same time as the number of parts.

In this way I could have a 3 part polygon, the first part could have 2 holes, the second part might have no holes, while the third part could have any number of holes (0 to n)

I would want to know this without me having to go through hoops, which I currently do.

It may not be as 'cool' as other stuff... but this is geometry, and fundamentally it is geometry that is used to answer questions.

Vector geometry has been ignored for too long. I can't even draw a 3D vertical cliff in 3D so that an xy coordinate can have 2 or more z-values. Not extrusion, ... actual geometry.

Rings, holes, they are the tip of the iceberg, but we should start somewhere.

Changes?????

Help file ...definitely.

Software behaviour... definitely

Re: Is a polygon with a hole a multipart polygon?

EidurEidsson — Thu, 11 Jan 2018 10:34:12 GMT

I suspect isMultipart (and its description) is inherited from Geometry, hence the problem (it works fine with multipoints and polylines! why not polygons too?).

A soft landing would be:

1) Keep partCount as it is

2) Keep isMultipart but change the description.and mark as deprecated

3) Add 0...* replacements

Hard landing (if I'm right about the inheritance):

Override isMultipart to exclude interior rings; keep the description

Re: Is a polygon with a hole a multipart polygon?

JoshuaBixby — Thu, 11 Jan 2018 13:50:21 GMT

Xander Bakker‌, I love the Freudian slip

"parts" of an inferior geometry dimension (like rings or lines)

Re: Is a polygon with a hole a multipart polygon?

DanPatterson_Retired — Thu, 11 Jan 2018 13:53:27 GMT

missed that one... very good Xander

Re: Is a polygon with a hole a multipart polygon?

JoshuaBixby — Thu, 11 Jan 2018 14:37:34 GMT

Xander Bakker‌, from what you have written, Jim B. appears open to changing the documentation, not the software/code itself. At this point I would be surprised to see any core updates/changes to the ArcPy Geometry classes themselves. ArcPy has been around long enough that changing the behavior of any existing property or method would create a backwards compatibility nightmare for them.

One of my biggest laments with ArcGIS Pro is that Esri didn't use it as an opportunity to really move ArcPy forward, ArcPy Pro if you will. Sure, the mapping module was redone because it had to be given the massive GUI changes, but so much of the rest of ArcPy was just forklifted over. Instead of ArcPy Pro, we got ArcGIS API for Python. Given the ever-growing overlap in functionality, it would be nice if Esri would share a coherent plan for both APIs moving forward, but we all know that is a bridge too far.

Even on the documentation front, it is a bit of a mess. I suspect the ArcPy isMultiPart property came from IGpDescribeGeometry.IsMultipart Property (ArcObjects .NET 10.5 SDK). Looking at the entire description for that property, it is as ambiguous and vague as ArcPy's documentation for that property:

Indicates whether the geometry object contains more than one part.

The IPolygon Interface (ArcObjects .NET 10.5 SDK) is fairly robust and includes properties like ExteriorRingCount and InteriorRingCount; but for whatever reasons, the level of nuance was lost in the ArcPy Geometry classes and we ended up with "parts." I think there are some flaws deep in the ArcPy concept/model and code on this one. Since I don't see Esri addressing the root issue(s), I think the best we can hope for is a footnote of sort that explains the edge case results.

Re: Is a polygon with a hole a multipart polygon?

XanderBakker — Thu, 11 Jan 2018 20:11:48 GMT

Hi Joshua Bixby , thanks for the extensive response and links provided. You are right, Jim did not mention that the software can be easily adjusted. For now an additional note in the help page to indicate that a polygon with one part (partCount=1) may be a multi-part (isMultipart=True) when the polygon contains one or more holes, since it will be based on more than one ring. At least that way it will reduce the confusion for end-users. I would like to see properties like ExteriorRingCount and InteriorRingCount later on in the API. That would be a great addition.

So, any suggestions for a text that could be added to the help of the isMultipart property of the polygon object as an additional note?

Re: Is a polygon with a hole a multipart polygon?

VinceAngelo — Fri, 12 Jan 2018 14:54:11 GMT

isMultipart appears to be broken for polygons (or at least using a different definition of "multipart polygon" than any other Esri component has ever used). At a minimum, this difference needs to be documented, with a workaround for obtaining the conventional meaning in a reliable manner (using partCount or WKT type). Unfortunately, large polygons (e.g., 1:100k Canada/US/Russia/Australia/Brazil) are very slow in generating Well-Known Text, so using the .WKT method to test for POLYGON/MULTIPOLYGON is a really inefficient means if determining "multi-ness".

- V

Re: Is a polygon with a hole a multipart polygon?

XanderBakker — Sat, 11 Dec 2021 15:31:59 GMT

Not sure how it performs on a large "multi-part" geometry with many vertices, but on the small example above one could do this to count the rings:

ring_count = len((eval(poly1.JSON.replace(u'null', u'0')))["rings"])
‍

Re: Is a polygon with a hole a multipart polygon?

DanPatterson_Retired — Sat, 11 Dec 2021 15:32:01 GMT

let's not forget, that there are bigger problems with the geometry model... a null point has X and Y coordinates of

import arcpy

arcpy.Point()
<Point (0.0, 0.0, #, #)>‍‍‍‍

maybe the basics before rings

Re: Is a polygon with a hole a multipart polygon?

VinceAngelo — Fri, 12 Jan 2018 16:07:05 GMT

That's not a null point so much as a nominal Point. A null point would be a PointGeometry with value of None.

Point corresponds to SE_POINT3D in the 'C' API, which is a simple struct type

Re: Is a polygon with a hole a multipart polygon?

DanPatterson_Retired — Fri, 12 Jan 2018 16:13:56 GMT

I know it is nominally null I just prefer Point(nan, nan, nan, nan)