topic Re: "Left-Handed" Geometry in a Spatial Dataframe (with shapely)-- Any way to fix? in ArcGIS API for Python Questions

"Left-Handed" Geometry in a Spatial Dataframe (with shapely)-- Any way to fix?

jcarlson — Wed, 23 Jun 2021 02:26:53 GMT

I'm trying to do some simple overlays between a couple of spatially enabled dataframes. The layers are coming from feature services, using query(... as_df=True).

When I attempt an overlay, I get the following message, or something like it:

TopologyException: Input geom 1 is invalid: Hole lies outside shell at or near point

Which, in many cases, is absolute nonsense, because the geometry in question has no holes. It's not even multipart!

A bit of digging reveals that this is an issue with the right-hand-rule, that the order of vertices in my polygons is causing this behavior. The thing is, I can't find any way to "reverse direction" on polygons, except maybe breaking apart the SHAPE column and reversing the arrays of vertices.

Before I go to all that trouble, I wanted to see if anyone else has encountered this. Any pointers or ideas?

EDIT: I'm in a custom python env, so my SDFs are coming in as shapely geometries. Arcpy is able to handle this, but if possible, I would like to keep this process workable with the ArcGIS Python API only. The resulting script is intended to be run in various settings and by different users, not all of whom are able to utilize arcpy.

Re: "Left-Handed" Geometry in a Spatial Dataframe (with shapely)-- Any way to fix?

DanPatterson — Wed, 23 Jun 2021 08:48:08 GMT

I would be surprised if the python api doesn't have a builtin method for doing this automatically, since having to use "orient" to change the polygon vertices order seems to be the way to go.

perhaps one of the other spatial representations does the reorientation behind the scenes

The Shapely User Manual — Shapely 1.7.1 documentation

pyqgis - Convert QGIS geometry into Shapely geometry to use orient method defined in Shapely - Geographic Information Systems Stack Exchange

Re: "Left-Handed" Geometry in a Spatial Dataframe (with shapely)-- Any way to fix?

jcarlson — Wed, 23 Jun 2021 15:15:34 GMT

I was surprised myself, but the only reference to polygon orientation in the API is:

Exterior rings are oriented clockwise, while holes are oriented counter-clockwise.

Simply a statement that they are oriented a certain way, but no mention of a way to re-orient a polygon, or what else to do when the orientation does not follow the proper order.

Orient is the thing I want, but annoyingly, shapely.polygon != shapely.Polygon. Also my features are being returned as MultiPolygon types, so lots of nested iteration to break those out and orient them correctly, then reassemble into a geometry series of multipolygons. Sheesh.

Re: "Left-Handed" Geometry in a Spatial Dataframe (with shapely)-- Any way to fix?

DanPatterson — Wed, 23 Jun 2021 19:30:28 GMT

this won't work shapely.polygon != shape.Polygon (I assume you are wanting to compare to arcpy

because shapely doesn't have M and Z defined by default.

I would just skip shapely and just indicate that geometry operations are open to arcpy people.

Re: "Left-Handed" Geometry in a Spatial Dataframe (with shapely)-- Any way to fix?

jcarlson — Wed, 23 Jun 2021 19:38:12 GMT

Ha, no, I just meant that the polygon submodule and the Polygon geometry object were literally not equal to one another, i.e., different things. (I should probably leave code conventions out of my narrative text).

The orient method takes a Polygon as an argument, so it would be

shapely.geometry.polygon.orient(Polygon(some WKT or something))

I'd hoped I could simply call orient on a Polygon object, Polygon.orient(), but no such luck. Definitely not comparing the actual shapes with anything else.