Geometry Validation: Catching polygons with less than 3 unique vertices

[joshuacortez]

This could be a feature to consider for Geowrangler Geometry Validation

I encountered an error where I tried to upload a geopandas dataframe to BQ and it said

GenericGBQException: Reason: 400 Error while reading data, error message: Invalid geography value for column 'geometry', error: Polygon loop should have at least 3 unique vertices, but only had 2; in WKB geography

It turns out there was a "polygon" that was actually a line. I verified it by computing the area which was actually 0.

'POLYGON ((122.95320551089915 11.473736609261481, 122.952381 11.4737421, 122.95320551089915 11.47373660926148, 122.95320551089915 11.473736609261481))'

The weird thing is it's not caught by is_valid on the epsg:4326 GeoSeries but it's caught by is_valid when the GeoSeries was projected to epsg:3123. I expected is_valid to return FALSE even if the polygon was not projected.

Perhaps this can be something geowrangler's geometry validation can also catch?

[jtmiclat]

We actually have a broken polygon for slither in data/broker.geojson

    {
      "type": "Feature",
      "properties": {
        "id": "slither"
      },
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [
              0.0,
              0.0
            ],
            [
              0.0,
              1.0
            ],
            [
              0.0,
              0.0
            ]
          ]
        ]
      }
    },

A way to detect this is by getting the polygon area.

Btw the polygon has an area. Is this correct?

In [36]: wkt.loads("POLYGON ((122.95320551089915 11.473736609261481, 122.952381 11.4737421, 122.95320551089915 11.47373660926148, 122.95320551089915 11.473736609261481))").area
Out[36]: 7.323127874328026e-19

[joshuacortez]

Oh I computed the area on the projected polygon. Maybe it's a precision thing. The sample broken polygon you mentioned seems good!

[joshuacortez]

I just noticed, it it supposed to be slither or sliver? Just learned that sliver is the term for the polygon I made in my example (got it from overlaying two geometries). I wonder if it makes sense to have a minimum threshold for slivers

[jtmiclat]

recognized source of error since overlay was first invented in the 1970s.
nice.

I think i was intending for sliver but got the words jumbled up. I think we can catch the area being 0 as a default and if the user has more of a threshold they can build their own validator

from geowrangler.validation import BaseValidator

class AreaValidator(BaseValidator):
    validator_column_name = "area_is_not_almost_zero"
    fix_available = False  # Telling the validator that there is no available fixes
    warning_message = "Found geometries with area almost zero"  # warning message
    geometry_types = ["Polygon", "MultiPolygon"]  # What kind of geometies to validate and fix

    def check(self, geometry):
        # Checks if the geometry is valid. If False, warn the user
        return geometry.area > 0.0000001 # or whatever threshold they have 

validated_gdf = AreaValidator().validate(gdf)

[joshuacortez]

Yep agree!