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Geometry-Validity.md

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Geometry Validity

TL;DR:

factory = RGeo::Cartesian.preferred_factory
bowtie_polygon = factory.polygon(factory.linear_ring([factory.point(0, 4),factory.point(4, 4),factory.point(2, 6),factory.point(0, 0),factory.point(0, 4)]))
polygon =
	case bowtie_polygon.invalid_reason
	when RGeo::Error::SELF_INTERSECTION
		bowtie_polygon.make_valid
	when nil # already valid
		bowtie_polygon
	else
		bowtie_polygon.check_validity! # Make sure we raise, undefined behaviour
	end


puts polygon.area
puts "Is the polygon valid? " + (polygon.valid? ? "yes" : "no")

RGeo embraces the concept of always giving correct results, or throwing a validity error (RGeo::Error::InvalidGeometry) if not possible.

Lets for instance take a bowtie shaped polygon:

factory = RGeo::Cartesian.preferred_factory
bowtie_polygon = factory.polygon(
	factory.linear_ring(
		[
			factory.point(0, 4),
			factory.point(4, 4),
			factory.point(2, 6),
			factory.point(0, 0),
			factory.point(0, 4)
		]
	)
)

There is no issue with creating this polygon, or viewing its representation for instance.

Bowtie polygon

However, its representation is invalid per specifications since it contains a Self-intersection. Computing its area would give us the result of 0, definitely not an expected result. Hence, the next code block results in a validation error:

bowtie_polygon.area # raises: Self-intersection (RGeo::Error::InvalidGeometry)

Errors are mapped to GEOS errors, and are all available in the codebase:

> constants = RGeo::Error.constants.grep(/\A[A-Z_]+\z/).sort
> size = constants.map(&:size).max
> puts constants.map { "#{_1.to_s.rjust(size)}: #{RGeo::Error.const_get(_1).inspect}" }
  DISCONNECTED_INTERIOR: "Interior is disconnected"
        DUPLICATE_RINGS: "Duplicate Rings"
     HOLE_OUTSIDE_SHELL: "Hole lies outside shell"
     INVALID_COORDINATE: "Invalid Coordinate"
           NESTED_HOLES: "Holes are nested"
          NESTED_SHELLS: "Nested shells"
         REPEATED_POINT: "Repeated Point"
 RING_SELF_INTERSECTION: "Ring Self-intersection"
      SELF_INTERSECTION: "Self-intersection"
         TOO_FEW_POINTS: "Too few distinct points in geometry component"
TOPOLOGY_VALIDATION_ERR: "Topology Validation Error"
          UNCLOSED_RING: "Ring is not closed"

Now if you really want to compute the area of our bowtie polygon, you have some options:

bowtie_polygon.unsafe_area # => 0
ok_polygon = bowtie_polygon.make_valid # => #<RGeo::Geos::CAPIMultiPolygonImpl:0x244 "MULTIPOLYGON (((0.0 0.0, 0.0 4.0, 1.3333333333333333 4.0, 0.0 0.0)), ((4.0 4.0, 1.3333333333333333 4.0, 2.0 6.0, 4.0 4.0)))">
ok_polygon.area.round # => 5

You can also play around with validity thanks to the RGeo::ImplHelper::ValidityCheck helper.

Note that not all methods on a geometry are checked for validity first. Tests like simple?, closed?, and empty? are not, and data accessors like num_points, factory, [], and interior_rings are not checked either. Only geometric analysis and predicate methods like length, buffer, intersection, crosses? and contains? are checked.