Can't fix invalid geometry

[Paul Meems]

I also posted this at https://stackoverflow.com/questions/48711333/cant-fix-invalid-geometry-using-nettopologysuite

I have this geometry:

MULTIPOLYGON (((6.8016626382085361 52.715309958659567, 6.8016978566354931 52.715300551948957, 6.8015610818087193 52.715189826862179, 6.8014185055170415 52.715082765798371, 6.8011385751008762 52.714892848902714, 6.80060419093019 52.714522597434573, 6.7999630427768381 52.714053843670676, 6.7997671025094686 52.713897979848007, 6.7995041016726177 52.713681792796635, 6.8008707444707825 52.71273529869638, 6.8018179126154372 52.713377474968908, 6.8019432523746257 52.713500919031091, 6.8019712703624577 52.71358956783984, 6.8019625919554079 52.713687641028628, 6.801900320844819 52.714051499553385, 6.8018106242715941 52.714620612080388, 6.801752965346215 52.714943969273726, 6.8016626382085361 52.715309958659567)))

It is not valid due to Self-Intersecting. Usually, this can be solved by using Buffer(0.0). But this time only a small part of the geometry is returned.

After reading https://github.com/NetTopologySuite/NetTopologySuite/issues/124 I tried

var pm = new NetTopologySuite.Geometries.PrecisionModel(1e10);
geom = new NetTopologySuite.Precision.GeometryPrecisionReducer(pm).Reduce(geom);

But the result is the same. Again only a small part of the original geometry.

When I use ST_MakeValid() in PostGIS I do get back the correct geometry.

Using https://arthur-e.github.io/Wicket/sandbox-gmaps3.html I can show you a screenshot of the original geometry:

NTS returns only a small triangle from the top, probably because of the location of the self-intersection.

How can I solve this with NetTopologySuite?

Thanks,

Paul

[FObermaier]

Maybe this helps: (https://stackoverflow.com/questions/31473553/is-there-a-way-to-convert-a-self-intersecting-polygon-to-a-multipolygon-in-jts)

    /// <summary>

    /// Extension method to validate polygons

    /// </summary>

    public static class ValidateGeometryExtension

    {

        /// <summary>

        /// Get or create a valid version of the geometry given. If the geometry is a 

        /// polygon or multi polygon, self intersections or inconsistencies are fixed. 

        /// Otherwise the geometry is returned.

        /// </summary>

        /// <param name="geom">The geometry to be fixed</param>

        /// <returns>The fixed geometry</returns>

        public static IGeometry Validate(this IGeometry geom)

        {

            if (geom is IPolygon)

            {

                if (geom.IsValid)

                {

                    geom.Normalize(); // validate does not pick up rings in the wrong order - this will fix that

                    return geom; // If the polygon is valid just return it

                }

                var polygonizer = new Polygonizer();

                AddPolygon((IPolygon)geom, polygonizer);

                return ToPolygonGeometry(polygonizer.GetPolygons(), geom.Factory);

            }

            if (geom is IMultiPolygon)

            {

                if (geom.IsValid)

                {

                    geom.Normalize(); // validate does not pick up rings in the wrong order - this will fix that

                    return geom; // If the multipolygon is valid just return it

                }

                var polygonizer = new Polygonizer();

                for (int n = geom.NumGeometries; n-- > 0;)

                {

                    AddPolygon((IPolygon)geom.GetGeometryN(n), polygonizer);

                }

                return ToPolygonGeometry(polygonizer.GetPolygons(), geom.Factory);

            }

            // ToDo other validations

            // Only care about polygons

            return geom; 

        }

        /// <summary>

        /// Add all line strings from the polygon given to the polygonizer given

        /// </summary>

        /// <param name="polygon">The polygon from which to extract the line strings</param>

        /// <param name="polygonizer">The polygonizer</param>

        static void AddPolygon(IPolygon polygon, Polygonizer polygonizer)

        {

            AddLineString(polygon.ExteriorRing, polygonizer);

            for (var n = polygon.NumInteriorRings; n-- > 0;)

            {

                AddLineString(polygon.GetInteriorRingN(n), polygonizer);

            }

        }

        /// <summary>

        /// Add the linestring given to the polygonizer

        /// </summary>

        /// <param name="lineString">The line string</param>

        /// <param name="polygonizer">The polygonizer</param>

        static void AddLineString(ILineString lineString, Polygonizer polygonizer)

        {

            if (lineString is ILinearRing)

            { // LinearRings are treated differently to line strings : we need a LineString NOT a LinearRing

                lineString = lineString.Factory.CreateLineString(lineString.CoordinateSequence);

            }

            // unioning the linestring with the point makes any self intersections explicit.

            var point = lineString.Factory.CreatePoint(lineString.GetCoordinateN(0));

            var toAdd = lineString.Union(point);

            //Add result to polygonizer

            polygonizer.Add(toAdd);

        }

        /// <summary>

        /// Get a geometry from a collection of polygons.

        /// </summary>

        /// <param name="polygons"></param>

        /// <param name="factory">Factory to create <see cref="IMultiPolygon"/>s</param>

        /// <returns><c>null</c> if there were no polygons, the polygon if there was only one, or a MultiPolygon containing all polygons otherwise</returns>

        static IGeometry ToPolygonGeometry(ICollection<IGeometry> polygons, IGeometryFactory factory)

        {

            switch (polygons.Count)

            {

                case 0:

                    return null; // No valid polygons!

                case 1:

                    return polygons.First(); // single polygon - no need to wrap

                default:

                    //polygons may still overlap! Need to sym difference them

                    IGeometry ret = (IPolygon)polygons.First().Clone();

                    foreach (var polygon in polygons.Skip(1))

                        ret = ret.SymmetricDifference(polygon);

                    return ret;

            }

        }

    }