[GIS] GeoTools JTS: Snap a Point to a polygon (equivalent to PostGis ST_ClosestPoint)

geotoolsjts-topology-suitesnapping

I'm using GeoTools JTS libraries in Java and I want to find the nearest point inside polygon shape to some single target location. This is exactly what in PostGis ST_ClosestPoint does.

I found this article discussing "Snap a Point to a Line".

So trivial solution will be:

-Dump polygon to lines
-Snap a Point for each above Line
-Calculate the distance between the point and "the snapped" point
-Take the one with minimum distance

How can I achieve my requirement in GeoTools in more "elegant" way?

EDIT:

Actually I'm not sure if the link to snap a point doing exactly what ST_Closest because I don't understand how it's actually extracting the closest point…

Best Answer

JTS's DistanceOP#nearestPoints might be an easy way. I have not tested it.

http://tsusiatsoftware.net/jts/javadoc/com/vividsolutions/jts/operation/distance/DistanceOp.html#nearestPoints%28com.vividsolutions.jts.geom.Geometry,%20com.vividsolutions.jts.geom.Geometry%29

Compute the the nearest points of two geometries. The points are presented in the same order as the input Geometries.

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[GIS] Finding Perpendicular Distance and Minimum Perpendicular Vector between Point and Line using GeoTools and JTS

Finally I found a java utility that do the job right away

http://biodiversityinformatics.amnh.org/open_source/pdc/documentation.php

[GIS] Finding distance in meters between point and path in lat/lon using GeoTools

The code from the previous question does in fact solve this problem once you have explored the issue a bit.

Basically your line doesn't have enough points so it "cuts" the corner across the more localised projection that is used to allow JTS to proceed as if the Earth is flat. This needs one additional line of code:

route = (LineString) Densifier.densify(route, .01);

The brown line is your original 3 point line and the blue line is a densified line (that has points added so it projects properly), shown in a local projection (EPSG:2470). As you can see the distance to the brown line to the point is much shorter than it should be.

So here is some suitably modified code:

public double getDistance(Point point, LineString line) {
double dist = -1.0;
try {
  String code = "AUTO:42001," + point.getX() + "," + point.getY();
  CoordinateReferenceSystem auto = CRS.decode(code);
  // auto = CRS.decode("epsg:2470");
  MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84, auto);
  Geometry g3 = JTS.transform(line, transform);
  Geometry g4 = JTS.transform(point, transform);

  Coordinate[] c = DistanceOp.nearestPoints(g4, g3);

  Coordinate c1 = new Coordinate();
  //System.out.println(c[1].distance(g4.getCoordinate()));
  JTS.transform(c[1], c1, transform.inverse());
  //System.out.println(geometryFactory.createPoint(c1));
  dist = JTS.orthodromicDistance(point.getCoordinate(), c1, DefaultGeographicCRS.WGS84);
} catch (Exception e) {
  e.printStackTrace();
}
return dist;

}

And incase you are wondering about the difference between the "flat distance" (c[1].distance(g4.getCoordinate())) is 760.731km and the "curved distance" (JTS.orthodromicDistance(point.getCoordinate(), c1, DefaultGeographicCRS.WGS84);) is 760.980km. For reference the distance tool in QGIS gives 760.919 km.

Best Answer

Related Solutions

[GIS] Finding Perpendicular Distance and Minimum Perpendicular Vector between Point and Line using GeoTools and JTS

[GIS] Finding distance in meters between point and path in lat/lon using GeoTools

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