GeoTools Linestring – Calculating Distance in Meters

distancegeotools

I want to use GeoTools for working with gis data. I'm most concerned about distance calculations, not so much about drawing a map.
What I still couldn't figure out is how to make distance calculations work smoothly on the WGS84 spheroid. As an example I want to calculate the distance (in meters) between two linestrings. The postgresql code would be

SELECT
  ST_Distance(
    ST_GeomFromText('LINESTRING(13.45 52.47,13.46 52.48)')::Geography,
    ST_GeomFromText('LINESTRING(13.00 52.00,13.1 52.2)')::Geography
)

The output is 38364.138314182

With geotools I would imagine something like

import org.geotools.geometry.jts.JTSFactoryFinder
import org.geotools.geometry.jts.WKTReader2,
import org.geotools.factory.Hints,
import org.geotools.referencing.crs.DefaultGeographicCRS

Hints hints = new Hints(Hints.CRS, DefaultGeographicCRS.WGS84)
GeometryFactory geometryFactory = JTSFactoryFinder.getGeometryFactory(hints)
WKTReader2 reader = new WKTReader2(geometryFactory)
Geometry g1 = reader.read('LINESTRING(13.45 52.47,13.46 52.48)')
Geometry g2 = reader.read('LINESTRING(13.00 52.00,13.1 52.2)')
System.out.println(g1.distance(g2))

Now the output is 0.442040722106004 which is the same as

SELECT
  ST_Distance(
    ST_GeomFromText('LINESTRING(13.45 52.47,13.46 52.48)'),
    ST_GeomFromText('LINESTRING(13.00 52.00,13.1 52.2)')
  )

which is the euclidean distance in degrees. For points I can use the OrthodromicDistanceCalculator but not for lines. Plus, it is very inconvenient to specify that distance calculations should be based on WGS84 over and over again.

Best Answer

Using the following code I can get an answer of 38355.3256m (which is probably close enough) - I imagine that with a more careful choice of central point a more "accurate" result might be obtained.

    Geometry g1 = reader.read("LINESTRING(13.45 52.47,13.46 52.48)");
    Geometry g2 = reader.read("LINESTRING(13.00 52.00,13.1 52.2)");
    System.out.println(g1.distance(g2));
    CoordinateReferenceSystem auto = CRS.decode("AUTO:42001,13.45,52.3");
    System.out.println(auto);
    MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84,
            auto);
    Geometry g3 = JTS.transform(g1, transform);
    Geometry g4 = JTS.transform(g2, transform);
    System.out.println(g3.distance(g4));

Related Solutions

[GIS] Find distance from a point to linestring

First of all you need to make sure that both your coastline features and points are in the GEOGRAPHY datatype and that the coordinates are lat/longs.

Alternatively make sure all your features are in the GEOMETRY datatype and all coordinates are projected (to Mercator, a National Grid etc.).

Then using the STDistance method you should be able to use a query similar to:

SELECT pointtable.ID,
MIN(pointtable.GEOG.STDistance(shapetable.GEOG)) as MinDistance
FROM pointtable, shapetable
GROUP BY pointtable.ID

GEOG field names should be replaced with the field names containing the GEOGRAPHY data.
ID field names should be replaced with the appropriate ID field for the table.

If you want to see the actual string represenations of a GEOGRAPHY field use SQL such as:

SELECT shapetable.ID,
shapetable.GEOG.STAsText()
FROM shapetable

[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] Find distance from a point to linestring

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

Related Question