RegionChooser: Relatively fast and efficient convex hull extractor from CityGML files.

src/eu/simstadt/regionchooser/ConvexHullCalculator.java

@@ -3,6 +3,10 @@
 import java.io.IOException;
 import java.nio.file.Path;
 import java.util.ArrayList;
+import org.osgeo.proj4j.BasicCoordinateTransform;
+import org.osgeo.proj4j.CRSFactory;
+import org.osgeo.proj4j.CoordinateReferenceSystem;
+import org.osgeo.proj4j.ProjCoordinate;
 import com.vividsolutions.jts.algorithm.ConvexHull;
 import com.vividsolutions.jts.geom.Coordinate;
 import com.vividsolutions.jts.geom.Geometry;
@@ -16,10 +20,10 @@
 
 public class ConvexHullCalculator
 {
-	//TODO: Convert coordinates to WGS84
 	public static Geometry calculateFromCityGML(Path citygmlPath)
 			throws NumberFormatException, XPathParseException, NavException, XPathEvalException, IOException {
 
+		GeometryFactory geometryFactory = new GeometryFactory();
 		ArrayList<Coordinate> allPoints = new ArrayList<Coordinate>();
 		CityGmlIterator citygml = new CityGmlIterator(citygmlPath);
 		for (BuildingXmlNode buildingXmlNode : citygml) {
@@ -30,8 +34,25 @@ public static Geometry calculateFromCityGML(Path citygmlPath)
 		}
 
 		ConvexHull ch = new com.vividsolutions.jts.algorithm.ConvexHull(
-				allPoints.toArray(new Coordinate[allPoints.size()]), new GeometryFactory());
-		return ch.getConvexHull();
-	}
+				allPoints.toArray(new Coordinate[allPoints.size()]), geometryFactory);
+
+		// Convert convex hull in original coordinates to WGS84 coordinates.
+		// NOTE: It would be much easier with import org.geotools.referencing.CRS; instead of Proj4J
+		// NOTE: It's faster to convert to WGS84 once the convex hull is calculated, because there are fewer points
+		Coordinate[] convexHullcoordinates = ch.getConvexHull().getCoordinates();
+
+		CRSFactory CRS_FACTORY = new CRSFactory();
+		CoordinateReferenceSystem wgs84 = CRS_FACTORY.createFromName("EPSG:4326");
+		CoordinateReferenceSystem originalCRS = citygml.getCRS();
+		BasicCoordinateTransform transformToWgs84 = new BasicCoordinateTransform(originalCRS, wgs84);
 
+		for (int i = 0; i < convexHullcoordinates.length; i++) {
+			ProjCoordinate wgs84Coordinate = transformToWgs84.transform(
+					new ProjCoordinate(convexHullcoordinates[i].x, convexHullcoordinates[i].y),
+					new ProjCoordinate());
+			convexHullcoordinates[i] = new Coordinate(wgs84Coordinate.x, wgs84Coordinate.y);
+		}
+
+		return geometryFactory.createPolygon(convexHullcoordinates);
+	}
 }

test/eu/simstadt/regionchooser/test/ConvexHullCalculatorTests.java

@@ -22,8 +22,7 @@ public void testExtractConvexHullFromOneBuilding() throws Throwable {
 		Path citygmlPath = repo.resolve("Gruenbuehl.proj/20140218_Gruenbuehl_LOD2_1building.gml");
 		Geometry hull = ConvexHullCalculator.calculateFromCityGML(citygmlPath);
 		assertEquals(hull.getCoordinates().length, 4 + 1); // Convex hull of a building should be a closed rectangle
-		//		Point someBuildingPoint = gf.createPoint(new Coordinate(3515960.36, 5415667.91)); // Original coordinates, GSK3
-		Point someBuildingPoint = gf.createPoint(new Coordinate(9.21656126408, 48.8782253597)); // WGS84
+		Point someBuildingPoint = gf.createPoint(new Coordinate(9.216845, 48.878196)); // WGS84
 		assertTrue("Hull should contain every building point", hull.contains(someBuildingPoint));
 	}