Allow to calculate area directly.

src/main/java/eu/simstadt/regionchooser/fast_xml_parser/ConvexHullCalculator.java

@@ -52,6 +52,26 @@ private ConvexHullCalculator() {
 	 */
 	public static Geometry calculateFromCityGML(Path citygmlPath) throws XPathParseException, IOException {
 
+		Polygon originalConvexHull = getConvexHull(citygmlPath);
+
+		CoordinateReferenceSystem originalCRS = RegionChooserUtils.crsFromCityGMLHeader(citygmlPath);
+		Polygon convexHull = RegionChooserUtils.changePolygonCRS(originalConvexHull, originalCRS,
+				RegionChooserUtils.WGS84);
+		convexHull.setUserData(originalCRS.toString());
+		return convexHull;
+	}
+
+	/**
+	 * @param citygmlPath
+	 * @return area of convex hull, in the same unit as original file. Hopefully m², but possibly ft² or even °².
+	 * @throws XPathParseException
+	 */
+	public static double calculateArea(Path citygmlPath) throws XPathParseException {
+		Polygon originalConvexHull = getConvexHull(citygmlPath);
+		return originalConvexHull.getArea();
+	}
+
+	private static Polygon getConvexHull(Path citygmlPath) throws XPathParseException {
 		GeometryFactory geometryFactory = new GeometryFactory();
 		ArrayList<Coordinate> allPoints = new ArrayList<>();
 		CityGmlIterator citygml = new CityGmlIterator(citygmlPath);
@@ -70,12 +90,13 @@ public static Geometry calculateFromCityGML(Path citygmlPath) throws XPathParseE
 		// Convert convex hull in original coordinates to WGS84 coordinates.
 		// NOTE: It's faster to convert to WGS84 once the convex hull is calculated, because there are fewer points
 		Polygon originalConvexHull = (Polygon) ch.getConvexHull();
+		return originalConvexHull;
+	}
 
-		CoordinateReferenceSystem originalCRS = RegionChooserUtils.crsFromCityGMLHeader(citygmlPath);
-		Polygon convexHull = RegionChooserUtils.changePolygonCRS(originalConvexHull, originalCRS,
-				RegionChooserUtils.WGS84);
-		convexHull.setUserData(originalCRS.toString());
-		return convexHull;
+	public static Geometry getArea(Path citygmlPath) throws XPathParseException, IOException {
+		Geometry geometry = calculateFromCityGML(citygmlPath);
+		RegionChooserUtils.changePolygonCRS((Polygon) geometry, RegionChooserUtils.WGS84, RegionChooserUtils.WGS84);
+		return geometry;
 	}
 
 	/**

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

@@ -4,7 +4,6 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
-
 import java.io.File;
 import java.io.IOException;
 import java.nio.file.Files;
@@ -46,6 +45,30 @@ public void testExtractConvexHullFromOneSmallRegion() throws IOException, XPathP
 		assertTrue(hull.contains(somewhereBetweenBuildings), "Hull should contain region between buildings");
 	}
 
+	@Test
+	public void testAreaFromOneSmallRegion() throws IOException, XPathParseException {
+		Path citygmlPath = repository.resolve("Gruenbuehl.proj/Gruenbuehl_LOD2_ALKIS_1010.gml");
+		double squareMeters = ConvexHullCalculator.calculateArea(citygmlPath);
+		double hectares = squareMeters / 10000;
+		assertEquals(19.0, hectares, 0.2);
+
+		citygmlPath = repository.resolve("NewYork.proj/FamilyCourt_LOD2_with_PLUTO_attributes.gml");
+		squareMeters = ConvexHullCalculator.calculateArea(citygmlPath);
+		// Checked with GoogleEarth
+		assertEquals(13000, squareMeters, 1000);
+
+		citygmlPath = repository.resolve("NewYork.proj/ManhattanSmall.gml");
+		squareMeters = ConvexHullCalculator.calculateArea(citygmlPath);
+		// Checked with GoogleEarth
+		double squareKilometers = squareMeters / 1000000;
+		assertEquals(1.0, squareKilometers, 0.1);
+
+		citygmlPath = repository.resolve("Stuttgart.proj/Stuttgart_LOD0_LOD1_small.gml");
+		squareMeters = ConvexHullCalculator.calculateArea(citygmlPath);
+		hectares = squareMeters / 10000;
+		assertEquals(47.5, hectares, 1);
+	}
+
 	@Test
 	public void testExtractConvexHullFromStoeckachNoBuildingPart() throws IOException, XPathParseException {
 		Path citygmlPath = repository.resolve("Stuttgart.proj/Stöckach_überarbeitete GML-NoBuildingPart.gml");