How to get WKT area?

Question

i use jts.jar, package is com.vividsolutions.jts.geom, to get a geometry's area. but the method getArea() does not give me the right result. my code is

public static void main(String[] args) throws ParseException {
        GeometryFactory geometryFactory = new GeometryFactory();
        WKTReader reader = new WKTReader(geometryFactory);
        String s = "MULTIPOLYGON (((114.273193 40.480272, 114.274645 
        MultiPolygon mpolygon = null;
            mpolygon = (MultiPolygon) reader.read(s);
            System.out.println(mpolygon.getArea());
    }

Answer 1

After finding this question when looking for the an answer to similar question, i thought i'd share my findings:

Description taken from blog post as it explains the problem you are trying to solve:

One of the recurring questions on GIS stackexchange is "What units is the area calculated by JTS getArea()". It doesn’t seem to matter how many times people say that these is no answer to this as it depends on the projection of your data. The question will not die, so I cooked up some code that should give a close-ish answer for most polygons.

As with most of these questions the trick is to convert your polygon to a flat cartesian plane (this is where JTS works best). We can use the GeoTools auto projection (assuming the polygon is small enough) and then simply call .getArea() method.

Solution:

public static void main(String[] args) throws Exception {
  GeometryFactory geometryFactory = new GeometryFactory();
  WKTReader reader = new WKTReader(geometryFactory);
  String s = "MULTIPOLYGON (((40 40, 20 45, 45 30, 40 40)), ((20 35, 10 30, 10 10, 30 5, 45 20, 20 35), (30 20, 20 15, 20 25, 30 20)))";
  MultiPolygon mpolygon = (MultiPolygon) reader.read(s);
  Point centroid = mpolygon.getCentroid();
  String autoCode = "AUTO:42001," + centroid.getX() + "," + centroid.getY();
  CoordinateReferenceSystem auto = CRS.decode(autoCode);
  MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84, auto);
  MultiPolygon projed = (MultiPolygon) JTS.transform(mpolygon, transform);
  Measure<Double, Area> measure = Measure.valueOf(projed.getArea(), SI.SQUARE_METRE);
  System.out.println(measure);
}

Output:

8.146055550674082E12 m²

Reference: https://blog.ianturton.com/geotools,/projections/2017/08/01/area-of-a-polygon.html