# Unit of Geotools' getArea function

I have Polygons with WGS84/EPSG:4326 coordinates
(like 13.57933804767844,52.5126298984133)
and use the Polygon.getArea function to obtain the area of the polygon.
But the result I get is something like 7.695432394893684E-8 .

There were similiar questions (see here, here or here)
but although some of them are marked as answered,
there is not an answer which helped me.

I would like to convert the result to square meter.
How can I do this?

• you will have to handle the projection yourself prior to running an area function. EPSG 4326 is a lat/lon GCS. Which means the coordinates are angles from mean not a surface coordinate. there are algorithms which allow some conversion to a surficial unit but from your question I would suggest you handle it through reprojection to a ground CRS with meters as the unit. There are many questions with this focus, and several with answers. gis.stackexchange.com/search?q=latitude+to+meters+area Dec 31 '14 at 21:46
• Can you show a real polygon? If not, can you check if your polygons roughly 30 metres each side? If so, just transform your coordinates into an equal area projection that has metres. Dec 31 '14 at 21:46
• @BradNesom is there a CRS which you could recommend me? @BradHards Im an not sure what you mean by show a real polygon but here are the coordinates of one polygon: `[ [ [ 13.632045970877657, 52.51028471023475 ], [ 13.631736612007554, 52.509817421791602 ], [ 13.632054749915945, 52.509817407463906 ], [ 13.632045970877657, 52.51028471023475 ] ] ]` this site tells me that it has an area of about 560 square meters. Jan 1 '15 at 0:40
• Maybe EPSG:5243. How much accuracy do you need? Jan 1 '15 at 1:04
• Well it doesn't have to be that accurate.. I have some polygons ranging from 100 m² to 10000 km² and maaaaany polygons around 1 m². The tiny polygons must be filtered out. Would EPSG:3068 be ok as well? Jan 1 '15 at 9:24

As all of the questions you have linked to say JTS does not care about the units of your data and treats them all alike - so the answer to your question is square degrees. Now that is rarely (if ever) a good unit to measure areas in as it changes with latitude.

So to answer your real question how do I determine the area of my lat/lon polygons in square metres (or other unit)? - first you need to reproject your polygon to a local projection this is so you have a unit of metres going into the calculation and also because JTS assumes that all of your calculations are on a flat cartesian plane.

``````private Measure<Double, Area> calcArea(SimpleFeature feature) {
Polygon p = (Polygon) feature.getDefaultGeometry();
Point centroid = p.getCentroid();
try {
String code = "AUTO:42001," + centroid.getX() + "," + centroid.getY();
CoordinateReferenceSystem auto = CRS.decode(code);

MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84, auto);

Polygon projed = (Polygon) JTS.transform(p, transform);
return Measure.valueOf(projed.getArea(), SI.SQUARE_METRE);
} catch (MismatchedDimensionException | TransformException | FactoryException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return Measure.valueOf(0.0, SI.SQUARE_METRE);
}
``````

Will give an approximate answer (and it be closer for smaller polygons), a little bit of playing with the javax units library will give the units you want.

``````Measure<Double, Area> a = me.calcArea(feature);
System.out.println(feature.getDefaultGeometry());
System.out.println(a);

@SuppressWarnings("unchecked")
Unit<Area> sq_km = (Unit<Area>) SI.KILOMETER.pow(2);
System.out.println(a.to(sq_km));
@SuppressWarnings("unchecked")
Unit<Area> sq_mile = (Unit<Area>) NonSI.MILE.times(NonSI.MILE);
System.out.println(a.to(sq_mile));
System.out.println(a.to(NonSI.HECTARE));
@SuppressWarnings("unchecked")
Unit<Area> acre = (Unit<Area>) NonSI.MILE.divide(8.0).times(NonSI.FOOT).times(66.0);
UnitFormat.getInstance().label(acre, "acre");
System.out.println(a.to(acre));
System.out.println(a.doubleValue(NonSI.HECTARE)+NonSI.HECTARE.toString());
``````