Area measurement not showing correct measurements

Question

I am trying to get the area of polygons drawn. But the area's I am getting is roughly 50% larger than they should be.

draw.on('drawend', funciton(e){
  var geom = e.feature.getGeometry();
  if(drawType === 'Polygon'){
    var rawArea = geom.getArea();
    var cleanArea = rawArea / 10000; // Convert to hectares
  }
});

Some examples:

• what should have been 1.18 ha was 1.87 ha
• what should have been 1.29 ha was 2.09 ha
• what should have been 1.45 ha was 2.31 ha

Answer 1

Look at how it's done in the measure example for the geodesic measurement option: http://openlayers.org/en/v3.6.0/examples/measure.html

var sourceProj = map.getView().getProjection();
var geom = /** @type {ol.geom.Polygon} */(polygon.clone().transform(
    sourceProj, 'EPSG:4326'));
var coordinates = geom.getLinearRing(0).getCoordinates();
area = Math.abs(wgs84Sphere.geodesicArea(coordinates));

Answer 2

getArea and getLength badly need their docs updating to explain that they do not give answers in normal units for Web Mercator maps.

See here https://github.com/openlayers/ol3/pull/3609 and here https://github.com/openlayers/ol3/issues/3533

setting the radius for a circle has similar issues.