How to Determine the Area of a Polygon in OpenLayers

Step-by-Step Guide

1. 1

   Step 1: Create a polygon feature.

   The Polygon constructor function needs an array of coordinate arrays; define this array in a variable first so that you can use it later.
   
   Simply copy the following line of code into your <script></script> element:. var coordinates = , , , ]; var polygon_feature = new ol.Feature({ geometry: new ol.geom.Polygon( ) });
2. 2

   Step 2: Add the feature to a vector layer.

   To add the polygon to the map, you need to add it to a source, which you add to a vector layer, which you can then add to the map: var vector_layer = new ol.layer.Vector({ source: new ol.source.Vector({ features: }) }) map.addLayer(vector_layer)
3. 3

   Step 3: Transform the feature’s geometry to use coordinates.

   var current_projection = new ol.proj.Projection({code: "EPSG:4326"}); var new_projection = tile_layer.getSource().getProjection(); polygon_feature.getGeometry().transform(current_projection, new_projection)
4. 4

   Step 4: Create a sphere to perform the calculation.

   The sphere should be the size of the Earth (should have a radius of
   6.3m meters).
   
   Technically, the sphere has a radius is equal to the semi-major axis of the WGS84 ellipsoid. var sphere = new ol.Sphere(6378137)
5. 5

   Step 5: Use the sphere to calculate the area using the geodesicArea() method.

   Because the method provides a value in square metres, divide by a million to get square kilometres. var area_m = sphere.geodesicArea(coordinates); var area_km = area_m / 1000 / 1000; console.log('area: '
   
   area_km, 'km²'); // CONSOLE: area:
   2317133.7166773956 km² , We know that it's correct because it appears to be approximately the same size as Algeria, which has an area of 2,381,741 km² (from Wikipedia).
6. 6

   Step 6: Check that the area answer makes sense.

Detailed Guide

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