8

So i would really like to modify this example: http://openlayers.org/en/v3.0.0/examples/tissot.html?q=circle

Problem is that when I try to apply it to my map it doesn't work, probably because I use OSM style which is not spherical:

var map = new ol.Map({
  layers: [
    new ol.layer.Tile({
        source: new ol.source.OSM()
      }),
    new ol.layer.Vector({
      source: vectorSource
    })
  ],
  renderer: 'canvas',
  target: 'map',
  view: new ol.View({
    center: ol.proj.transform([2.1833, 41.3833], 'EPSG:4326', 'EPSG:3857'),
    zoom: 2
  })
});

And for the question: how to create circular polygon? As I can see, there are two choices:

  1. Somehow convert geom.Circle in geom.Polygon, which I can't do, based on my noob level

  2. Create my own function to do that, something like openlayers 2:

    OpenLayers.Geometry.Polygon.createRegularPolygon = function(origin, radius, sides, rotation) {
    var angle = Math.PI * ((1/sides) - (1/2)); if(rotation) { angle += (rotation / 180) * Math.PI; } var rotatedAngle, x, y; var points = []; for(var i=0; i

Has anyone got to the same point in their GIS master life? Which one is better? or is there a hidden path, that only Wise and Strong ones can see? Help me please!

11

As others have correctly pointed out, the GeoJSON and WKT formats do not support circular geometries in any way. Circles are not in the KML specification either. This makes it a pain when trying to write out a map frame to KML, GeoJSON or WKT.

OpenLayers v3 does have a way to approximate a circle with a polygon, but it isn't pretty as it involves converting to great circle distance. See the API reference for the ol.geom.Polygon.circular method here: http://openlayers.org/en/v3.5.0/apidoc/ol.geom.Polygon.html#circular

An example of the ol.geom.Polygon.circular method is shown below, but can also be seen in this OpenLayers tissot indicatrix example.

 var lowpoly = ol.geom.Polygon.circular(
  /* WGS84 Sphere */
  new ol.Sphere(6378137),
  circle.getCenter(),
  circle.getRadius(),
  /* Number of verticies */
  12);

Alternatively, you can use fromCircle which, at the time of this post, is experimental.

var lowpoly = ol.geom.Polygon.fromCircle(
  circle,
  /* Number of verticies (optional) */
  12,
  /* Start angle (optional) */
  90
);

For something that you can play with, see my jsFiddle here: https://jsfiddle.net/a1syw4od/9/

  • I could propose an edit, but maybe it's easier if you fix it: the last example should use ol.geom.Polygon.fromCircle rather than ol.geom.Polygon.circular. Cheers! – Arjan Mar 13 '18 at 15:19
0

If you are interested in creating a simple circular polygon then use:

new ol.geom.Circle(
            ol.proj.transform([longitude, latitude], 'EPSG:4326', 'EPSG:3857'),
            radius_meters),
            'XY'
        )

Source: http://openlayers.org/en/v3.0.0/apidoc/ol.geom.Circle.html (remove the stable tick at the top of the page)

  • This is valid, but currently has problems if you intend to try and convert the feature into GeoJSON. See github.com/openlayers/ol3/pull/3237#issuecomment-78475413 – Darren Reid May 5 '15 at 8:01
  • also when you try to extract geometry in wkt using writeWKT. – Suraj May 5 '15 at 15:05
  • 1
    ol.geom.Circle is not a polygon, it is a simple geometry represented by a radius and center. (see the base class of ol.geom.Circle is ol.geom.SimpleGeometry not ol.geom.Polygon) GeoJSON, WKT, and KML do not have circular geometries in their specification and require conversion before formatting. Please see my answer for a way to approximate an ol.geom.Circle by a true polygon. – nagytech May 22 '15 at 11:56

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