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Generate geodesic circles for range rings.

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Turns out that OL3's circles aren't geodesic circles so the
ranges are all off if you're anywhere but on the equator.
So generate our own LineStrings that approximate a circle.

Should fix #133
This commit is contained in:
Oliver Jowett 2016-07-24 16:13:04 +01:00
parent a06b13d42a
commit d3565b407e
1 changed files with 27 additions and 1 deletions
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28
public_html/script.js
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@ -316,6 +316,30 @@ function end_load_history() {
} }
// Make a LineString with 'points'-number points
// that is a closed circle on the sphere such that the
// great circle distance from 'center' to each point is
// 'radius' meters
function make_geodesic_circle(center, radius, points) {
var angularDistance = radius / 6378137.0;
var lon1 = center[0] * Math.PI / 180.0;
var lat1 = center[1] * Math.PI / 180.0;
var geom = new ol.geom.LineString();
for (var i = 0; i <= points; ++i) {
var bearing = i * 2 * Math.PI / points;
var lat2 = Math.asin( Math.sin(lat1)*Math.cos(angularDistance) +
Math.cos(lat1)*Math.sin(angularDistance)*Math.cos(bearing) );
var lon2 = lon1 + Math.atan2(Math.sin(bearing)*Math.sin(angularDistance)*Math.cos(lat1),
Math.cos(angularDistance)-Math.sin(lat1)*Math.sin(lat2));
lat2 = lat2 * 180.0 / Math.PI;
lon2 = lon2 * 180.0 / Math.PI;
geom.appendCoordinate([lon2, lat2]);
}
return geom;
}
// Initalizes the map and starts up our timers to call various functions // Initalizes the map and starts up our timers to call various functions
function initialize_map() { function initialize_map() {
// Load stored map settings if present // Load stored map settings if present
@ -489,7 +513,9 @@ function initialize_map() {
distance *= 1.852; distance *= 1.852;
} }
var feature = new ol.Feature(new ol.geom.Circle(ol.proj.fromLonLat(SitePosition), distance)); var circle = make_geodesic_circle(SitePosition, distance, 360);
circle.transform('EPSG:4326', 'EPSG:3857');
var feature = new ol.Feature(circle);
feature.setStyle(circleStyle); feature.setStyle(circleStyle);
StaticFeatures.push(feature); StaticFeatures.push(feature);
} }