如何在OpenLayers 3中通过地图边缘绘制测地线？

Question

I'm trying to draw geodesic line with arc.js . It works fine when difference between points' longitudes is less than 180. Otherwise, eg longitudes are 179 and -179, OpenLayers draws line with greatest distance.

So I found a solution for straight line:

1. Checking if difference between longitudes is more than 180
2. Calculating intersection of needed line and map edge (finding temporary point with longitude 180 or -180)
3. Creating ol.geom.MultiLineString with array [[firstPoint, temporaryPoint], [temporaryPoint, secondPoint]]
4. Creating feature with line

And it works fine. But it's pretty complicated to make this trick for geodesic line with arc.js . The main problem is in calculation of intersection.

I supposed to find solution in OpenLayers documentation or examples but there is no any example with map edge intersection.

Answer 1

After hours of investigation I found a solution. That looks like a trick but it works fine.

First of all here is an original code from arc.js documentation :

var start = { x: currentPoint.longitude, y: currentPoint.latitude },
    end = { x: nextPoint.longitude, y: nextPoint.latitude },
    generator = new arc.GreatCircle(start, end),
    arcLine = generator.Arc(1000 ,{offset:10});

The main issue is arc.js can't calculate coordinates for line that intersects International Date Line . So I decided to move points into one tile before calculating Great Circle .

I had to find longitudes offset:

var dateLineOffset = 180 - currentPoint.longitude;

It also could be nextPoint.longitude . Depends on what point is placed left.

After that you can use arc.js to generate coordinates:

var start = { x: -180, y: currentPoint.latitude },
    end = { x: nextPoint.longitude + dateLineOffset, y: nextPoint.latitude },
    generator = new arc.GreatCircle(start, end),
    arcLine = generator.Arc(1000 ,{offset:10});

Then you need to iterate generated coordinates and fix offset. In my case I used map .

var coordinatesWithOffset = arcLine.geometries[0].coords,
    geodesicLineCoordinates = coordinatesWithOffset.map(function(coord) {
        return ol.proj.fromLonLat([coord[0] - dateLineOffset, coord[1]]);
    }),
    geodesicLine = ol.geom.LineString(geodesicLineCoordinates);

That's it. geodesicLine will contain proper coordinates.