Determine if a polygon intersects itself in OpenLayers

Question

I let the users draw polygons and then I try to save them out to the SQL Server 2008 SQLGeography field, but it is giving me an error:

Error 24200: The specified input does not represent a valid geography instance.

I think the issue is because the polygon intersects itself and SQL doesn't accept this behavior. Is it possible to do a check in OpenLayers to see if the WKT from the polygon would be accepted by the SQLGeography.STGeomFromText function?

Answer 1

There are no built-in checks for this in OpenLayers, but it should be possible to implement one quite easily.

The Polygon class has a Components property which holds several LinearRings, the first ring beeing the outer ring and the consecutive ones (if any) represents holes.

To find self-intersections you could make a function that loops the induvidual points of each ring, forming linestrings and checking intersections.

I think an approach like this should work (this only checks if the outer ring is self-intersecting, but it should be quite easy to extend it to holes, just build segments from the holes and push to the segments array)

var format = new OpenLayers.Format.WKT();

var f1=format.read("POLYGON(1 1, 4 1, 4 4, 1 4, 2 5, 1 1)");
var f2=format.read("POLYGON(1 1, 4 1, 4 4, 1 4, 1 1)");

function checkSelfIntersection(polygon){
    console.log(polygon);
    if(polygon.CLASS_NAME=="OpenLayers.Geometry.Polygon"){
        //checking only outer ring
        var outer = polygon.components[0].components;           
        var segments = [];
        for(var i=1;i<outer.length;i++){
            var segment= new OpenLayers.Geometry.LineString([outer[i-1].clone(),outer  [i].clone()]);
            segments.push(segment);               
        }   
        for(var j=0;j<segments.length;j++){    
            if(segmentIntersects(segments[j],segments)){
               return true;
            }
        }                     
    }    
    return false;
}

function segmentIntersects(segment,segments){
    for(var i=0;i<segments.length;i++){
        if(!segments[i].equals(segment)){
            if(segments[i].intersects(segment) && !startOrStopEquals(segments[i],segment)){
                return true;
           }            
       }    
    }
    return false;    
}    

function startOrStopEquals(segment1,segment2){

    if(segment1.components[0].equals(segment2.components[0])){
        return true;
    }
    if(segment1.components[0].equals(segment2.components[1])){
        return true;
    }
    if(segment1.components[1].equals(segment2.components[0])){
        return true;
    }
    if(segment1.components[1].equals(segment2.components[1])){
        return true;
    }
    return false;
}

console.log(checkSelfIntersection(f1.geometry));
console.log(checkSelfIntersection(f2.geometry));

Answer 2

The excellent Java Topology Suite has been/is being ported to Javascript:

https://github.com/bjornharrtell/jsts

you could try using its Geometry.isValid() method to test the polygon for correctness before submitting it to the server.

Answer 3

Don't know if you found an answer for this, but I found this link very helpful and it was a combination that worked for me. http://www.beginningspatial.com/fixing_invalid_geography_data

Hope it helps