Question

I would like to save a circle in a sql-server 2008 geography field, using c#.

In c# I have a latitude, a longitude and a radius but I just can’t find a way to calculate the polygon that would represent the circle and create a SqlGeography from it.

I have tried to following function to create the polygon:

    private List<Coordinate> getCirclePoints(Coordinate center, int radius, int speed)  //speed 1: draws 360 sides, 2 draws 180 etc...
    {
        var centerLat = (center.Latitude * Math.PI) / 180.0;  //rad
        var centerLng = (center.Longitude * Math.PI) / 180.0; //rad
        var dist = (float)radius / 6371.0;             //d = angular distance covered on earth's surface
        var circlePoints = new List<Coordinate>();
        for (int x = 0; x <= 360; x += speed)
        {
            var brng = x * Math.PI / 180.0;         //rad
            var latitude = Math.Asin(Math.Sin(centerLat) * Math.Cos(dist) + Math.Cos(centerLat) * Math.Sin(dist) * Math.Cos(brng));
            var longitude = ((centerLng + Math.Atan2(Math.Sin(brng) * Math.Sin(dist) * Math.Cos(centerLat), Math.Cos(dist) - Math.Sin(centerLat) * Math.Sin(latitude))) * 180.0) / Math.PI;
            circlePoints.Add(new Coordinate((latitude * 180.0) / Math.PI, longitude));
        }
        return circlePoints;
    }

And then try to convert this List<Coordinate> to a parsable string:

        var s = "POLYGON((" + string.Join(",", points.ConvertAll(p => p.Longitude + " " + p.Latitude).ToArray()) + "))";
        var poly = SqlGeography.STPolyFromText(new System.Data.SqlTypes.SqlChars((SqlString)s), 4326);

But it always complains the polygon has to be on a single hemisphere, where I’m sure it is the case.

Am I on the right track at all? Is there any other (simpler) way to do this?

Answer 1

OK, found the answer on my own. The trick is to create a point

var point = SqlGeography.Point(latitude, longitude, 4326);

Then create a buffer around the point

var poly = point.BufferWithTolerance(radiusInMeter, 0.01, true); //0.01 is to simplify the polygon to keep only a few sides

Then you could simply create a SqlCommand and add the polygon as parameter:

var param = new SqlParameter(@"Polygon", poly);
param.UdtTypeName = "Geography";
command.Parameters.Add(param);

Hope that will help someone else in the future!