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POCreate a SqlGeography polygon-circle from a center and radius
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  1. POCreate a SqlGeography polygon-circle from a center and radius
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    copied!<p>I would like to save a circle in a sql-server 2008 geography field, using c#.</p> <p>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 <code>SqlGeography</code> from it.</p> <p>I have tried to following function to create the polygon:</p> <pre><code> private List&lt;Coordinate&gt; 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&lt;Coordinate&gt;(); for (int x = 0; x &lt;= 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; } </code></pre> <p>And then try to convert this <code>List&lt;Coordinate&gt;</code> to a parsable string:</p> <pre><code> var s = "POLYGON((" + string.Join(",", points.ConvertAll(p =&gt; p.Longitude + " " + p.Latitude).ToArray()) + "))"; var poly = SqlGeography.STPolyFromText(new System.Data.SqlTypes.SqlChars((SqlString)s), 4326); </code></pre> <p>But it always complains the polygon has to be on a single hemisphere, where I'm sure it is the case.</p> <p>Am I on the right track at all? Is there any other (simpler) way to do this?</p>
 

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