Check if dbgeometry dbgeometry/dbgeography point is within a polygon

Question

I have a problem I hope you guys can help me solve.

I have got a DbGeometry point (or DbGeography, I can use both) and I would like to check if this is within a DbGeometry Polygon (alternatively a DbGeography).

I am doing this at the moment:

var dbZones = new List<WasteManager.Database.Zone>();
foreach(var zone in zones)
        {
            var res = from z in DatabaseContext.Zones
                   let boundary =
                       !z.BoundaryGeometry.IsValid
                           ? SqlSpatialFunctions.MakeValid(z.BoundaryGeometry)
                           : z.BoundaryGeometry
                      where z.ID == zone.ID && point.Within(boundary)
                      select z;

            if(res.FirstOrDefault() != null) dbZones.Add(res.FirstOrDefault());

        }

So I iterate through zones (EF entity of my db) and check if this point I have is within this boundary.

Problem is that it does not return any result, but I know that that point is within that boundary because I created manually the boundary and the point to be inside that boundary.

Can anyone tell me if what I am doing is wrong, if there is another way to do this or whatever else?

Much appreciate.

Manuel

Answer 1

I would like to add a comment to Nick Strupat.

You should be carefull with ring orientation. SQL Server uses left-handed orientation, which means that if you are walking along the perimeter of a polygon, your left hand should be on the inside of the polygon and your right hand on the outside (counter-clockwise or anti-clockwise). I got the “ring orientation” error because I drew my polygon in the opposite direction (clockwise, or right-handed) which meant that SQL Server was treating the whole surface of the earth EXCEPT FOR my polygon as the area of the polygon.

To check if a point is in the polygon you should always use point.Intersects(polygon) and not !point.Intersects(polygon).

There is a solution to check if your polygon is ok or not by checking the size of the Area, For more information go to :

https://blog.falafel.com/ring-orientation-sql-spatial/

Here is my code based on the blog explanation :

    private bool isInside(DbGeography polygon, double longitude, double latitude)
    {
        DbGeography point = DbGeography.FromText(string.Format("POINT({1} {0})", latitude.ToString().Replace(',', '.'), longitude.ToString().Replace(',','.')), DbGeography.DefaultCoordinateSystemId);

        // If the polygon area is larger than an earth hemisphere (510 Trillion m2 / 2), we know it needs to be fixed
        if (polygon.Area.HasValue && polygon.Area.Value > 255000000000000L)
        {
            // Convert our DbGeography polygon into a SqlGeography object for the ReorientObject() call
            SqlGeography sqlPolygon = SqlGeography.STGeomFromWKB(new System.Data.SqlTypes.SqlBytes(polygon.AsBinary()), DbGeography.DefaultCoordinateSystemId);

            // ReorientObject will flip the polygon so the outside becomes the inside
            sqlPolygon = sqlPolygon.ReorientObject();

            // Convert the SqlGeography object back into DbGeography
            polygon = DbGeography.FromBinary(sqlPolygon.STAsBinary().Value);

        }
        return point.Intersects(polygon);
    }