How to determine on which side of a line a polygon feature falls?

Question

I have parcel data intersecting line data. In the parcel data there are some parcels that don't intersect the line. How could I programmatically figure out if the non-intersecting parcel is on the Right side or the Left side of the line? Thanks.

Answer 1

Use the IHitTest interface. Your query point will be the polygon centroid and the input geometry will be the line. One of the outputs will be a boolean (bRightSide) which will tell you what side of the line you're on.

Answer 2

You can use the dot product for this

/// <summary>
/// Used to indicate the orientation of an object in space 
/// with respect to another object
/// </summary>
public enum OrientationType
{
    Left,
    Right,
    Coincident,
    Unknown
}


/// <summary>
    /// Determines if a point is oriented left, right or coincident with
    /// a directed line. 
    /// Line direction is determined by its From and To points.
    /// </summary>
    /// <param name="p">The point to test.</param>
    /// <param name="segment">The line dividing the space</param>
    /// <returns>An OrientationType indicating the orientation.</returns>
    public static OrientationType GetPointOrientation(IPoint p, ISegment segment)
    {

        OrientationType result = OrientationType.Unknown;

        double Ax = segment.FromPoint.X;
        double Ay = segment.FromPoint.Y;
        double Bx = segment.ToPoint.X;
        double By = segment.ToPoint.Y;
        double Px = p.X;
        double Py = p.Y;

        double nDotV = ((Ay - By) * (Px - Ax)) + ((Bx - Ax) * (Py - Ay));

        if (nDotV < 0)
        {
            result = OrientationType.Right;//opposite direction to normal vector
        }
        else if (nDotV > 0)
        {
            result = OrientationType.Left;
        }
        else if (nDotV == 0)
        {
            result = OrientationType.Coincident;
        }

        return result;
    }