How to find the shortest distance of a point outside of a polygon starting at the polygon's edge?

Question

Consider the following:

• Point A DbGeography (office address)
• Point B DbGeography (customer's address outside of office's service area)
• Polygon C DbGeography (office's service area)

Using the above points and polygon, how can I find the closest distance of B to C 's edge? I assume that first I need to find the line between A and B , then find where the line intersects C (= D ) and then calculate the distance from D to B ?

Since my usage of SQL Server's spatial capabilities is limited and I'm using Entity Framework, I'm not sure how to express that in code. I also assume that I'd have to use SqlGeography for this since DbGeography is kind of limited. I'd probably end up writing an extension to DbGeography .

I'd appreciate any suggestions (would love code examples) of how I can accomplish the above task.

Answer 1

So, after messing around with this for the past three hours, I found a solution. Here's the code for anyone who cares:

public static class DbGeographyExtensions {
    /// <summary>
    /// Returns a double? value containing the shortest distance from the edge of this polygon to the specified point
    /// </summary>
    public static double? ToShortestDistanceFromPoint(
        this DbGeography polygon,
        DbGeography point) {
        if ((polygon != null)
            && (point != null)) {
            /// Convert the DbGeography to SqlGeography
            SqlGeography sqlPolygon = SqlGeography.STPolyFromText(new SqlChars(polygon.AsText()), polygon.CoordinateSystemId);
            SqlGeography sqlPoint = SqlGeography.STPointFromText(new SqlChars(point.AsText()), point.CoordinateSystemId);

            /// Get the shortest line from the edge of the polygon to the point
            SqlGeography shortestPoint = sqlPoint.ShortestLineTo(sqlPolygon);

            /// Return the length of the line (distance returns 0 because it excludes the area of the line)
            return (double?)shortestPoint.STLength();
        }

        return null;
    }
}