how to get nearest point on a SqlGeometry object from another SqlGeometry object?

Question

I have a set of line and polygon object (SqlGeometry type) and a point object (SqlGeometry type). How can we find the the nearest point on each line from the given point object? Are there any API for doing this operation?

Answer 1

I'm not sure if this is possible directly in SQL Server 2008:

http://social.msdn.microsoft.com/Forums/en/sqlspatial/thread/cb094fb8-07ba-4219-8d3d-572874c271b5

The workaround suggested in that thread is:

declare @g geometry = 'LINESTRING(0 0, 10 10)' 
declare @h geometry = 'POINT(0 10)' 

select @h.STBuffer(@h.STDistance(@g)).STIntersection(@g).ToString()

Otherwise you would have to write a script to read the geometry from your database and use separate spatial libraries.

Answer 2

If you are interested in actually finding the nearest point on the line (otherwise called a node) you can turn each line into a set of points with the same lineid. Then query for the closest and calc the distance.

If instead you are trying to calc the distance from a point to the nearest line - stdistance http://msdn.microsoft.com/en-us/library/bb933808.aspx I guess the problem that the other answer addresses is what to put in your where clause though you could use stdistance to specify a distance above which you don't care such as

Where pointGeom.stdistance(lineGeom) < "distance you care about"

Answer 3

Here a sample presenting possible solution using SqlGeometry and C#, no SQL Server is required:

using System;
using Microsoft.SqlServer.Types;
namespace MySqlGeometryTest
{
    class ReportNearestPointTest
    {
        static void ReportNearestPoint(string wktPoint, string wktGeom)
        {
            SqlGeometry point = SqlGeometry.Parse(wktPoint);
            SqlGeometry geom = SqlGeometry.Parse(wktGeom);
            double distance = point.STDistance(geom).Value;
            SqlGeometry pointBuffer = point.STBuffer(distance);
            SqlGeometry pointResult = pointBuffer.STIntersection(geom);
            string wktResult = new string(pointResult.STAsText().Value);
            Console.WriteLine(wktResult);
        }

        static void Main(string[] args)
        {
            ReportNearestPoint("POINT(10 10)", "MULTIPOINT (80 70, 20 20, 200 170, 140 120)");
            ReportNearestPoint("POINT(110 200)", "LINESTRING (90 80, 160 150, 300 150, 340 150, 340 240)");
            ReportNearestPoint("POINT(0 0)", "POLYGON((10 20, 10 10, 20 10, 20 20, 10 20))");
            ReportNearestPoint("POINT(70 170)", "POLYGON ((110 230, 80 160, 20 160, 20 20, 200 20, 200 160, 140 160, 110 230))");
        }
    }
}

The program output:

POINT (20 20)
POINT (160 150)
POINT (10 10)
POINT (70 160)