CODEWITHSUNDEEP
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HaydenKai
HaydenKai

Reputation: 880

Calculate distance along line, closest to other point (aka "project")

I have a LineString (string of coordinates) made up of many points and I want to calculate the distance along the line (from the first point to the last, directionality is important) that is closest to a Point much like this says it does: "project()" calculation

It would be great if I did not have to use any third party libraries, I realize some exist that would do this easily.

Upvotes: 1

Views: 2039

Answers (1)

sorifiend
sorifiend

Reputation: 6307

Linear Referencing is what you are looking for. From your comment it does sound complicated, but it is still a seriously simple piece of math/code when you break it down and then just add a loop to run through your points to work out the closest segment.

Below is a nice code pulled from a google search.... You can use it to find the closest point on a line segment using: Util.getClosestPointOnSegment(Point ss, Point se, Point p)

Code:

import java.awt.Point;

public class Util{

  /**
   * Returns closest point on segment to point
   * @param ss - segment start point
   * @param se - segment end point
   * @param p - point to found closest point on segment
   * @return closest point on segment to p
   */
  public static Point getClosestPointOnSegment(Point ss, Point se, Point p)
  {
    return getClosestPointOnSegment(ss.x, ss.y, se.x, se.y, p.x, p.y);
  }

  /**
   * Returns closest point on segment to point
   * @param sx1 - segment x coord 1
   * @param sy1 - segment y coord 1
   * @param sx2 - segment x coord 2
   * @param sy2 - segment y coord 2
   * @param px - point x coord
   * @param py - point y coord
   * @return closets point on segment to point
   */
  public static Point getClosestPointOnSegment(int sx1, int sy1, int sx2, int sy2, int px, int py)
  {
    double xDelta = sx2 - sx1;
    double yDelta = sy2 - sy1;

    if ((xDelta == 0) && (yDelta == 0))
    {
      throw new IllegalArgumentException("Segment start equals segment end");
    }

    double u = ((px - sx1) * xDelta + (py - sy1) * yDelta) / (xDelta * xDelta + yDelta * yDelta);

    final Point closestPoint;
    if (u < 0)
    {
      closestPoint = new Point(sx1, sy1);
    }
    else if (u > 1)
    {
      closestPoint = new Point(sx2, sy2);
    }
    else
    {
      closestPoint = new Point((int) Math.round(sx1 + u * xDelta), (int) Math.round(sy1 + u * yDelta));
    }
    return closestPoint;
  }
}

Obviously you still need to write some code to work out the closest segment to check, and still add the lengths, but this will get you started.

Example taken from:

http://www.java2s.com/Code/Java/2D-Graphics-GUI/Returnsclosestpointonsegmenttopoint.htm

Upvotes: 1

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