A Simple Geo Fencing Using Polygon Method

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Introduction

One of the important feature of GPS Tracking software using GPS Tracking devices is geo-fencing and its ability to help keep track of assets. Geo-fencing allows users of a GPS Tracking Solution to draw zones (i.e., a Geo Fence) around places of work, customer’s sites and secure areas. These geo fences when crossed by a GPS equipped vehicle or person can trigger a warning to the user or operator via SMS or Email.

Geo Fence

A Geo fence is a virtual perimeter on a geographic area using a location-based service, so that when the geo fencing device enters or exits the area, a notification is generated. The notification can contain information about the location of the device and might be sent to a mobile telephone or an email account. Reference: http://en.wikipedia.org/wiki/Geofence.

Background

For geo-fencing, I used Polygonal geo-fencing method where a polygon is drawn around the route or area. Using this method, GPS Tracking devices can be tracked either inside or outside of the polygon.

Determining a Point

The function will return true if the point X,Y is inside the polygon, or false if it is not. If the point is exactly on the edge of the polygon, then the function may return true or false. Thanks for the article “Determining Whether A Point Is Inside A Complex Polygon”.

public bool FindPoint(double X, double Y)
{
            int sides = this.Count() - 1;
            int j = sides - 1;
            bool pointStatus = false;
            for (int i = 0; i < sides; i++)
            {
                if (myPts[i].Y < Y && myPts[j].Y >= Y || 
			myPts[j].Y < Y && myPts[i].Y >= Y)
                {
                    if (myPts[i].X + (Y - myPts[i].Y) / 
			(myPts[j].Y - myPts[i].Y) * (myPts[j].X - myPts[i].X) < X)
                    {
                        pointStatus = !pointStatus ;                        
                    }
                }
                j = i;
            }
            return pointStatus;
}

Creating a Polygon

On the map, draw a polygon to the area which is to be geo-fenced and capture the corner points of the polygon and store into XML file (see: PolygonPoints.XML). loadData() function will create a polygon using defined corner points in the XML file.

private void loadData()
{
    DataSet ds = new DataSet();
    ds.ReadXml("PolygonPoints.XML");

    foreach (DataRow dr in ds.Tables[0].Rows)
    {
        Point p = new Point();

        //Convert Latitude into degrees
        String Lat = dr[0].ToString();
        double LatSec = Double.Parse(Lat.Substring(4, 4)) / 6000;
        double LatMin = (Double.Parse(Lat.Substring(2, 2)) + LatSec) / 60;
        p.X = Double.Parse(Lat.Substring(0, 2)) + LatMin;

        //Convert Longitude into degrees
        String Long = dr[1].ToString();
        double LongSec = Double.Parse(Long.Substring(5, 4)) / 6000;
        double LongMin = (Double.Parse(Long.Substring(3, 2)) + LongSec) / 60;
        p.Y = Double.Parse(Long.Substring(0, 3)) + LongMin;
        points.Add(p);              
    }
} 

Sample Code

When you run and enter latitude and longitude outside the polygon, then a message shows point not found in the route and otherwise it shows point found in the route.

PolyGon myRoute = new PolyGon(points);
bool stat = myRoute.FindPoint(Double.Parse(txtLat.Text.ToString()), 
		Double.Parse(txtLang.Text.ToString()));
if(stat)
{
    lblResult.Text = "Point found in the route";
}
else
    lblResult.Text = "Point not found in the route";