How to check if a point is near or inside polygon?

Question

I am trying to find a simple library which can do point in polygon. Earlier i did hit Java Spatial Index library ...could not figure out.

Tried openmap..looked simple didn't work.

I am totally new to GIS..my application has polygons. i need to search if my point isin a polygon or new polygon(s)

in open map i wrote as follows.

package abc.poi;

import com.bbn.openmap.geo.Geo;
import com.bbn.openmap.geo.OMGeo;

public class PoiTest {

    public static void main(String[] arg) {
        Geo geo1 = new Geo(1,1);
        Geo geo2 = new Geo(3,2);
        Geo geo3 = new Geo(4,4);
        Geo geo4 = new Geo(3,4);
        Geo geo5 = new Geo(4,3);
        Geo geo6 = new Geo(1,1);

        Geo listGeo[] = {geo1,geo2,geo3,geo4,geo5,geo6};

        OMGeo.Polygon poly = new OMGeo.Polygon(listGeo);

        System.out.println(poly.isPointInside(new Geo(2,2)));


    }
}

It always returns false. What i am missing ?

Answer 1

Well I don't know the openmap library, but your polygon has self-intersections, and openmap likely doesnt account for this scenario.

Also I might suggest Openlayers if you're developing a web-based mapping application.

Answer 2

You can code it yourself in python. Its like 30 lines of code, max. I did it myself. I can outline the algorithm below, but you need to optimize for bigger data set.

Basic idea: If a point is inside a polygon, the sum of the angles subtended by the line segments at the point must be equal to 360. If it is outside, the summation will be less than 360.

If you have holes, than you need to check for if its in a hole or not. So first check if a point is inside a polygon. Then convert holes in that polygons into polygons and check if its outside of each polygon(i.e. holes). A flow chart is here:

If you need more help with implementation, I can dig in old folders and check if I still have the implementation of this algo.

Answer 3

import java.util.Date;
import java.util.Scanner;
public class polygon
{

public static void main(String args[])
{
    System.out.println("PROGRAMMED BY:\nName:BHAGIRATH BHATT");

    System.out.println(" *******Program to check if a given point lies inside or outside of any polygon******** ");

Scanner a=new Scanner(System.in);
System.out.println("Enter the number of sides of the polygon..");
int side=a.nextInt();
if (side<=2)
{
    System.out.println("You have not formed a polygon.So using this program is of no meaning.Run it again with a valid number of sides");
}
else
{

double x[]= new double [side];
double y[]= new double[side];
double lengthtopoint[]=new double [side];
double sidelength[]=new double [side];
double angles[]=new double [side];

for(int u=0;u<=desirednumber;u++)
{
System.out.println("\nEnter the "+(u+1)+" coordinate for which you need to check");
System.out.println("Xcheck= ");
double xcheck=a.nextDouble();
System.out.println("Ycheck= ");
double ycheck=a.nextDouble();

for(int i=0;i<side;i++)
{
System.out.println("Enter x for vertex "+i);
x[i]=a.nextDouble();
System.out.println("Enter y for vertex "+i);
y[i]=a.nextDouble();
lengthtopoint[i]=Math.sqrt(((x[i]-xcheck)*(x[i]-xcheck))+((y[i]-ycheck)*(y[i]-ycheck)));
//System.out.println("length of line joining given check point and vertex "+i+" is " +lengthtopoint[i]);

}
System.out.println("List of all coordinates");  
for(int j=0;j<side;j++)
{   

System.out.println("Coordinate of vertex "+j+"= " +x[j]+" ,"+y[j] );

}   
for(int k=0;k<side-1;k++)
{   
sidelength[k]=Math.sqrt(((x[k+1]-x[k])*(x[k+1]-x[k]))+((y[k+1]-y[k])*(y[k+1]-y[k])));
//System.out.println("length of side "+k +(k+1)+ " is " +sidelength[k]);
}
sidelength[side-1]=Math.sqrt(((x[0]-x[side-1])*(x[0]-x[side-1]))+((y[0]-y[side-1])*(y[0]-y[side-1])));
//System.out.println("length of side "+(side-1)+"0 is " +sidelength[side-1]);

/*for(int w=0;w<side-1;w++)
{   
System.out.println("length  is " +sidelength[w]);
}*/
for(int l=0;l<side-1;l++)
{
angles[l] =((180/(Math.PI)))*Math.acos(((lengthtopoint[l]*lengthtopoint[l])+(lengthtopoint[l+1]*lengthtopoint[l+1])-(sidelength[l]*sidelength[l]))/(2*lengthtopoint[l]*lengthtopoint[l+1]));
//System.out.println("Angle= " +angles[l]); 
}

angles[side-1] =((180/(Math.PI)))*Math.acos(((lengthtopoint[side-1]*lengthtopoint[side-1])+(lengthtopoint[0]*lengthtopoint[0])-(sidelength[side-1]*sidelength[side-1]))/(2*lengthtopoint[side-1]*lengthtopoint[0]));    
//System.out.println("Angle= " +angles[side-1]);    

double sum=0;
for(int m=0;m<side;m++)
{
sum=sum+angles[m];
}

System.out.println("The sum of all the angles is  " +sum);  

if (sum==360)
{
System.out.println(" The  point"+(xcheck)+","+(ycheck) +"lies inside  polygon ");   
}
else if(sum<360)
    System.out.println(" The point"+(xcheck)+","+(ycheck)+"lies outside the polygon");
{

}

}


}

}
}