[udig-devel] How to find whether three coordinates are collinear

[vishnu soman <vishnuit@xxxxxxxxx>]

Hi All,

 

I am sure that my questions is a very generic one , probably a pure java problem. However i have been trying to find a way to identify whether three coordinates are collinear using the same logic of finding it doesnt seem to work on examples which take 'Points' as inputs.

Two approaches can be

1. Find the area of the triangle forming the three coordinates/points . If they are on the same line; the value of area must be zero.

2. Split the line joining these coordinates into two and find the individual slopes for the same. If they are on the same line, the slopes will be same.

 

The following is the method i am trying.

 

private

 boolean

collinearCheck( Coordinate endPointOne , Coordinate intersection,Coordinate endPointTwo ){

boolean isCollenear = false;

 

//Area of triangle approach

double area = (Math.round(endPointOne.x )* (Math.round(intersection.y) - Math.round(endPointTwo.y)) + Math.round(intersection.x )* (Math.round(endPointTwo.y) - Math.round(endPointOne.y)) +

Math.round(endPointTwo.x) * (Math.round(endPointOne.y) - Math.round(intersection.y)));

if((endPointOne.x * (intersection.y - endPointTwo.y) + intersection.x * (endPointTwo.y - endPointOne.y) +

endPointTwo.x * (endPointOne.y - intersection.y))<= 0)

if(Math.round(area) <= 0)

{

isCollenear = true;

}

 

// Slope Approach

double numeratorOne = Math.round(intersection.y) - Math.round(endPointOne.y);

double denominatorOne = Math.round(intersection.x) - Math.round(endPointOne.x);

double numeratorTwo = Math.round(endPointTwo.y) - Math.round(intersection.y);

double denominatorTwo = Math.round(endPointTwo.x) - Math.round(intersection.x);

double result1 = Math.round(numeratorOne/denominatorOne);

double result2 = Math.round(numeratorTwo/denominatorTwo);

if(result1== 0 && result2==0){

isCollenear = true;

}

return

isCollenear;

}

In both these cases while using coordinates as inputs ; even for simingly collinear situations i end up getting values like 4 etc for the area. And for situations that are clearly non collinear; i end up getting the same value for slope.

 

Is there a way I can obtain the clear notifier of collinearity using any udig construct? Is my approach correct?

The sample value for coordinates i pass to the method are

Coordinate endPointOne  = -26.666666666666686, 32.38095238095238  .... etc

 

Looking forward for your inputs.

 

 

Thanks & Regards

VS