C# code to determine if the polygon vertices are in the clockwise direction

In some GIS polygon data formats such as the ESRI shape file format, the holes of a polygon are stored in the counter-clockwise direction while the outer polygon is stored in the clockwise direction. If you are writing a program to read a polygon geometry containing holes, it may be necessary to determine the holes by checking whether the vertices are in the counter-clockwise or clockwise direction.

The C# code snippet below is useful to determine whether the vertices of a polygon are in the clockwise direction or counter clockwise direction. Simply pass in the polygon vertices into the function as an array of PointF structures where the first and last member of the array are the same point. The function will return true if the vertices are in the clockwise direction and false if they are in the counter-clockwise direction.

private bool IsClockwisePolygon(PointF[] polygon)

{

bool isClockwise = false;

double sum = 0;

for ( int i = 0; i < polygon.Length-1; i++)

{

sum += (polygon[i + 1].X - polygon[i].X) * (polygon[i + 1].Y + polygon[i].Y);

}

isClockwise = (sum > 0) ? true : false;

return isClockwise;

}