The POLY_AREA function returns the area of a polygon given the coordinates of its vertices. This value is always positive.
It is assumed that the polygon has n vertices with n sides and the edges connect the vertices in the order:
[(x1,y1), (x2,y2), ... , (x n ,y n ), (x1,y1)]
such that the last vertex is connected to the first vertex.
This routine is written in the IDL language. Its source code can be found in the file
subdirectory of the IDL distribution.