;; | ----------------------------------------------------------------------------
;; | GE_polyarea
;; | ----------------------------------------------------------------------------
;; | Function : Calculate the area of a polygon
;; | Argument : ptlist - list of points forming the polygon
;; | Return   : The area
;; | Update   : March 6, 1998
;; | e-mail   : rakesh.rao@4d-technologies.com 
;; | Web      : www.4d-technologies.com
;; | ----------------------------------------------------------------------------


(defun GE_polyarea(ptlist / nverts cnt area tmp pt1 pt2)

(setq
	cnt 0
	nverts (length ptlist)
	area 0.0
)

(while (< cnt (1- nverts))
(setq
	pt1 (nth cnt ptlist)
	pt2 (nth (1+ cnt) ptlist)
	area (+ area (* (cadr pt1) (car pt2)))
	cnt (1+ cnt)
)
)

(setq
	pt1 (nth (1- nverts) ptlist)
	pt2 (nth 0 ptlist)
	area (+ area (* (cadr pt1) (car pt2)))
	cnt 0
	tmp 0.0
)
    
(while (< cnt (1- nverts))
(setq
	pt1 (nth cnt ptlist)
	pt2 (nth (1+ cnt) ptlist)
	tmp (+ tmp (* (cadr pt2) (car pt1)))
	cnt (1+ cnt)
)
)

(setq
	pt1 (nth 0 ptlist)
	pt2 (nth (1- nverts) ptlist)
	tmp (+ tmp (* (cadr pt1) (car pt2)))
	area (abs (* 0.5 (- area tmp)))
)
)