function area = quadrilateral_area ( xlist, ylist )

  xlist1 = [ xlist(1), xlist(2), xlist(3) ];
  ylist1 = [ ylist(1), ylist(2), ylist(3) ];
  area1 = triangle_area ( xlist1, ylist1 );

  xlist2 = [ xlist(1), xlist(3), xlist(4) ];
  ylist2 = [ ylist(1), ylist(3), ylist(4) ];
  area2 = triangle_area ( xlist2, ylist2 );

  area = area1 + area2;

  return
end