subroutine chleng(x,y,n)
*     ********************************************************
*     check the equal-distanced condition between neighbouring
*     points by calculating the deviation from the mean
*     ********************************************************
      integer n,j
      double precision x(0:8192),y(0:8192),dist(8192)
      double precision h,hp,err,ave
      common/gridsz/h,hp
      ave = 0.D0
      err = 0.D0
      do 10 j=1,n
         dist(j) = dsqrt( (x(j)-x(j-1))**2 + 
     +                    (y(j)-y(j-1))**2 )
         ave = ave + dist(j)
 10   continue
      ave = ave*h
      do 20 j=1,n
         err = err + (dist(j)-ave)**2
 20   continue
      err = dsqrt(err*h)
      print *,'equal distance error = ',err
      end