c  decimate matrix subroutine
c  This subroutine decimates the input matrix by depth in mulitples of the 
c    input resolution, created 9/96 
c
      subroutine decimate_matrix(grid,grerror,nx,ny,xres,yres,
     1    xleft,xright,ytop,ybot)
      parameter (ngx=1000,ngy=700)
      character*1, quest
      integer nx,ny,ifactor,i,j,k
      real xres,yres,xleft,xright,ytop,ybot
      real grid(ngy,ngx),newgrid(ngy,ngx)
      real grerror(ngy,ngx),newgriderr(ngy,ngx)

c  Ask user if they want to decimate
      print *,'Input matrix pressure resolution is:  ',yres
      print *,' '
      print *,"Do you want to decimate this matrix in pressure? (y/n)"
      read *,quest
c  if the user says 'yes'
      if (quest .eq. "y") then
c  ask user for the decimation ifactor 
       iyres = int(yres)
       print *,'Enter output vertical resolution in multiples of ',iyres 
       read *,nyres
       ifactor = nyres/iyres
	  if ((ifactor .lt. 1) .or. (ifactor .ge. (ybot/yres))) then
            print *,'error: no decimation'
            return
          endif
c
c  create a new, vertically decimated grid
      i = 0 
      dy = float(ifactor)*yres
c      print *,'ifactor ',ifactor,nyres,iyres,dy
      ybnew = ytop

c     do while (((i*ifactor)+1) .lt. (ybot/yres))
      do while (((i*ifactor)) .le. (ybot/yres))
       do j = 1, nx
          newgrid(i+1,j)=grid((ifactor*i)+1,j)
          newgriderr(i+1,j)=grerror((ifactor*i)+1,j)
       enddo
       i = i + 1
       ybnew = ybnew + dy
      enddo
c  end-of-while
c
c  replace the old grid with the new grid
	  do j = 1, i
	    do k = 1, nx
	      grid(j,k)=newgrid(j,k)
	      grerror(j,k)=newgriderr(j,k)
	    enddo
	  enddo
c  set the new y dimension, yres and also ybot if necessary
	  ny = i
      yres = dy
      ybot = ybnew - dy 
c     print *,'new ',ybot,yres
c
      end if 
	
      return
      end