derm_cart Subroutine

  pure subroutine derm_cart(self, f, dfm, char)
    use constants, only: pi
    implicit none
    class(abstract_eqfile_cart_geo_type), intent(in) :: self
    real, dimension(:,:), intent(in) :: f
    real, dimension(:,:,:), intent(out) :: dfm
    character(1), intent(in) :: char
    integer :: i, j, nw, nh
    nw = self%nw ; nh = self%nh
    i = 1
    dfm(i,:,1) = -0.5 * (3 * f(i,:) - 4 * f(i+1,:) + f(i+2,:))

    i = nw
    dfm(i,:,1) = 0.5 * (3 * f(i,:) - 4 * f(i-1,:) + f(i-2,:))

    j = 1
    dfm(:,j,2) = -0.5 * (3 * f(:,j) - 4 * f(:,j+1) + f(:,j+2))

    j = nh
    dfm(:,j,2) = 0.5 * (3 * f(:,j)-4 * f(:,j-1) + f(:,j-2))

    do i = 2, nw-1
       dfm(i,:,1) = 0.5 * (f(i+1,:) - f(i-1,:))
    end do

    do j = 2, nh-1
       dfm(:,j,2) = 0.5 * (f(:, j+1) - f(:, j-1))
    end do

    if (char == 'T') then !Could a similar approach be ported to derm_eqfile?
       where(dfm(:, :, 2) > pi * 0.5)
          dfm(:, :, 2) = dfm(:, :, 2) - pi
       end where
    end if
  end subroutine derm_cart