distrl Subroutine

    subroutine distrl(x0, y0, x1, y1, x2, y2, xm, ym, dm, wm)
        !! The subroutine calculates the minimum distance from a given receptor
        !! point to a given line source.
        real, intent(in) :: x0 !! Receptor point x-coordinate
        real, intent(in) :: y0 !! Receptor point y-coordinate
        real, intent(in) :: x1 !! Line source x-coordinate 1
        real, intent(in) :: y1 !! Line source y-coordinate 1
        real, intent(in) :: x2 !! Line source x-coordinate 2
        real, intent(in) :: y2 !! Line source y-coordinate 2
        real, intent(out) :: xm !! Minimum distance x-coordinate 
        real, intent(out) :: ym !! Minimum distance y-coordinate
        real, intent(out) :: dm !! Minimum distance

        ! Local variables
        real :: num, denum, wm

        if (x1 == x2 .and. y1 == y2) then
            wm = 0.5
        else
            num = (x0 - x1)*(x2 - x1) + (y0 - y1)*(y2 - y1)
            denum = (x2 - x1)*(x2 - x1) + (y2 - y1)*(y2 - y1)
            wm = num/denum
        end if

        wm = min(wm, 1.0)
        wm = max(wm, 0.0)

        xm = (1.0 - wm)*x1 + wm*x2
        ym = (1.0 - wm)*y1 + wm*y2
        dm = sqrt((x0 - xm)*(x0 - xm) + (y0 - ym)*(y0 - ym))
    end subroutine distrl