For surface-based ducts, the thickness of the duct = the height of the inversion.

For elevated ducts, the lower boundary of the duct is usually less defined than the upper boundary (the inversion level), so LUF determination is less reliable.

Unlike an ionospheric skip MUF (maximum usable frequency) which is a sharp hard frequency limit determined by the critical angle, the ducting LUF instead is a soft frequency limit below which a gradual decline in signal strength occurs.