Avigation easements are instruments developed to convey airspace rights over another property and are typically acquired by an airport whenever air rights and/or obstruction removal is required for airspace protection (for example tree removal or trimming). Easements can also be used to control land use around an airport when fee acquisition is not feasible (RPZ protection).  FAA AC 150/5100-17, chapter 2, section 2-15 (b) describes Avigation Rights to be appraised and acquired.