Looks like the 2nd one didn't come through as a clickable link. Sorry about that. ----- Mike, It's all in the CFRs, the Code of Federal Regulations. More specifically, FAR Part 77 - Objects Affecting Navigable Airspace. The more general information you're looking for is in section 77.23, Standards For Determining Obstructions: http://tinyurl.com/jt5wf And airports are surrounded by imaginary "surfaces" dependent upon their runways and approaches; these deal with the airspace immediately surrounding airports, in section 77.25: http://tinyurl.com/e56rg Horonjeff/McKelvey's "Planning & Design of Airports", fourth edition, describes it in detail in Chapter 7 as well. Greg Rendell greg@xxxxxxxxxxxxxxx KPHL