Approach Slope : Imaginary areas to be kept clear of obstructions extending out and away from the approach ends of runways at either 20:1 (visual) or 50:1 (precision instrument), and at 7:1 along the side transition areas
Ilan Kelman (ed.), Vulnerability and Resilience Definitions and References, Version 2, 2007. http://www.islandvulnerability.org/docs/vulnres.pd f, (Vulnr and Resilience - Brit) {524}