ICAO ANNEX4

11.3.3 A scale indication shall be given.

11.3.3.1 Except where this is not practicable, a distance circlewith a radius of 20 km (10 NM) centred on a DME located on or closeto the aerodrome, or on the aerodrome reference point where nosuitable DME is available, shall be shown; its radius shall beindicated on the circumference.

原文地址：http://www.dispatcher.cc/2011/05/26/20km%e8%8c%83%e5%9b%b4%e5%9c%88/

被问道国内航图上那个20KM的圈是什么？我也不知道。感谢Uncle Wong的文章帮我找到出处。原来他在2011年就知道了。