The reference plane representing the average elevation of the finished ground level measured at a distance of 3 m (10 ft) from all exterior walls of the building. NFPA 1141, 2003 ed
National Fire Protection Association, Illustrated Dictionary of Fire Service Terms, Jones & Bartlett Publishers, MA. USA 2006. https://books.google.com.kw/books?id=W0WIRa3Gl4QC& printsec=frontcover&source=gbs_ge_summary_r&cad=0# v=onepage&q&f=false, (Fr Srvc - Amer) {141}