TWO SURVEY CREW EMPLOYEES WERE USING A JLG MANLIFT DURING A CENTER LINE CONTROL STAKE OUT. THE JLG FELL OVER THE HIGH WALL FROM THE UPPER FLOOR TO THE NEXT LOWER FLOOR AT A LOCATION WHERE THE FACE WAS NOT SLOPED & THE CREW DID NOT KNOW THE TOP OF THE HIGH WALL WAS NOT BARRICADED. NO WARNING SIGNS WERE POSTED.