The campus safety committee has asked Professor Ibrahim to study bicycling on his campus. He trains two observers and has them observe the number of cyclists and their safety at various points around campus. He randomly selects 10 observation locations from the places bicycles can be ridden on campus and randomly selects five 1-hour spans of time for each place. He has his observers make observations at each of the 10 places for each of the five time spans. Which type of sampling is he using