Is it difficult for you to get up in the morning ?  Hiroyuki's bed will solve your problem ! Here is how it works .
   The bed is connected to an alarm clock . First , the alarm clock rings . You have a few minutes to wake up . Next , a tape recorder in the bed plays soft music . The tape recorder in Hiroyuki's bed plays a recording of his girlfriend . She whispered in a soft voice , “Wake up , darling , please .”  After minutes later , Hiroyuki hears a recording of his boss . His boss shouts , “Wake up at once , or you'll be late !”
If you don't get up after the second recording , a mechanical “foot” in the bed will kick you in the head . The bed waits a few more minutes . What ! You’re still in bed ! Slowly the top of the bed rises higher and higher . The foot of the bed goes lower and lower . Finally you slide off the bed and onto the floor . You are out of bed awake !
   Hiroyuki's bed is not in stores . There is only one bed--- the bed Hiroyuki made for a contest（竞赛）. Maybe someday a company will make Hiroyuki’s bed and sell it in stores . Maybe people will buy millions of beds . Then Hiroyuki will be rich . If the bed makes Hiroyuki rich , he won’t need to work , and he won't need to get up early !

某大学利用地理信息技术绘制了广州市公交等时线图（等时线是指在相同时间内利用公交可达最大范围的边际线），用于研究“如何解决城市交通拥堵”问题。下面是以广州市天河区作为出发地点，绘制的当地12时、14时、16时、18时四个不同时刻下时间粒度为1小时的等时线分布图。制作公交等时线图主要依靠（     ）