This glitch can be used in single player.This will allow you to get on tall buildings. This glitch wont really help you but its a fun glitch to do. Find a police car. Then find a large building with a flat wall(may be hard to find one but there are some).
Manage to get the police car side ways on the wall. You can usually do this by ramming the wall with the side of your car. Once you do this, drive forward and up. You will begin to climb the wall. Once you reach the top edge of the building, the car will lean forward. When it does then get out -- sometimes the car will not make it up with you. Now you will be at the top of the building, and if it is tall enough you can view thw whole city all around. Enjoy!