Chalk Pavement Illusions - How are they done?

Jullian Beaver is an artist famous for his chalk pavement drawings that give a fantastic impression of depth. The illusion is achieved by trickery - the illusuion depends on one looking from a particular perspective.

Here is a famous snap (courtesy of another blog) from the perspective the artist does not want you to look from, and another from the direction he designed the drawing to be viewed from.