GPR & RT accuracy?

How accurate are GPR & RT technology in determining depth or 'z' coordinates?

The article 'Determining what lies beneath the ground is essential in avoiding damage incidents,' from the Damage Prevention PROfessional- Fall 2010 issue raised valid questions and feedback. Here is a blurb from the article but you can also read more here. 

"When nonmetallic facilities are involved, a number of options are available including acoustical locators, ground penetrating radar (GPR) and radar tomography (RT). Combining these technologies with geographic positioning systems (GPS) will allow most underground utilities to be located, providing X and Y coordinates. Vacuum excavation is then utilized to provide exact X, Y and Z coordinates."


What are your thoughts on GPR & RT technologies in locating depth?