GNSS localization in an urban environment remains a key issue that must be solved in order to enable innovative driver assistance systems or cooperative applications. This key issue exists not only in terms of accuracy but also in terms of integrity. Actually, one aims at reducing as much as possible the error mainly due to multipath on buildings and provide in the mean time a certain guarantee on the user position exactness. This article shows how a simple geometric modeling, named 'Urban Trench', of the city environment can improve significantly the positioning accuracy. Authors also propose an algorithm for computing an urban trench protection level, with a much higher reliability compared to conventional protection level whose open sky assumption is violated. Experimental results are given in the cities of Nantes, Paris, and Toulouse, France.