Global Navigation Satellite Systems (GNSS) provide accurate and reliable positioning solutions in open field environments. However, the positioning performance is not the same in dense urban areas, where satellite signals are blocked or reflected by tall buildings. A 3D city model, "Urban Trench", is introduced to simulate blockage and reflection of GNSS signals. The "Urban Trench" model assesses the reflection environment of the city and the non-light-of-sight (NLOS) ranging errors are corrected, based on satellite elevation and a 3D surface model. Subsequently, the metric of NLOS signal exclusion using an elevation-enhanced map is developed and tested using real vehicular data in the test urban network of Nantes. A GPS&GLONASS-constellation single-frequency receiver is used during the experiment. The performance of both systems, stand alone and in combination as dual-constellation, are presented, compared and evaluated, with and without "Urban Trench" model implementation. Additionally, a fault detection and exclusion test is applied, to check and enhance the integrity of the output.