2.6. Levelling and differential GPS measurements

Use of geodetic tools are planned in order to quantify the geomorphic expression of faults. IPSN has acquired a differential GPS in order to measure environmental parameters with a precise positioning. This tool is also powerful to measure simply x, y, and z coordinates (geographic coordinates) with an accuracy of about 20 cm in a real time session and 5 cm in a post processing session. For our purpose, and based on field experience, 20 cm accuracy is sufficient. This tool consists of two GPS receptors cards : one fixed and emmiting his antenna position to the mobile antenna wich is connected to a PC, recording it's position at regular time intervals. The path can be visualized by the mobile operator on a real time map.

In the firts phase of targeting, this tool has been used to locate precisely geophysics profiles, in order to make altimetric profiles and to map fault traces in quarries (Achenheim, Riedseltz).

Example of positioning of geophysical profiles with Differential GPS:

1. Base positioned at Mundolsheim (left) and mobile processing (right)

2. Path followed for positioning geophysical profiles (graduations every 50 meters)