Topographic effects on levelling comparisons: case studies of Saint-Paul-de-Fenouillet (Eastern Pyrenees) and Arudy (Western Pyrenees), France.
Rigo, A; Cushing, M.
COMPTES RENDUS DE L ACADEMIE DES SCIENCES SERIE II FASCICULE A-SCIENCES DE LA TERRE ET DES PLANETES, 329: (10) 697-704.
Differences between repeated levelling surveys measure vertical movements. We show that these comparisons are correlated with the topography of the measured lines. This correlation constitutes an important additional source of error in levelling. We analyse levelling lines in two regions in the French Pyrenees where earthquakes occurred recently (Saint-Paul-de-Fenouillet and Arudy) and correct the levelling comparisons using the correlation between the topographic slope and the measured tilt. This correction reduces the vertical signature by a factor of 2. This demonstrates that it is absolutely necessary to take into account the topographic effect in analysing and interpreting levelling comparisons.