We combine ALOS-PALSAR coherence images with airborne LiDAR data, both acquired over the Piton de la Fournaise volcano (Reunion Island, France), to study the main errors affecting repeat-pass InSAR measurements and understand their causes. The high resolution DTM generated using LiDAR data is used to subtract out the topographic contribution from the interferogram and to improve the radar coherence maps. The relationship between LiDAR intensity and radar coherence is then analyzed over several typical volcanic surfaces: it helps to evaluate the coherence loss terms. Additionally, the geometric and physical properties of these surfaces have been measured in situ. Coherence deteriorates over pyroclastic deposits and rough lava flows due to volume and surface scattering. In the presence of vegetation, it is directly related to plant density: the higher the Leaf Area Index (LAI), the lower the coherence. The accuracy of InSAR measurements strongly decreases for LAI higher than 7.