Abstract. Terrestrial laser scanning (TLS) is one of the most
promising surveying techniques for rockslope characteriza-
tion and monitoring. Landslide and rockfall movements can
be detected by means of comparison of sequential scans. One
of the most pressing challenges of natural hazards is com-
bined temporal and spatial prediction of rockfall. An outdoor
experiment was performed to ascertain whether the TLS in-
strumental error is small enough to enable detection of pre-
cursory displacements of millimetric magnitude. This con-
sists of a known displacement of three objects relative to a
stable surface. Results show that millimetric changes cannot
be detected by the analysis of the unprocessed datasets. Dis-
placement measurement are improved considerably by ap-
plying Nearest Neighbour (NN) averaging, which reduces
the error (1σ ) up to a factor of 6. This technique was ap-
plied to displacements prior to the April 2007 rockfall event
at Castellfollit de la Roca, Spain. The maximum precursory
displacement measured was 45 mm, approximately 2.5 times
the standard deviation of the model comparison, hampering
the distinction between actual displacement and instrumen-
tal error using conventional methodologies. Encouragingly,
the precursory displacement was clearly detected by apply-
ing the NN averaging method. These results show that mil-
limetric displacements prior to failure can be detected using
TLS.