1053 
INVESTIGATIONS ABOUT THE ACCURACY OF TARGET MEASUREMENT FOR 
DEFORMATION MONITORING 
Mario Alba, Fabio Roncoroni, Marco Scaioni 
Politecnico di Milano, DIIAR, Polo Régionale di Lecco, via M. d’Oggiono 18/a, 23900 Lecco, Italy - (mario.alba, 
fabio.roncoroni, marco.scaioni)@polimi.it 
Commission V, WG V/3 
KEY WORDS: Laser scanning, Calibration, Accuracy analysis, Deformation, Civil engineering, Retro-reflecting target 
ABSTRACT: 
Experience in applications of TLS for deformation monitoring of concrete structures have shown the need of further exhaustive 
study about the quality of retro-reflecting target measurement. Indeed, this factor is highly influencing the georeferencing that is of 
primary importance to compare scans acquired at different epochs. Here some tests carried out by means of a RIEGL LMS-Z420/ 
laser scanner are reported. In addition, several algorithms for automatic retro-reflecting target measurement have been applied, and 
new ones proposed. Results obtained from lab experiments have revealed a good repeatability on target measurement, while the 
accuracy is highly affected by a bias on the measured range. This mainly depends on the distance of the target from TLS, and on the 
angle of incidence of laser beam. In order to compensate for this error, two approaches have been successfully tested: the estimation 
of a corrective function, and the use of an algorithm for automatic retro-reflecting target measurement which locally estimates the 
bias on the range. 
1. INTRODUCTION 
Terrestrial Laser Scanning (TLS) is today widely used in many 
application fields, with different capability to satisfy end-user 
requirements. Broadly speaking, modem instmments (Lemmens, 
2007) are highly capable to provide a 3-D geometric 
reconstruction of objects and surrounding environment through 
fast acquisition of dense clouds of unspecific points. Several 
viable strategies can be followed for georeferencing scans 
captured from different points of view into the same reference 
frame. Currently these account for the use of targets (Valanis & 
Tsakiri, 2004), surface matching algorithms (Akca, 2007), 
direct georeferencing (Gordon & Lichti, 2004), and 
contemporary coregistration of images and 3-Dviews. 
More complex is the information extraction task. In all cases 
where surfaces can be modelled by elementary geometric 
primitives, the process of object reconstruction can be 
completed in an almost automatic way. This happens when 
natural surfaces (e.g. ground, rock faces, water basins, open pit 
mines) have to be reconstructed by TIN models, or regular 
shapes have to be fit into data (e.g. in modeling of industrial 
plants). In the cultural heritage field, much work is still to do in 
object modeling, despite of the amount of information stored in 
laser scanner data (Fassi, 2007). 
Recently the number of people interesting in application of TLS 
for deformation monitoring is quickly increasing. This is 
motivated by the fact that this technique would allow to analyse 
deformations of whole surfaces instead of single points, like 
current monitoring geodetic and photogrammetric systems do. 
Common requirements of these applications are: (i) the high 
accuracy and point density; (ii) the registration of all adopted 
scans at different epochs into the same reference frame in order 
to detect changes of the object shape. So far, the former issue 
has been coped with in different ways, which however always 
try to exploit the redundant observations to reduce the effect of 
measurement errors in surface reconstruction. Approach to do 
this are based on fitting object surfaces to a set of several planar 
patches (Lindembergh & Pfeifer, 2005), or by interpolating the 
whole surface with a regular geometric shape, if possible 
(Schneider, 2006; Van Gosliga et al., 2006). In case irregular 
surfaces have to be compared, the use of surface matching 
algorithms can be adopted to detect local deformation 
(Monserrat & Crosetto, 2008). The latter problem might 
concern manifold aspects, related to the size of the object under 
investigation, the portion of the object which may change its 
shape and the presence of stable the period and the frequency of 
measurements, the need of registering these into a given 
external reference frame, for example referred to the local 
plumb line. In general, the widespread solution to define a 
permanent reference frame common to several scans taken at 
diverse epochs is based on retro-reflective targets (RRT). 
During some experimental tests of deformation monitoring of 
civil engineering structures, the demand of exhaustive study 
about the precision of this kind of targets has arisen, being this 
factor highly influencing the quality of georeferencing that is of 
primary importance to compare scans acquired at different 
epochs. Because this subject has been poorly investigated so far, 
some experimental tests have been carried out to this aim. 
The paper is organized on a preliminary decription of RRTs and 
their use in common TLS practise (Sec. 2). In section 3, the 
experimental tests that have been leaded is described. Data 
analysis and presentation of the outline problems, as well as the 
proposed solution, is the subject of section 4; here also a 
comparative review of the algorithm which are usually adopted 
for the automatic measurement of RRTs is reported. Finally, 
conclusions and further developments are drawn in section 5.