The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
1054 
2. RETRO-REFLECTIVE TARGET OVERVIEW 
The use of RRTs for scan registration is a standardized and 
operational process, which is usually recommended by different 
instrument vendors. The automatic recognition and 
measurement of such targets is implemented in several 
terrestrial laser scanners and in their companion data processing 
softwares. Instruments are capable to make a preliminary 
localization of the rough target position, and consequently a 
high resolution scanning is performed in the nearby of it. As 
alternative, the user might manually aim the scanner head 
towards targets’ approximate position, or these can be achieved 
by a list of point coordinates. This latest solution is worth for 
monitoring applications, where the same targets are measured at 
different epochs, and then their rough position is already known. 
However, strategy and algorithms adopted by specific 
instruments to carry out RRT measurement is generally 
unknown, in the knowledge of the authors. 
Some instruments can use only targets of a specific material and 
shape (e.g. Leica); others (Trimble) are capable to work with 
different kinds of RRTs, but they guarantee the best precision 
only by using proprietary targets. Finally, other scanners are 
able to use indifferently RRs featuring other shape (plane, 
cylindrical, spherical) and size. 
For the above-mentioned grounds, a common method to set the 
best dimension of target as a function of the distance from the 
instrument stand-point is difficult to be found. On the other 
hand, some general factors exist which influence the precision 
of measurement of all ToF scanners: 
• the laser beam-width divergence; 
• the maximum angular resolution; 
• the incidence angle of laser beam on the target surface; 
• the target intensity response. 
In particular, hereafter some experiences about RRT 
measurement with the Riegl LMS-Z420/ laser scanner will be 
reported. Technical documentation on this ToF long-range 
instrument can be directly found on the vendor website. 
2.1 RR target measurement with Riegl LMS-Z420/ 
The Riegl LMS-Z420/ laser scanner can be georeferenced on 
the basis of RR targets, which are used as Ground Control 
Points (GCP) to compute a 3-D roto-translation from the 
Intrinsic Reference System (IRS) of each scan to the Ground 
RS (GRS). During the data collection stage, the approximate 
coordinates of each RRTt are automatically identified by data 
acquisition control software Riegl Riscan Pro. This task is 
accomplished by analysing the points featuring the highest 
intensity response in a preliminary low-resolution scan. Thank 
to the non Lambertian behaviour of retro-reflecting materials, 
laser beams are reflected towards the scanner head with a very 
high intensity that outstands from the surrounding points. In a 
second stage, targets are scanned at higher resolution, according 
to the distance from the scanner. This solution allows it to 
capture the surface around each target at a higher point density 
w.r.t. the remaining parts of the object to be scanned. By this 
approach, the precision of target measurement is improved, and 
consequently that of georeferencing as well. 
From the analysis of several targets captured with LMS-Z420/ 
scanner, it is possible to outstand 4 different strategies that are 
followed for RRT scanning. Parameters influencing the 
selection of a specific strategy are the size (i) of the target and 
its distance (ii) from the sensor. These values are a priori 
available before high-resolution scanning: the former (i) 
because the user has to select the specific kind of target to be 
used (shape and dimensions); the latter (ii) it’s already known 
from preliminary low-resolution scanning. First the Riegl 
system scans a squared window (with side D) large about 5 
times the largest dimension d of the target around its 
approximate location whit a resolution that depends on the 
distance. From this step on, different strategies are applied: 
1. up to about 4 m: the TLS scans the window whit a 
grid of 100x100 points, resulting in a horizontal and 
vertical spatial grid resolutions of: 
s H = s y = D/100 ~ d/20 (1) 
2. from 4 to 32 m: the horizontal scan resolution is 
selected as in the previous case 1; in the vertical 
direction, the surface is scanned at the maximum 
angular (and then spatial) resolution; 
3. from 32 to 60 m: the system adopts a scan window at 
the maximum angular resolutions in both directions;, 
4. over 60 m: a grid of 65x65 points is scanned at the 
maximum angular resolution, disregarding the target 
size. Obviously, the dimensions of the scan window 
increases with a fix proportion w.r.t. to the range. 
This strategy has been implemented to assure a correct scanning 
of target windows in common practitioner applications, where 
accuracy in surface reconstruction of a few cm is enough (e.g. 
in open pit surveys). In special applications for deformation 
monitoring, where a higher accuracy in georeferencing is 
needed, the size of the target must be accurately selected 
according to the strategy that will be applied to perform its 
measurement. In particular, the size of the target should not 
exceed the scan window size, otherwise it would not be possible 
to detect the target centre. On the contrary, if the target size is 
too small, this would be measured with a not sufficient number 
of points. The same should be checked out by analysing the 
laser footprint. 
3. DESCRIPTION OF EXPERIMENTS 
In this section we will give a presentation of different tests 
carried out by means of a Riegl LMS-Z420/ laser scanner in 
order to assess problems in the measurement of RRTs. Here the 
target search and scanning has been performed by using the 
data acquisition control software Riscan Pro. 
3.1 Test 1: long-range measurements 
The first test has been carried out to evaluate the precision of 
target measurement according to different ranges from sensor- 
to-object, up to 300 m. Influence of laser beam angle of 
incidence, laser footprint, and type and dimension of RRTs are 
analysed. The measurements have been carried out in an 
outdoor site due to the required long-range. 
During the first test a timber frame of 1 x2 m has been adopted, 
where 11 targets of different size and shape covered by retro- 
reflecting paper have been fixed. All targets can be grouped 
into the following 3 categories: 
1. square foil with 4 cm (no. 8), 5 cm (no. 4), and 6 cm 
(no. 1,3,9,11) side; 
2. circular foil with 10, 20 and 30 cm diameter (no.2,5,6, 
respectively);