The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008
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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);