The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
up to 81 gon. However, problems concerning the range 
measurements on RRTs still occur also for short distances. 
Thus, data acquired in Test Ex3.1 have been used to further 
study the relation between the offset from coordinate centre of 
target and the plane fitting the point-cloud of target scan 
without using the more reflective part. 
The specific scan of each target has been exported and 
processed by the algorithm “intersect” (see Sub-sec. 4.1). 
Finally the short distance from the gravity centre G of each 
target and its related background plane ;ris computed. 
The results (Figure 7) presents a growth of the bias as far as a 
distance of 18 m, then the trend keeps constant and slightly 
decreases after 28 m. This results is not in disagrement with that 
obtained in case of long-range distances (Test 1) and 
summarized in figure 5. 
In a second time, data coming from Test Ex3.2 considering also 
tilted RRTs have been analysed. Here the coordinate of target 
centres measured in Riscan software have been used. In each 
experiment, the coordinate of target in the position facing the 
TLS have been assumed as reference. 
Figure 6. Planimetric (A) and altimétrie (B) residuals on targets 
measured from stand-points 100 (red vectors) and 200 (blue 
vectors) 
Figure 7. Red points represent arget off-plane in function of the 
distance, evaluated in close-range field (Test Ex3.1). Blue 
points are residuals after the data interpolation by function (2), 
which is drawn as black line. 
The computed discrepancies between the reference coordinates 
and those measured in other tilted positions (see table 8) have 
resulted very small, and only a systematic error in range 
direction has been outlined. According to this result, the same 
offset calculated in Test Ex3.1 has been recomputed. Figure 9 
shows that the error in range as function of the incidence angle 
can be attributed to off-plane bias of each RRT, as confirmed 
by the high linear correlation (p=-0.93) between off-plane bias 
and error in range. 
After the analysis of experiment results, two different ways 
implemented to reduce the error in range during the RRT 
measurement will be described in sub-section 4.3. 
Rotations 
Intensity 
10-1| ' 
Range 
[mm| 
Theta 
Igonl 
Phi 
Igonl 
Off-plane 
bias [mm| 
H 
mean 
-0.150 
3 
-0.003 
-0.001 
11 
±G 
0.172 
5 
0.005 
0.004 
3 
V 
mean 
0.000 
5 
0.002 
0.000 
10 
±a 
0.001 
5 
0.006 
0.001 
3 
3-D 
mean 
-0.193 
7 
0.003 
0.001 
10 
±o 
0.188 
15 
0.008 
0.003 
5 
Table 8. Statistics computed on differences between RRTs 
measured in tilted positions w.r.t. position directly facing the 
TLS. In rows entitles as “H” and “V”, the full set of tilted 
positions in horizontal and vertical directions are summarized, 
respectively; in the row named“3-D” contemporary rotations in 
both directions are considered 
Figure 9. Range error (red) and off-plane bias (bleu) in 
function of the incidence angle 
4.2.2 Analysis of repeatability 
In order to evaluate the repeatability of the RR target scanning, 
the standard deviation of coordinates and other parameter of 
each target have been computed on the whole set of 9 repeated 
scans (Test Ex2.1). The RMSE of cartesian coordinate has 
always resulted less the 1 mm (according to the GRS shown in 
Figure 5, ±0.6 mm in X and ±0.8 mm in Y and Z). Considering 
spherical coordinates, RMSE has resulted as ±4.0 and ±9.2 
mgon for and angles, respectively, and ±1.1 mm for r. 
The average number of pixel for each target has been 173, 
while the RMSE of intensity ±1.4% of the full range. These 
findings show a very good repeatability of target measurement 
in a close-range environments, especially if it is compared to 
that can be obtained from other topographic instruments. Even 
though only targets at distances under 10 m have been 
considered here, the results can be extended linearly to a longer 
ranges. 
A second analysis of repeatibility has been carried out by 
considering data acquired from Test Ex2.2, where 3 different 
scans rotated of 120 deg have been captured from stand-points 
100 and 200. Analysis of absolute residuals w.r.t. GCP 
coordinates have been already reported in Par. 4.1.1. Here the 
repeatability of target measurement from the same stand-point 
(without changing the instrument setup), but with different