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