destruction of control points. There are two kinds of connection
targets are used to join the point cloud and control network:
Figurel show planar targets, a reflection patch (a) is stitch on
the Leica HDS target (b) with its centers overlapped, the scan
site could be measured fast and precisely with this target.
(a) (b)
Figurel reflection patch on HDS4500 targets
Figure2 show sphere targets: the small reflected ball (Sphere
targets of TCA2003 i.e.) is set on the same height with scan
spheres, thus the scan target could be measured with high
accuracy.
TCA target SITE
Figure2 Concentric of TCA target and scan target
2.2 Terrestrial LIDAR Scanning
The principles of field work scan sites need to ensure in
condition of the inter-visibility among sites, Scan sites used are
as less as possible to cover complete object data, and the layout
of sites should take the combined determination of control work
into account. The setting of scanning parameters is under the
principle to keep data utilization and scanning efficiency
balance, and guarantee to acquire sufficient density of data in
the shortest time, different density is set according to the scan
distance. The whole process of field work is shown in Figure 3.
| Design of Control | | Control Network |
ge I ;
EE Network | Surveying ED
Field | & & # Scanning |
observation d # [oo ome
| ; s scc] |. Contribution of
# Design of Scanning > Stations
Figure3 Scanning Process of Large Steel structure Building
2.3 Key Techniques in Data processing
Internal data Processing mainly consists of data registration,
segmentation, feature extraction and comparative analysis of
data. The goal of data registration is mainly to put field work
scanning data into a complete object and converse into the
datum system of survey or design, multi-stations are registered
with control points; data segmentation is to separate surveyed
object out for feature extraction; point cloud data measurement
information is hidden in the point cloud, accuracy of feature
points collected directly through point cloud data is relatively
low, but high accuracy after converting data into a mold,
feature extraction is to extract measured specific feature point
from point cloud; Finally, get the results via the overall
comparative analysis of data.
3. EXTRACTION OF GEOMETRIC FEATURE POINTS
Steel-structure building construction survey is focus on finding
the right feature points for comparative analysis, which are
extracted from local geometric features of Steel construction.
There are generally three types of extracted geometric feature:
planar patch features, spherical and rod-shaped features.
Common methods of these three characteristics to obtain
feature points are introduced as following.
3.1 Extraction of corner points
There are many box interfaces in large Steel structure building
construction, parts of those interfaces are always approximate to
be planar patch features, four corners of the box interfaces get
the design coordinates. There is chamfer in the interface of
Steel structure in actual construction, the coordinates of the
corner can’t be measured precisely, and then the intersection
technology of planar patch features can be used to access the
precise feature corner.
According to the planar equation a, +a x+a,y=z ,
points on the planar construct equation as follows:
n
0=> (a, +ax +ay, +z) (D
i=0
In which a,,a,,a, are planar parameters, which can be
solved by the least square method.
In order to extract the vertex coordinates, the ridge of port need
to be extracted firstly, then the intersection of ridge and
interface plane is the vertex of port. As general steel structures
all have certain thickness, when extracting the feature points, the
differences of both internal and external surface of interface in
the measurement results should be considered to ensure the
accuracy of the results. Figure 4 shows corner feature extraction
results.
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