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(a) Box Interface 
  
(b) Corner point 
Figure4 feature extraction of box vetexs and corners 
3.2 Extraction of Geometric Centre 
There are more rod-shaped and spheres in steel structure, and 
these characteristics are common used in actual construction 
survey. For the spherical characteristics, centers are usually 
extracted as feature points. For the rod-shaped components, the 
axis of its intersection can be seen as feature points. 
The expression for sphere is: 
  
-Rz0 (2 
  
X,,X,,Z, R are spherical parameters, 0 = (zz 
is spherical center, and R is spherical radius. The general 
form expresses is as follows: 
F(x,y,z2)=x"+y’ +2’ +c x+cy+c,z +c, =0 (3) 
Among them, we have already known the radius of R, the center 
coordinates can be solved with least-square method. 
Spatial cylinder could be expressed as: 
  
[(x—x,)m— (y= y IT 
F(xy,z)e MIG-»)n-G-z)m]-R-0 (9 
tH z=2zY +(x x nl 
In equation (4) » X4J,,2,.7, n,l,R as rod-shaped 
surface parameters , O= (x, Vo>Zo) says a point of 
rod-shaped surface axis, a =(m,n,l) is the unit vector of 
rod-shaped surface axis, R is rod-shaped surface radius. 
There must be exists some noise points in the point-set selected 
for fitting the specific geometry. Generally, the fitting error of 
the specific geometry is {mm Figure 6 shows the rods 
intersection of the results and the extracted point in the point 
clouds. 
  
Figure 6 Rod-Shaped Components and its fitting center 
4. APPLICATION EXAMPLES 
4.1 National Stadium (Bird’s Nest) 
The Beijing National Stadium, known as the Bird's Nest is the 
main stadium for the 2008 Summer Olympics. The stadium is 
330 meters long and 220 meters wide, and is 69.2 meters tall. 
The 250,000 square meters stadium is to be built with 36 km of 
unwrapped steel, with a combined weight of 45,000 tones. 
Twenty-four columns are installed firstly, and then the 
connection part are assembled on the ground and installed in the 
air. The ports of columns need to be measured and compared 
with connection part to make sure precise installation. 
Figure7 shows the install field and point cloud of installed 
columns.