The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
1342 
3. WORK PROCESS 
3.1 Exact center of the wall located 
immediately to guarantee the quality. This work includes GPS 
and IMU data preprocess to get trajectory, laser and image 
coverage checking, image definition checking etc. 
Because the corridor is narrow we have to locate the wall’s 
exact center so to cover every part of the wall. We use 1:50000 
relief map and the historical photography data to get the basic 
location of the wall. At the same time we use GPS to get the 
comers’ coordinates so as to locate the wall exactly. 
3.2 Airborne laser system 
In order to get enough laser points of the wall we design three 
lines for LiDAR system that are one just above the wall, each 
one for both sides they are 75m to 120m from the wall. 
Followings are the parameters: 
■ Altitude above ground = 300m(average elevation is 
about 35 m) 
■ Airplane speed = 150 km/h 
■ Scanner angle = 45 degree 
■ Laser frequence = 100,000hz 
■ Focal length = 50mm 
The point spacing is 10cm to 30cm. The pixel size on ground is 
about 7cm. 
There are two synchronized ground GPS receiver when flying, 
which place at the control points of south-east comer of the wall 
and south to Big-Stone bridge respectively. For the definition of 
photo we choose clear day and check the picture quality when 
work. For the covered west parts we acquire the data when sun 
angle is wide enough and shadow is most small. 
Figure 2: Laser data of the wall 
3.3 Oblique picture taking 
Figure 3: DEM of the wall 
Figure 4: DOM (weng cheng) 
3.6 DEM and DOM making 
Firstly we classify laser data by LiDAR software and eliminate 
the points which are not belong to surface. Then we make DEM 
through surface laser points. The wall points are classify to 
surface. 
On the base of classified laser data we micro rectify each 
pictures and mosaic them automatically then produce 
orthophotos. According to requirement we do some mix work to 
pictures’ color to get high quality DOM. 
In order to get the side texture of the wall we design two lines 
which are 350m inside and outside of the center of the wall. For 
the west parts they are covered by mass trees we add one more 
line which is 225m to the west of the wall. 
Based on DEM and DOM data we can achieve 3D browse, 
distance, acreage and volume measurement, section cutting and 
making. 
3.4 Terrestrial laser scan 
The parts of wall which are covered by tress LiDAR and 
oblique camera cannot their ideal textures. Therefore we use 
terrestrial laser scanner to get them. 
3.5 Data checking 
After data acquiring we will do some preprocess work