The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
river, streams and lake’s natural blockade, and vanished wall, 
etc. Thirdly, the status of destroying makes it difficult to 
identify and map the MGW for some cases. 
In order to resolve the above problems, the method has been 
developed based on the test and the theory’s support of stereo 
measurement, and can be summarized as 4 steps: 
1) Data collection and processing: During this period, aerial 
photos, high resolution satellite images, digital maps, analog 
maps and MGW related documents will be captured and sorted 
out; 
2) Interpretation and identification of the Great Wall based on 
ortho-image: Firstly, 1:10k DOM and 1: 50k DOM covering the 
Great Wall and the vector data which be derived from digitizing 
the Great Wall elements based on 1:50k DRG are sorted in the 
computer to set up the Great Wall base imagery database. 
Secondly, existence, the status of preservation, other auxiliary 
elements, such as watch towers and battle forts, and attribute 
classification will be interpreted and identified with the support 
of the experts; 
3) Stereo measuring: based on the results of field investigation 
and control, the stereo measure of the Great Wall will be carried 
out. 
4) Length calculation and accuracy assessment: the surface and 
the projection length of the MGW for total and segments will be 
calculated by the photogrammetric approach through defining 
the length of the MGW and stereo measurement and the 
measuring accuracy will be assessed by the accuracy model. 
Figure 1. The method of Image-based Measurement of the 
MGW 
3. IDENTIFICATION AND AFFIRMING OF THE MGW 
3.1 Identifying the MGW Based on Ortho-image Data Base 
The Great Wall made by human being according to definite 
regulations, such as height, width, structure, materials, function, 
etc. The most MGW in Beijing, Hebei, Tianjin, Shanxi, Shanxi, 
Neimengu are the stone and brick structure and built on the top 
of the mountains and the most MGW in Ningxia, Gansu, 
Qinghai are sand and fence structure and built on the surface of 
the plain. 
In national foundation data base, there are the part of Great Wall 
thematic vector data in the 250k data base. Extracting the 
thematic vector data from 250k data base can get the Great 
Wall’s base data, then get 1:50k base map guiding by the 250k 
data base. Digitizing the Great Wall element from 1:50k DRG, 
can get the 1:50k Great Wall vector data. The ortho-image 
database has been set up based on the Great Wall vector 
data,l:10kDOM and l:50kDOM,etc. 
Figure 2. The Work Flow of Identifying the MGW 
Using the references such as Cultural Heritage historical map, 
document, and recordings, and 1:10k base map, Cultural 
Heritage and surveying and mapping experts identify and affirm 
jointly the Great Wall based on the ortho-image database, 
generating the Ming Great Wall elementary distribution 
map(see figure 3). 
Figure 3. The MGW Elementary Distribution Map 
3.2 Affirming the MGW in the Field 
During the process of the MGW resources investigation in the 
field, guiding by the Ming elementary distribution map, the 
field investigator will locate the main inflexion of the MGW by 
the GPS handset, describe the investigation information on the 
aerial enlarge photo, fill in 10 kinds of resources investigation 
forms to record the attribute of the wall and battle forts and 
watch towers, etc, such as wall registration form describing 
building up age, location (x,y,z), building materials of the wall, 
and GPS registration form describing the numbering, 
coordination, relations, and name etc. 
The important job will be carried out in the field investigation is 
to resolve the location and the extent of the steep mountainous 
place wall, river, streams and lake’s natural blockade, and 
vanished wall, etc. Through consulting the obvious surface 
feature around, the field investigator needs to express the result 
judged in field on the aerial enlarge ortho-photo, meanwhile; 
record the corresponding characters in detail by filling in the 
investigation forms . 
4. STEREO MEASURE OF THE MGW 
4.1 The Length Definition of The MGW 
The vertical profile of the MGW can be defined as the curved 
surface which the central axis through the MGW intersects 
vertically the horizontal plane, the track line of the MGW can 
be defined as the three-dimensional space curves which the 
vertical profile intersects vertically the surface of ground, the 
track line length can be defined as the length of the earth's 
surface or the surface length of the MGW. The red line in figure 
stands for the surface length of the the MGW.