(3) No field works except image acquisition 3.2 Image Acquisition by a Digital Camera 
It takes a great deal of time and cost to conduct a ground 
survey of control points. A ground survey of control points Image acquisition by a digital camera is the most important 
is overinvestment in respect of accuracy in most cases of operation of our proposed method. There are three ways of 
digital camera application. Moreover, in case of geological image acquisition by a digital camera in the method. 
survey at a construction site, there is a limited time for field 
works at a site. According to our method no field works 
except image acquisition are necessary at a site and it is 
easy for an amateur photographer to take necessary images. 
(4) Provision of products of the requested quality 
Several precision levels of a geometrically corrected image 
are requested according to diverse application fields. 
Higher precisely corrected images are required to make an 
appropriate preservation or restoration plan of a cultural 
heritage. On the other hand, geological survey at a 
construction site allows rather lower spatial accuracy of a 
rectified image of a slope. Since our method adopts three 
ways of image acquisition by a digital camera mentioned 
below, geometrically corrected images with several 
precision levels can be created with appropriate workload 
corresponding to user requests. 
(1) Conventional stereo image acquisition 
Camera axes of the adjacent images are parallel to each 
other, and the area covered by each image overlaps part of 
the coverage of its adjacent image to take a stereo pair 
image covering all over the object. A scale such as a staff 
in leveling is taken in a pair of stereo images to give a scale 
of a geometrically corrected image. This is the highest cost 
way. 
(2) Image acquisition for a plane object 
When the object can be treated as a plane, a digital surface 
model (DSM) is not necessary to create a geometrically 
corrected image. Therefore overlapping ratio of the adja- 
cent images can be less than 50 percent. Our proposed 
method is as follows: A pair of scales such as staffs in 
leveling laid in parallel on a target plane is photographed in 
both sides of an image as shown in Figure 2. A scale 
photographed in the left side of the image is photographed 
in the right side of the left neighbor image, and a scale 
photographed in the right side of the image is photographed 
in the left side of the right neighbor image. Figure 3 3) 
illustrates acquisition of a series of images with a pair of 
scales. Image 1 and Image 3 have Scale I in the left side 
and Scale II in the right side. In reverse, Image 2 and Image 
4 have Scale I in the right side and Scale II in the left side. 
3. OPERATIONS OF THE METHOD 
3.1 Operation Flow 
Operation flow of our method is similar to one of a conven- 
tional method by analog/analytical photogrammetry. Figure 1 
shows the flow of the standard operation of the method. 
These movements of a pair of scales are the same as the 
movements of staffs in leveling. This gives geometric 
Il controls such as length and linearity in orientation of images. 
Furthermore this ensures that there is no gap between 
adjacent images. Proposed overlapping ratio of the adjacent 
images is 5 to 10 percent. This is the lowest cost way. 
  
L Image acquisition by a digital camera d At site 
  
  
| Measurement of image coordinates of points | 
Ju 
ü Orientation of images In office 
  
  
  
    
    
   
| Creation of a geometrically corrected image | 
  
Left Center Right 
Figure 1. Operation flow 
Pos 
Figure 2. Image acquisition for a plane object 
as ( 
cor 
  
  
  
  
Coverage of Image 3 
Coverage of Image 1 
Coverage of Image 2 
Coverage of Image 4 poi 
Figure 3. Acquisition of a series of images with a pair of scales 
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