The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. 34, Part XXX 
2.4 RPM 
Figure 2 shows the overall concept of the RPM. In Figure 2(a), 
a pair-wise matching point is extracted on master stereo model 
A through a matching process, and its ground coordinate is 
calculated using a forward colinearity equation (Schenk 1999). 
In Figure 2(b), the ground coordinate of a DEM point is 
projected to neighbor stereo model B using back projection 
based on a colinearity equation. The reliability is then evaluated 
based on the similarity of the texture pattern at the projected 
point on stereo model B. Here, points with high reliability are 
adopted, while those with low reliability are rejected. 
Stereo Stereo 
Stereo Model B Stereo Model B 
Model A Model A 
    
   
   
O Point on Left Image * DEM Point 
Bm Point on Right Image O Point on Left Image 
% DEM Point € Point on Right Image 
(a)Matching point (b) Reverse projection 
on master stereo model on neighbor stereo model B 
Aero photo by GSI Japan (2008) 
Figure 2 Matching Process and RPM 
In this research, the normalized correlation value, which is 
stable against the tone or contrast of the image, is adopted as an 
indicator of the similarity. In addition, the threshold is set to 0.4, 
which is generally assumed to be a fairy strong correlation. 
Further, when multiple stereo models are available for a 
projection, the reliability can be evaluated as follows: 
The DEM point is projected to each neighbor stereo model, 
and the similarity of each model is evaluated. 
When the similarity is in the upper threshold, one vote is 
given to each stereo model. 
When the similarity is in the upper threshold, one vote is 
given to each stereo model 
When a point is rejected, its altitude is interpolated from the 
adopted points of its neighbor by creating a triangle network 
(TIN), after the RPM is completed for all points, and the 
process moves to the next stage. 
2.2 Coarse-to-fine approach 
The coarse-to-fine approach enables a fast point search by 
creating a few stages of gradually scaled images and restricting 
the search areas for matching. However, a problem exists in that, 
when a mismatched point is generated during the middle of the 
process, the mismatch cannot be recovered until the end of the 
matching process is reached to the end of the matching process. 
Therefore, mismatched points need to be filtered during the 
middle of the process. Here, to remove mismatched points, the 
RPM filters out points with low reliability during each stage of 
the coarse-to-fine approach. 
In this research, the number of stages for the coarse-to-fine 
approach is set to 6, which means creating gradually scaled 
images of Stage 6 (1:32) to Stage 1 (1:1). The search range used 
during the matching process begins with the lowest resolution 
of 1:25, with a substitution of the values in equation (1) : s = 
6.0 pixels, Z,= 0 m, AZ = +100 m, bo= 600 m, Hp = 1920 m m 
(bo and Hp are calculated at a photo scale of 1:16000, with an 
overlap rate of 60% and a camera focal length of 120 mm), 
where s is the length of the search area; by, the baseline length; 
Hp, the height to ground;AZ, the uncertainty of the elevation; 
and Z,, the approximate elevation. 
HAZ - 
0 2 
(H, 74, 
2.3 Reference of the Digital Terrain Model 
During the matching process for the lowest resolution stage in 
the coarse-to-fine approach described in Section 2.2, 
mismatched points with a large gap from the correct altitude arc 
often created. It has been speculated that these mismatched 
points are caused from a search of the entire range, as shown in 
Figure 3 (a). In this case, the uncertainty of the altitude is 
significant. On the other hand, as shown in Figure 3(b), when 
the approximate altitudes of the DEM points are known, large 
mismatches are prevented by restricting the search range around 
each altitude. 
Flight 
Height 
   
  
Base 
Height 3 a 
(a) Entire Search (b) Restriction of the 
; search range 
— Actual altitude 8 
e"  Knownaltitude b> Search range 
Ocorrect matched point | )mismatching 
Figure 3. Matching process on the lowest resolution stage 
in the coarse-to-fine approach 
Digital terrain models (DTM) have been recently prepared for 
use at the global scale. Thus, the approximate altitude for every 
point is acquired in advance for stereo matching, based on 
DTM as a reference. 
In this research, during the lowest resolution stage of the 
coarse-to-fine approach, the approximate altitude for the points 
are acquired from the “Digital Map 50 m Grid (Elevation)” 
published by GSI Japan. With this information, the search 
ranges are restricted around the DTM altitude and large 
mismatches are prevented prior to the matching process.In this 
research, the number of resolution stages for the coarse-to-fine 
approach is set to 4 when using the “Digital Map 50 m Grid 
(Elevation) as a reference, which means creating low- 
resolution images of Stage 4 (1:8) to Stage 1(1:1). With a 
substitution of the values in equation (1), and under the same 
conditions described in Section 2.2, the uncertainty of an 
elevation is minimized to 20 m, which is the accuracy of the 
*Digital Map 50 m Grid (Elevation)," at the lowest resolution 
of 1:5. The search range around the DTM, as shown in Figure 3 
(b), is set to 20 m, which is the same as the elevation 
uncertainty of the elevation. 
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