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Thus, as in the quantitative evaluation, the stereo models of the
previous and following strips are judged to be the best
combination for the RPM from the viewpoint of processing cost.
3.2.2.2 Comparison of the restricted application of the
RPM in the coarse-to-fine approach
For the RPM using stereo models of the previous and following
strips, changes in the accuracy of the DEM are examined based
on the restricted application of the RPM in the coarse-to-fine
approach. The qualitative evaluation of the DEM accuracy is
performed by checking the distribution of mismatched points on
the color-shaded maps.
Figure 7 shows the changes in the color-shaded maps for arcas
A and B, with variations in the restricted use of the RPM in the
coarse-to-fine approach.
Figures 7(a) and 7(b) show color-shaded maps for mountainous
arcas and rice fields, respectively. Mismatched points are
generated in the squared areas without the use of the RPM.
Additionally, it was confirmed that these mismatched points are
removed for both areas when (1) the RPM is used with all
resolution stages, (3) the RPM is used with a high-resolution
stage, and (4) the RPM is used with DTM as a reference.
Ortho W/O
Photo RPM (1) @) (3) (4)
(a) Mountain (area A).
Aero photo by GSI Japan (2008)
0 200m (b) Rice fields (areaB)
Figure 7 Comparison of elevation with the
RPM Changing Resolution Stage in coarse-to-fine approach
Here, as in the quantitative evaluation, from the qualitative
evaluation of the restricted application of the RPM, it was
confirmed that these same cases improve the accuracy of the
DEM. Based on the processing time for cach case shown in
Table 6, from a qualitative examination, it was judged that (4)
the RPM with DTM as a reference using only high-resolution
stages in the coarse-to-fine approach is the most applicable
method.
3.3 Examination
In this section, changes in the accuracy of the DEM are
examined, with variations in the stereo model combinations for
the RPM, and with variations in the application of the RPM in
the coarse-to-fine approach.
3.3.1 Changing the stereo model combinations for the RPM
From the quantitative and qualitative evaluations in section 3.2,
the accuracy of the DEM was confirmed to be better when the
RPM is used with all stereo models, or with only stereo models
of the previous and following strips, as compared with stereo
models of only the current strip.
This result is speculated to have been caused from changes in
the usable area of the projection based on variations in the
stereo model combinations for the RPM, as the RPM, from its
mechanization, cannot evaluate the reliability of the matched
points when there is no stereo model to project, and because the
reliability of the matched points depends on the amount of
usable area for the projection.
Table 7 shows the amount of usable area for the RPM and the
reduction in the number of mismatched points for area D, where
aerial photos are taken under the normal condition of a 60%
overlap and 30% sidelap. It was confirmed that the amount of
usable area for the RPM is proportionally related with the
reduction in the number of mismatched points.
# of Mismatched #of Rates of
Usable
area pts. in usable area reduced reduced
(96) W/O With mismatch | mismatch
2 RPM | RPM ed pts. ed pts
All Stereo 0
Model 78.0 121 90 31 25.6%
CUR Strip 43.0 38 34 4 11.8%
PREV and o
FF Strips 58.2 105 78 27 25.7%
Table7 Amount of usable area for the RPM and
the reduction in the number of mismatched points ( Area D )
From this result, the reduction in the number of mismatched
points is the highest when the RPM is used with all stereo
models. However, a few gaps exist in the rate of reduction of
mismatched points when comparing the use of the RPM with all
stereo models and with stereo models of only the previous and
following strips. In addition, based on the processing cost with
an increasing number of stereo models for the RPM, it was
determined that the use of the RPM with the stereo models of
the previous and following strips is the most effective method.
3.3.7 Restriction of the RPM in the coarse-to-fine approach
From the evaluation described in section 3.2, it was confirmed
that the RPM with a restriction in the high-resolution stages in
the coarse-to-fine approach, and the RPM using the DTM as a
reference, both improved the accuracy. Figure 8 shows the
changes in the color-shaded maps under the coarse-to-fine
approach for each use of the RPM. From this figure, it can be
confirmed that the mismatched points generated in the square
areas are not filtered during the resolution stage when the RPM
is not used.
Furthermore, when the RPM is used with DTM as a reference
during the lowest-resolution stage, large mismatched points are
removed during stage 4 prior to the matching process during the
higher resolution stage. Therefore, it is speculated that
unnecessary searches are prevented and that the processing time
is drastically reduced.
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