sub-pixel correlation method. First order polynomial
coefficients based on MSS pixel coordinates were
calculated. The polynomial coefficients were applied to
AVHRR coordinates. Systematically corrected AVHRR
coordinates were rectified using the polynomial
coefficients.
(4) Detection of Parallax and Computation of Elevation
Parallaxes were detected by sub-pixel correlation
method. As indicated in the figure 6, an intersection of
two looking vectors from the ground points is calculated.
In practice two vectors does not intersect, thus a center
of a minimum segment of a line between two vectors
was calculated.
(5) Evaluation
Elevations were evaluated using 3-arc second DEM
produced by USGS.
3. TEST RESULTS
3.1 Registration Accuracy of a Stereopair
MSS scene was rectified with root mean square error
(RMSE) of 64-m. AVHRR coordinates were rectified
using the MSS scene as reference images. Thirteen
AVHRR pixels were selected as GCPs and image-to-
image correlation was achieved. Table 1 shows the
result. RMSE changes slightly in case of more than 5 by
5 reference window. It seems that RMSE 100-m would
be reached a limit, considering conditions such as MSS
IFOV, MSS rectification RMSE, incomplete way of
AVHRR pixel simulation by MSS pixels etc.
3.2 Accuracy of Generating DEM
Elevations of all points in AVHRR scene corresponding
to MSS scene (185-km by 170-km) except sea area were
calculated. The number of points was 19380. Some
points were rejected as miss-matching points.
Conditions of miss-matching points are (1) correlation
coefficient is less than 0.5; (2) distance between two
looking vectors is more than 500-m; (3) the difference
between an original elevation and an elevation after
applied median filter is more than 500-m. Except these
points, elevations were evaluated with USGS DEM.
Table 2 shows the result.
4. DISCUSSION AND CONCLUSION
Elevation accuracy of a map is defined normally by the
contour interval. According to National Map Accuracy
Standards by USGS, 90% linear error (LE) is within
half of contour interval. For example 1:1,000,000 scale
map has usually contour line with intervals of 500-m or
1000 feet (300-m); the RMSE is 152-m or 91-m
respectively. RMSE 320-m, the result of this
experiment, does not meet the criterion. In case of DCW,
the source map scale is 1:1,000,000 but 90% LE is +
610-m (Defense Mapping Agency, 1992). This criterion
38
is corresponding to RMSE 371-m. This fact indicates
that the proposed method has potential to fill elevation
information in contour blank regions within the
accuracy of DCW; however the accuracy of DCW might
be better than the criterion in most of regions where
contours existing.
Since RMSE of elevation is 320-m and the mean of B/H
ratio is approximately 0.5, it means registration
accuracy of a stereopair is less than 160-m. This shows
that it is possible to rectify AVHRR using GCPs even
from non coastal pixels.
As future plans, one of a refinement way of DEM
accuracy is using multi-direction's AVHRR scenes
providing the off-nadir view and MSS scene providing
nadir view like triplet matching. Another way is to use
sensor with higher resolution and wide FOV. Satellites,
ADEOS-II and EOS-AMI, which will be launched in
1998, will have a sensor of 250-m resolution on the
ground with wide FOV. The sensors names are GLI
and MODIS respectively. By applying the same method
to a stereopair of these sensor and Landsat TM, more
accurate DEM generation is expected.
ACKNOWLEGEMNT
This research was done during the author's stay at
EROS Data Center, US Geological Survey / UENP
GRID Sioux Falls, supported by STA Long-Term
Scholarship program of the Science and Technology
Agency, Government of Japan.
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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Length (km)
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Figure