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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008
a) Perform bundle block adjustment on a block of virtual
images.
b) Compute mean image residuals per square cell (about
256x256 pixels). Each image cell should have at least 40
points in order to have a reliable correction grid.
c) Compute some sort of smoothing of the trend surface with
either low-pass Gaussian kernel or least square surface
splines.
d) Refine image coordinates with this “Correction Grid”
using bi-linear interpolation.
e) Repeat steps (a-d) 3 to 4 times until maximum residual
trend increment per cell drops to lower than 0.5[um],
f) Use the correction grids in the DMC Postprocessing
software to generate “distortion free” virtual images.
Repeat steps (a-d) 3 to 4 times until the maximum residual trend
increment per cell drops lower than 0.5[um], Use the correction
grids in the DMC Postprocessing software to generate
“distortion free” virtual images.
Alternatively, correction grids can be generated from self-
calibration bundle adjustments. Exported correction grids
should cover the entire virtual image format. Otherwise, image
coordinates outside the correction grid must be extrapolated,
which will give wrong results. These correction grids can be
imported in the DMC Postprocessing software for refining
virtual images, in the image observation refinement process,
and in the object to image projection (real-time math model of
the digital photogrammetry workstations).
5. NUMERICAL RESULTS
5.1 DMC50 Calibration and Testing
In the following sections, only results of the DMC50 block are
presented. Results and conclusions of other test blocks were
similar; therefore, they are not reported here.
Ima^eStation Automatic Triangulation is used to generate
tie/pass points (Madani, 2001) for the DMC50 block. This
block is then adjusted by using GPS with block shift correction
(no IMU is used) and 10 microns for standard deviation of
image points (tight constraints in object space and loose ones in
image space force all the errors to go into the image space,
where it will be captured by the collocation model). The general
adjustment statistics are given in Table 2 and distributions of
control/check points are given in Figure 3.
Sigma =2.9, RMS x=2.7,
RMS y = 2.6 [urn]
Xfm]
Y[m]
Z[m]
RMS of 8 Control Points
0.017
0.029
0.019
RMS of 6 Check Points
0.020
0.032
0.047
MAX of 8 Control Points
0.035
0.043
0.030
MAX of 6 Check Points
0.036
0.049
0.067
RMS GPS
0.007
0.009
0.019
GPS Block Shift
-0.028
-0.033
0.218
6.16 6.160 617 6.175 6 10 6 100 6 19
«10*
Figure 3. Distribution of control “red circles”, check “blue
squares”, and photo centers “red circles”
5.1.1 Correction Grid generation
For a reliable estimation of the residual trend surface that is
almost free from influence of clusters of small outliers, the
recommended density of image residuals per 256x256-pixel cell
should be between 20 to 40 points. A histogram of image points
per cell and spatial distribution of cumulative redundancy
number per cell (which serves as weight factor in collocation fit)
are given in Figure 4.
mean count: 58; max count: 97
Redundancy Number
50 I
150
100
50
50
100
10 20 30
Figure 4. Histogram of image points and cumulative
redundancy number distribution per cell
The collocation fit has converged to a trend surface after 4
iterations performed with the previously adjusted reference
block. The estimated systematic distortion and the remaining
residual trend are given in Figure 5.
Dilionxxi PmtiT r.i* X 3.03|ijm) mu Y 3 S3|um! Owwtw P«n«m m.i X 0 «Hum) m« Y 0 U[urn!
J0 •» fl » <0 «
RmsX“ 1 03|UfTi] PmfY» 1 67(umj
« -20 0 M 4fi £C
HHum). Rm»Y < D 13j«m|
Table 2. DMC50 general adjustment statistics
Figure 5. DMC50 VIR correction grid and the remaining
distortion trend