The digital photogrammetric workstation of Dipnet system
Experiments are conducted to test the precision of stereoscopic observation and orientation with digitized aerial photos, which was taken with 153.79mm focal length 13000 flight alight altitude and photos scale 80000. Digitizing was carried out in the PDS microdensitometer with 50m step and the photo coordinates were measured on DIPNET system with DSS-11 device.
After orientation 6 control points were taken.
The rms of vertical parallax is 1.0m.
The rms of position is 10.0m (left photos)
The rms of levation is 3.0m
Tests using Finnis digitized photos pair with 213.59mm focal lenth 300m flight altitude and photos scale 15000(digitizing) with 50m step) shows:
After orientation (7 control points taken).
The rms of position is 2.2m(left photos) and 3.1m (right photos).
The rms of elevation is 0.67m.
The availability are verified.
The Main Functions and Algorithms:
In the near future the traditional photogrammetry based on projective geometry would be combined with image processing technology for the application of pattern recognition and computer vision to solve the interpretation problems of geometry/feature. It is in this sense that the digital photo grammetric workstation is the complement of an integrated digital geographic information processing system.
The digital photogrammetric workstation possessesthe following fuctions:
- digitized photos measurement.
- stereoscopic measurement. (for photos and spot image pair)
- Point determination of a stereo-model.
- photos orientation.
- SPOT image pair orientation.
- stereo-model orientation.
- digital rectification.
- digital mosaic.
- grey extraction for orthophotos.
- Correlation calculation for sattellite image pair.
- Correlator-aided DEM measurement.
- Correlator-aided DEM measurement (for SPOT image pair).
- Interpolation.
- Plotting.
- Other functions.
For the problem of auto correlation very important research topics are the initial value and "correlation loss" determination. In our system, a geometric constraint condition is adopted to calculate search area. The experiment shows that initial value could be determined by using this condition.
For each conjugate point pair, in searching range, the correlation will be conducted taking a image window along epipolar line, and the direction of epipolar line can be determined with formula (1):
tang
q = [A3(Ys'-Ys)-A2(Zs'-Zs)]xi' + [A1(Zs'-Zs)-A3(Xs'-Xs)]yi' + [A2)Xs]-Xs)-A1 (Ys'-Ys) zi' /
[B3(Ys'-Ys)-B2(Zs'-Zs)]xi'+[B1(Zs'-Zs)-B3(Xs'-Xz)]yi' + [B2(Xz'-Zs)-B1 (Ys'-Ys)]zi'
.............................(1a)
tang
q' = [A2' (Zs'-Zs)-A3' (Ys'-Ys)]xi+[A3'(Xs'-Xs)-A1'(Zs'-Zs)]yi + [A1'(Ys'-Ys)-A2' (Xs'-Xs)]zi /
[B2'(Zs'-Zs)-B3'(Ys'-Ys)xi+[B3'(Xs'-Xs)-B1' (Zs'-Zs)]yi +[B1'(Ys'-Ys)-B2' (Xs'-Xs)]zi
.............................(1b)
In the formula (1a), (1b)
q,
q' are the direction angles of epipolar line of an image point in left or right photos respectively.
About 1-2% "correlation loss" points are emerged and it results from the poor texture area of photos. On DIPNET system, the threshold of standard errors, mean value and correlation coefficient are adopted to judge the "correlation loss". The correlator-aided DEM measurement software developed in DIP NET system, has been successfully applied to DEM data capture application.
With similar principle, the DEM data or contour lines could be produced starting on SPOT image pair processing. It uses multi-instantaneous exposure center geometry and pseudo-epipolar correlation. The basic mathematic models are a follows:
Xst = Xst Ø +A (t-t Ø)
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Yst = Yst Ø +B (t-t Ø)
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Zst = Zst Ø +C (t-t Ø)
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jst = jst Ø + A (t-t Ø)
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Wst = WestØ + A (t-t Ø)
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Kst = Kst Ø + A (t-t Ø)
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Xti = Xst Ø + Nxti
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Yti = Yst Ø = Nyti
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| Zti = Zest Ø = Nzti |
The problem is that the orbit parameters and altitude parameters are correlative, so, very often the iteration is not convergent. In SPOT stereoscopic DEM measurement program, for accelerate the speed of convergency, a group of initial values as pseudo observational value are joined.
