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The digital photogrammetric
workstation of Dipnet system
Yang Ming Hui, Ren Wei Chun, Liu Zhao De, Liu Yu Xian
Research Institute of Surveying and Mapping, Beijing, China
Yang Ming Hui, Ren Wei Chun, Liu Zhao De, Liu Yu Xian
Research Institute of Surveying and Mapping, Beijing, China
Abstract
This article introduces the digital photogrammetric workstation of DIPNET 900 system designed by RISM, China. Because of the rapid development of computer technology and remote sensing application, the digital photogrammetric processing method as an advanced geographic information acquisition tool serving for geographic information processing and acquisition is combined with image and graphic processing.
A digital photogrammetric workstation should be consist of a powerful function workstation with additional stereoscopic viewer and precious image digitizer.
In the article, the system configuration, processing functions and the main algorithms are presented. Concerning the algorithms, the correlation is the important problem, here in, an approach for initial value determination and correlation loss resolving is discussed.
The digital photogrammetric workstation of Dipnet system:
In 1988, the Research Institute of S & M of China developed the DIPNET system for remote sensing and cartography application. At that time, we had already predicted the possibility of such development that image processing, computer aided cartography, and digital photogrammetric processing would be joined together and integrated system could be used to process GIS data in vector and raster form. This is the reason why we named the system as Digital (geographic) Information Processing Network System (DIPNET).
For a complete geographic information application system, the four sub-systems are indispensable: the data collection, data processing, data management and analysis & application. Among them, the quite important things are the data collection. Processing and updating. Moreover, nowadays the dynamic characteristics are most vital for an information system. Therefore, as an efficient data processing method in aerial and satellite remote sensing, the digital photogrammetric processing should take the important position in GIS data processing. The RISM has developed a digital photogrammetric workstation (Figure 1) as a special important component of DIPNET 900 system.
Figure 1.
Digital Photogrammetric workstation of DIPNET 900 series has different basic image processing and geometric processing functions. With the support of topo-graphic database and control point database, it can be used to rectify satellite image and digitized aerial photos for photomap or orthophotomap production. The aerial photos Digitizer (IDS-11) is used to transform an optical image to digitized photos.
In the workstation "Digital Plotter", which is a combination of digital stereoscopic supervision device (DD-11) + a powerful workstation with high speed image operation function, exists and is used to observe or measure a stereoscopic pairs, which could be aerial photos or SPOT image. It can produce DEM data and output contour lines.
Therefore, the workstation of DIPNET 900 has almost the same functions that analytical plotter and orthoprojector have. The processing function of a GIS will be enhanced.
Digital photos measurement system:
It consists of a stereoscopic viewer and a special microcomputer workstation.
The stereoscopic viewer is mounted on the base of workstation. A binocular fixed on pair of parallel rod rotable is used to help the operator observing the image pair with optical floating marks with 12 images magnification of measurement system. The precision of digital photos measurement depends on the step adopted in image scanning. According to the estimation, 25m step could be adopted to medium scale mapping, and 12.5m step will be used for large scale mapping. Therefore for a stereoscopic pair, at least 128M bytes (25m step) up to 512M bytes data (12.5m step) must be stored in the hard disc and 32M/64M main memory is demanded.
In digital plotter' of DIPNET 900, detail mapping and relief drawing is separated. Details mapping is very simple to carry out in moving display bitmap with cursor or mouse. For relief drawing, auto-correlator aided DEM measurement conducted while relative object windows are moving in order, and the observed object points fitting with the floating marks will be emerged in the center of each window, in this way the satisfied accuracy of stereo view can be attained with a simple stereoscop device.
In addition, more than 14 mips CPU speed and 5M bytes transfer rate for disk data is available and the best feature is that 24 image bitmaps exist in the workstation.
A large quantity of DEM calculation is carried out with correlation method. On the screen, the image for a pair of conjugate points will be displayed. If "correlation loss" occurs, the digital workstation delivers a sound of warning, and the operator interruption is permitted.
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.
The rms of vertical parallax is 1.0m.
The rms of position is 10.0m (left photos)
the rms of levation is 3.0m
After orientation (7 control points taken).
the rms of elevation is 0.67m.
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 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):
tangq = [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)
tangq' = [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) |
| Yst = Yst Ø +B (t-t Ø) |
| Zst = Zst Ø +C (t-t Ø) |
| jst = jst Ø +
A (t-t Ø) |
| Wst = WestØ + A (t-t Ø) |
| Kst = Kst Ø + A (t-t Ø) |
| Xti = Xst Ø + Nxti |
| Yti = Yst Ø = Nyti |
| 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.