Topographic Map Revision in Digital Photogrammetric Environment
Methodology
Workflow is as follows,
The first step in the map revision process was selecting most recent 1:20,000 scaled aerial photographs for scanning (no matter whether these are diapositives or contacts). Target area was covered two strips with six aerial photographs. All these were scanned 25 micron resolution with sophisticated VX 400 scanner. The scanning surface should be clean and dirt free to produce good quality images. With changing the look up table for each image, output image quality can be improved dramatically. It is important to have all fiducial marks in image for inner orientation. The next phase is to input scanned images and parameters such as camera calibration data and ground control points' coordinates to VirtuZo NT Workstation for further processing.
Next phase is to orient each model. The interior orientation is based on the calibration values and the measured coordinates of the fiducial marks. After completing the inner orientation of a model relative orientation can be performed continuously to obtain the stereoscopic vision. Then performed absolute orientation to transform the stereo model from photogrammetric coordinate system to ground coordinate system.
Once the relative orientation completed, can be specified the working area and do the automatic epipolar resampling to removed the Y- parallaxes from the original imagery. After creating epipolar image, software provides facility to do image matching automatically. The image matching method technique using to VirtuZoNT is an area and feature based, bridge mode technique integrating with the dynamic programming and least squares matching algorithms. As the image matching finished, can be entered editing environment. Software provides variety of editing tools for editing where necessary. Selecting options namely PRODUCT - DEM & ORTHO IMAGE provides by the software, Ortho Imagery can be created for each model. After creating a number of ortho images within a block, single continuous composite image for entire target area can be performed.
Then overlaid the existing vector data on to the composite ortho image. There is an interface between VirtuZo and Microstation software. In this connection, setting lower left & upper right coordinates of vector data set on to an ortho image, overlaid image can be get easily. After that digitizing performed on screen in Microststation environment as per 1: 10,000 data library. Separate level was assigned for new and changed information. Ultimately updated digital vector map available in acceptable accuracy.
Discussion
The 1:10,000 map revision method, which has been adopted by the Survey Department, would take at least one month to revise the particular target area. Even if we spent large amount of money for instrumentation, as a long time basis it would be benefited a lot because it will take only a few days to revise a map. Total time taken to produce ortho image and map revision for target area was three working days with the full time involvement of one person.
In future, revision of 1:10,000 map series in the Survey Department should be as mentioned earlier also target areas be selected according to the prioritized method (revision promptly by factors other than mere change in the landscape as a example of demanding from the customer or knowledge of future development). With regard to developing countries such as Sri Lanka, this system is very suitable because no need to spend much money for hardware/software maintain as well as no need of skill full persons to operate. Eventually, emphasis of developing such a system is benefited country a lot.
Conclusion
This paper is mainly dealt with revision of 1:10,000 map series in digital photogrammetric environment. Map revision is only one product from ortho imagery. As ortho imagery contains all terrain features in details, they can play vital role in land resources survey than conventional old paper maps. Further to map revision ortho images can be used as invaluable large information for earth resource planners as well as geological information system (GIS).
