K. S. K. Wijayawardana
Assistant Superintendent of Surveys
Photogrammetric Unit, Survey Department
P.O.Box. 506, Narahenpita, Sri Lanka
Email: [email protected] Survey Department is the national surveying & mapping organisation that provides topographical data to other institutions and individuals for their needs. These data are available in both hard copy and soft copy format. The history of topographic map production goes back to early 1900s. At that time, the department established geodetic triangulation network and then the entire country was mapped using plane table method at the scale of one inch to one mile, this is popularly known as One inch map series. During the period of 1980 – 1997, the department has produced 1: 50,000 topographical maps for entire country by combining these one inch maps and 1: 40,000 aerial photographs. Since 1992 topographical maps of 1: 10,000 scale have been produced using 1:20,000 aerial photographs that were taken at early 90’s. It implies that most of the available data do not provide updated or recent changes in development. The negative effects of delaying updating have been clearly demonstrated in planning and decision making stage. Updating of topographical maps in traditional way would take long period of time as well as money. As users are more concerned about updated 1:10,000 data, the updating of 1:10,000 map series using digital photogrammetric method is described here.
The target area, Horana is situated in the Kalutara District of Western Province of Sri Lanka. Its geographical location is latitude 60 41′ 34″ N & 60 43′ 10″ N and longitude 800 02′ 53″E & 800 05′ 03″ E and that covers 16 km2. Area has been rapidly developed recently as introducing a new industrial zone. Simultaneously other facilities have been developed such as residential areas, commercial areas and new roads etc. In this connection necessity of revision of existing information is very vital.
Data and software/hardware used
Digital vector data set in particular area that was extracted in 1994 as per 1: 10000 data library and recently taken aerial photographs with a scale of 1: 20,000 for scanning were used Sophisticated software such as Vexcel, VirtuZo and MicrostationSE to photographs scanning, orthophoto generation and data extraction respectively were utilised. Data collection and analysis were done by using soft copy photogrammetric instrument with 750MHZ 512MB RAM computer system.
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 was 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 was 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 absolute orientation to transform the stereo model from photogrammetric coordinate system to ground coordinate system was performed.
Once the relative orientation was completed, the working area was specified and the automatic epipolar resampling to remove the Y- parallaxes from the original imagery was done. After creating epipolar image, software provides facility to do image matching automatically. The image matching method technique using VirtuZoNT is an area and feature based, bridge mode technique integrating with the dynamic programming and least squares matching algorithms. As the image matching was finished, editing environment was created. Software provides variety of editing tools for editing where necessary. Selecting options namely PRODUCT – DEM & ORTHO IMAGE provided 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 the existing vector data on to the composite ortho image was overlaid. There is an interface between VirtuZo and Microstation software. In this connection, setting lower left and upper right coordinates of vector data set on to an ortho image, overlaid image can easily be obtained. After that digitisation was 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 became available in acceptable accuracy.
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 spend large amount of money for instrumentation on a long time basis, it would be benefitted a lot because it takes only a few days to revise a map. During its process, 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 also target areas, selected according to the prioritised method (revision promptly by factors other than mere change in the landscape as an 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 there is no need to spend much money for hardware/software maintenance as well as no need of skilled persons to operate. Eventually, emphasis of developing such a system is beneficial to the nation as a whole.