WFM 6202: Remote Sensing and GIS in Water Management

1. Akm Saiful Islam WFM 6202: Remote Sensing and GIS in Water Management [Part-B: Geographic Information System (GIS)] Lecture-3: Input of Geospatial Data Institute of Water and Flood Management (IWFM) Bangladesh University of Engineering and Technology (BUET) December, 2006

2. Data Acquisition As data acquisition or data input of geospatial data in digital format is most expensive (about 80% of the total GIS project cost) and procedures are time consuming in GIS, the data sources for data acquisitions should be carefully selected for specific purposes.

3. Required Data Sources for GIS • Analog mapsTopographic maps with contours and other terrain features and thematic maps with respect to defined object classes are digitized by digitizers manually or by scanners semi-automatically. • Aerial photographsAnalytical or digital photogrammetry is rather expensive but the best method for updating • Satellite imageSatellite images or data are available for land use classification, digital elevation model (DEM), updating highway network etc. • Ground survey with GPSTotal station together with GPS (Global Positioning System) will modernize the ground survey. • Reports and publicationsSocial economic data are usually listed in the reports of statistics and census with respect to administration units.

4. Compare data acquisition methods

5. Digitizers for Vector Data Input The digitizing operation • Step 1 : affixed a map to a digitizing table. • Step 2 : add control points or tics at four corners. • Step 3 : digitize map contents according to the map layers • Step 4 : editing errors and clean the dataset Step 5 : convert from digitizer coordinates to map coordinates to store in a spatial database. Tablet digitizers with a free cursor connected with a personal computer are the most common device for digitizing spatial features with the planimetric coordinates from analog maps.

6. Major problems of map digitization • The map will stretch or shrink day by day which makes the newly digitized points slightly off from the previous points. • The map itself has errors. • Discrepancies across neighboring map sheets will produce disconnectivity.

7. Selecting a digitizing technique

8. Scanners for Raster Data Input • Scanner are used to convert from analog maps or photographs to digital image data in raster format. • Mechanical ScannerIt is called drum scanner since a map or an image placed on a drum is digitized mechanically with rotation of the drum and shift of the sensor. It is accurate but slow. • Video CameraVideo camera with CRT (cathode ray tube) is often used to digitize a small part of map of firm. This is not very accurate but cheap. • CCD CameraArea CCD camera (called digital still camera) instead of video camera will be also convenient to acquire digital image data. It is more stable and accurate than video camera. • CCD ScannerFlat bed type or roll feed type scanner with linear CCD (charge coupled device) is now commonly used to digitize analog maps in raster format, either in mono-tone or color mode. It is accurate but expensive.

9. Major Types of Scanner

10. Performance of major scanners

11. I/O of scanning process (a) original document in black (with scanner resolution in green) (b) scanned document in gray scale pixel (0=black, 255= white)

12. Digital Mapping by Aerial Photogrammetry • Analytical PhotogrammetryThough computer systems are used for aerial triangulation, measuring map data, editing and output with pen plotter, a stereo pair of analog films are set up in a stereo plotter and the operator will manually read terrain features through stereo photogrammetric plotter called analytical plotter. • Digital PhotogrammetryIn digital photogrammetry, aerial films are converted into digital image data with high resolution (5-25mm). Digital elevation model (DEM) is automatically generated with stereo matching using digital photogrammetric workstation.

13. Procedures of Aerial Phtogrammetry

14. Remote Sensing with Satellite Imagery • Satellite remote sensing is a modern technology to obtain digital image data of the terrain surface in the electro-magnetic region of visible, infrared and microwave. • Multi-spectral bands including visible, near-infrared and/or thermal infrared are most commonly used for production of land use map, soil map, geological map, agricultureal map, forest map etc. at the scale of 1:50,000 ~ 250,000. • A lot of earth observation satellites for example landsat, SPOT, ERS-1, JERS-1, IRS, Radarsat etc. are available.

15. Earth Observation Satellites

16. High Resolution Satellites

17. Advanced Technologies for Primary Data Acquisition Following advanced technologies will be useful for future GIS. • Electronic Plane Surveying SystemAn integrated system of total station with automated tracking function, kinematics global positioning system (GPS) and a pen computer will replace the conventional plane surveying. • Mobile Mapping SystemDifferent sensors such as GPS, INS (inertia navigation system), more than two digital cameras, voice recorder etc. are fixed on a vehicle in order to map objects in close range. • Laser ScannerAirborne laser scanner together with GPS and INS will measure directly the terrain releif or DEM with the height accuracy of 10 cm up to the altitude of 1,000 m. • SAR InterferometerSAR (synthetic apperture radar) inter-ferometry is a new technology to produce DEM automatically by special interferometric processing of a pair of SAR images.

18. Advanced Technologies

19. Advanced Technologies (contd..)

21. Rasterization • Conversion between raster and vector data is very useful in practical applications of GIS. • Rasterization refers to conversion from vector to raster data. • Raster format is more convennient to produce color coded polygon maps such as color coded land use map, while map digitizing in vector format is more easier to trace only the boundary. • Rasterization is also useful to integrate GIS with remote sensing becaues remote sensing images are in raster format.

22. Conversion Vector to Raster • trapezoid area can be applied to convert vectorized polygon to rasterized polygon with grid cells

23. Vectorization • Vectorization refers to conversion from raster to vector data, which is often called raster vector conversion.

24. Raster Vector Conversion • Schematic of the raster vector conversion by which left and right polygons are identified