Raster data model


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Raster data model

  1. 1. Raster Data Model (Chang’s Chapter 7)Elements of the Raster Data Model Raster model divides the area into grid cells or pixel. Each grid cell is filled with the measured attribute values. It can represent points, lines and area (Figure 7.1). Resolution depends on real world area represented by each grid cell. AGS 722 AGS 722 1
  2. 2. Raster Data ModelThe larger the area represented, the lowerthe resolution of data.Cells are identified by their positions in thegrid.Raster data is geo-referenced by: • Real world coordinates of the reference point • Cell size in real world distance • Use the upper-left or lower-left corner of grid as the reference point. AGS 722 IDRISI Metadata AGS 722 2
  3. 3. Raster Data Model Storage requirement is high. Ex: If the area is 100 km x 100 km and cell size is 10 m. It needs 10,000 rows x 10,000 columns or 100,000,000 pixels. If one byte is used per pixel, it requires 100 MB storage AGS 722 Types of Raster Data1. Satellite Imagery Remotely sensed satellite data are recorded in raster format. Spatial resolution varies: • 30 m. for Landsat 4 and 5 (use the Thematic Mapper scanner), and Landsat 7 (use Enhanced Thematic Mapper-Plus, ETM+ scanner). AGS 722 3
  4. 4. • 20 m. for SPOT images (Multi-spectral sensor), and 10 m. for SPOT Panchromatic sensor). • 4 m. and 1 m. for IKONOS Multi-spectral and Panchromatic images respectively.The pixel value in a satellite image representslight energy reflected or emitted from theEarth’s surface. AGS 722The measurement of light energy is based onelectromagnetic spectrum.Panchromatic images are comprised of asingle spectral band.Multi-spectral images have multiple bands. – Landsat TM has 7 band.Land use, land cover and hydrography canbe classified from image processing system.Satellite images can be diaplayed in blackand white or in color. AGS 722 4
  5. 5. Landsat TM Bands AGS 722Composite Color Images AGS 722 5
  6. 6. 2. Digital Elevation Models (DEM) DEM consists of an array of uniformly spaced elevation data. DEM are produced from: – a stereoplotter and aerial photograph with overlapping areas. – Satellite imagery such as SPOT stereo model using special software. AGS 7223. Digital Orthophotos Prepared from aerial photograph or other remotely sensed data. Displacement caused by camera tilt and terrain relief has been removed. They are geo-referenced and can be registered with topographic and other maps. AGS 722 6
  7. 7. Digital Orthophoto AGS 7224. Binary Scanned Files Scanned image containing values of 1 and 0. Maps to be digitized are typically scanned at 300 or 400 dpi (dots per inch).5. Graphic Files Maps, photographs and images can be stored as digital graphic files. – e.g. TIFF (Tagged Image File Format), GIF (Graphic Interchangeable Format), JPEG (Joint Photographic Exports Group), etc. – GeoTIFF is a geo-referenced version of TIFF format. AGS 722 7
  8. 8. Raster Data Structure Refers to storage of raster data so that it can be processed by the computer.Cell-by Cell Encoding A raster model is stored as a matrix. Its cell values are written into a file by row and column. (Figure 7.2) Ideal to store the cell values that change continuously, e.g.,DEM. AGS 722 AGS 722 8
  9. 9. For multi-spectral satellite image, eachcell has more than one value, data arestored in either of the following formats. – The band interleaved by line (.bil): this method stores the 1st value of every row sequentially, followed by the second value of every row, and so on in one image. AGS 722 Multi-band Satellite Data Structure .bsq .bil Figure 7.x .bip AGS 722 9
  10. 10. The Band Sequential (.bsq) method: stores values of each band sequentially in one image.The Band Interleave by Pixel (.bip): each row of an image is stored sequentially, row 1 all bands, row 2 all bands, and so on. (See Figure 7.x) AGS 722 Multi-band Satellite Data Structure .bsq .bil Figure 7.x .bip AGS 722 10
  11. 11. Run-length Encoding Records the cells by row and by group Each group includes a cell value and the number of cells with that value. If all cells in a row contain the same value, only one group is recorded, hence save computer memory. See Figure 7.3. AGS 722 AGS 722 11
  12. 12. Chain Code Method Represent the boundary of a region by using a series of cardinal directions and cells. – Ex: N1 means moving north by 1 cell, S4 means moving south by 4 cells. See Figure 7.4 AGS 722 AGS 722 12
  13. 13. Block Code Method Uses square blocks to represent the region. – A unit square represents 1 cell. – 4-square block represents 2 x 2 cells – 9-square block represents 3 x 3 cells, and so on. Each square block is coded only with the location of a cell (lower left of the block), and the side length of the block. See Figure 7.5 AGS 722 AGS 722 13
  14. 14. Quad Tree Method Uses recursive decomposition to divide a grid into a hierarchy of quadrants. (Figure 7.6). A quadrant having cells with the same value will not be sub-divided, and it is stored as a leaf node. Leaf nodes are coded with the value homogeneous quadrant. A quadrant having different cell values will be subdivided until a quadrant at the finer level contains only one value. AGS 722 AGS 722 14
  15. 15. This method is efficient for storing andprocessing data.Different raster GIS software use differentmethod of storing data. – IDRISI and GRASS use either cell-by-cell or run length encoding method. – SPANS uses a quad-tree data structure. AGS 722 Data CompressionRefers to the reduction of raster datavolumes.Run length encoding method may reach 10:1compression ratio.TIFF and GIF files use lossless compressionwhich allows the original image to beprecisely reconstructed. AGS 722 15
  16. 16. Data CompressionJPEG files use lossy compression which canachieve high compression ratios but can notreconstruct the original image fully.MrSid (Multi-resolution Seamless ImageDatabase) has capability of recalling imagedata at different resolution or scales and alsocan compress a large image. AGS 722 Projection of Raster DataProjected raster data are based on rows andcolumns but the rows and columns aremeasured in real-world coordinates. – Ex: • Rows: 463, Columns: 318, Cell size: 30 m • UTM coordinates at the lower left corner: 499995, 5177175 • UTM coordinates at the upper right corner: 509535, 5191065 AGS 722 16
  17. 17. • The cell in Row 1 and Column 1 at the upper left corner has UTM coordinates of 499995, 5191035.Data Conversion Conversion of vector to raster data is called rasterization. Conversion of raster to vector data is called vectorization. (Figure 7.8) Both require use of computer algorithms which most GIS software have. AGS 722 AGS 722 17
  18. 18. Integration of Raster and Vector DataCan take place in data display, dataprocessing, data conversion, or data analysis.DEM are input data to extract topographicfeatures such as contour, drainage network,watersheds, etc.Most GIS packages allow simultaneousdisplay of raster and vector data.Data conversion must be performed first if theanalysis of both raster and vector data isrequired. AGS 722 18