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Data Models for Computer Cartography

Data Models for Computer Cartography. Data Models for Computer Cartography. The nature of geographical data Spatial database concepts Data input to cartographic databases Digital cartographic databases. The nature of geographical data. Geographical position Attributes Spatial relationship

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Data Models for Computer Cartography

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  1. Data Models for Computer Cartography

  2. Data Models for Computer Cartography • The nature of geographical data • Spatial database concepts • Data input to cartographic databases • Digital cartographic databases Data Models for Computer Cartography

  3. The nature of geographical data • Geographical position • Attributes • Spatial relationship • Time Data Models for Computer Cartography

  4. Spatial database concepts • Relationship of spatial resolution to scale. • In the past, a map’s scale greatly influenced map content and data resolution. • Digital databases are scaleless in a theoretical sense. • In a practical sense, scale is still a critical factor. Data Models for Computer Cartography

  5. Spatial precision and accuracy • Spatial precision is a measure of how exactly a location is specified without any reference to its true value. • Most and least significant digits. • Spatial accuracy refers to a measure of how close a recorded location comes to its true value. Data Models for Computer Cartography

  6. Relationships Data Models for Computer Cartography

  7. Fundamental spatial models • Points:locations of, e.g., oil and water wells, weather stations, cities on a small scale map • Lines:centre lines of, e.g., railways, highways, natural streams • Polygons:enclosed regions such as reservoirs, lakes, local government areas Data Models for Computer Cartography

  8. Spatial data models • There are two fundamental approaches towards the representation of the spatial component of geographical information — the raster and vector models. • In both models, the spatial information is represented using finite, discrete homogeneous units. • In the raster model, the homogeneous units are grid cells (or pixels). • In the vector model, the homogeneous units are points, lines and polygons. Data Models for Computer Cartography

  9. Real World R E E 6575000 E H R E E H E E R P P E E E R P P P Eucalypts R P P R Pine Forest R R H River H R House R R 6555000 Raster Representation Vector Representation 561000 581000 The raster and vector models Data Models for Computer Cartography

  10. The raster data model • Divides the entire study area into a regular grid of cells • Each cell contains a single value • Is space-filling since every location in the study area corresponds to a cell in the raster Data Models for Computer Cartography

  11. Y (row) X (column) The raster coordinates Data Models for Computer Cartography

  12. The vector data model • Based on vectors. • The fundamental primitive is points. • Objects are created by connecting points with straight lines (or arcs). • Areas are defined by sets of lines. Data Models for Computer Cartography

  13. Y (7, 10) 10 (9, 8) (5, 7) (1, 7) (8, 7) (3, 6) (7, 5) (9, 5) (3, 4) A (5, 3) Y = 3 (2, 3) X 0 X = 2 10 The vector coordinates Data Models for Computer Cartography

  14. Map ID Area(ha) Perimeter(m) Soil TypeID 21 4 435 880 21 4 9 210 580 25 11 628 1140 21 11 21 252 650 15 9 Soil TypeID Name pH … … 15 Black soil 6.5 21 Brown soil 6.0 25 Red soil 5.0 Data-map link Data Models for Computer Cartography

  15. Data input to cartographic databases • Keyboard entry • Manual digitising • Scanning • Input of existing digital files Data Models for Computer Cartography

  16. Manual digitising • Most widely used method to date for entering spatial data from maps • The efficiency depends on the quality of the digitising software and skill of the operator • Time consuming and error prone Data Models for Computer Cartography

  17. Digitiser hardware • The position of an indicator is detected by the computer and interpreted as pairs of x, y coordinates. • Control buttons on the cursor that permit control of the system. • The current most popular digitiser is contemporary tablets using a grid of wires embedded in the tablet to generate a magnetic field which is detected by the cursor. • The accuracy of such a tablet is typically better than 0.1 mm. Data Models for Computer Cartography

  18. Digitiser hardware (cont.) Data Models for Computer Cartography

  19. The digitising operation • The map is affixed to a digitising table. • Three of more control points are digitised for each map sheet. • They should be easily identified on the map. • The coordinates of these control points will be known. • The control points are used to calculate the necessary mathematical transformations to convert all coordinates to the final system. • The more control points, the better. • Digitising the map contents using either the point or stream mode. Data Models for Computer Cartography

  20. Scanning • A digital image of the map is produced by moving an electronic detector across the map surface • Spot size: the size of the map area viewed by the detector, commonly 20 microns (0.02 mm) • Need to be processed or edited to improve quality • Need to be converted to vector • Tagging for linking attributes • The scanned images can be directly used for map production • Remotely sensed data are in fact produced by scanning the earth surface from the sensor Data Models for Computer Cartography

  21. Scanning hardware Flat-bed optical scanner. The scannerhead includes a charge-coupleddevice (CCD) that is moved over themap being scanned. Drum scanner. The Y axis of themap is given by the rotation of thedrum. L: laser; D: detector Data Models for Computer Cartography

  22. Scanning operation • Scanning • Image editing and cleaning • Vectorising the scanned image • Adding attributes Data Models for Computer Cartography

  23. Scanning Scannedimage Original map Editing & Cleaning Vectorising Resulting vector data Scanning operation (cont.) Data Models for Computer Cartography

  24. Digital cartographic databases • Digital chart of world • CIESIN - consortium for international earth science information network • Data centre for human interactions in the environment • Provide large amount of cartographic data sets, including China (e.g. China dimensions data collection) • Web address: http://www.ciesin.org • Geography Network • Geography data available in the world: http://www.geographynetwork.com • MapWorld • National Administration of Surveying, Mapping and Geoinformationof China: China data setshttp://www.tianditu.cn/map/index.html Data Models for Computer Cartography

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