Data Models for Computer Cartography

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