2 The nature of geographical data Spatial database concepts Data input to cartographic databases Digital cartographic databases
Data Models for Computer Cartography3 The nature of geographical data Geographical position Attributes Spatial relationship Time
Data Models for Computer Cartography4 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 Cartography5 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 Cartography6 Relationships
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 Cartography8 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 Cartography9 E E E R H R R R R R R R R R P R P P P P P E E E H E E House River Pine Forest Eucalypts E P E H H Real World Raster RepresentationVector Representation The raster and vector models
Data Models for Computer Cartography10 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 Cartography11 The raster coordinates X (column) Y (row)
Data Models for Computer Cartography12 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 Cartography13 The vector coordinates 0 10 X Y X = 2 Y = 3 (2, 3) A (1, 7) (3, 6) (3, 4) (5, 3) (7, 10) (5, 7) (7, 5) (9, 5) (8, 7) (9, 8)
Data Models for Computer Cartography Map IDArea (ha) Perimeter (m) Soil Type ID Soil Type ID NamepH 15Black soil6.5 21Brown soil6.0 25Red soil5.0 … Data-map link
Data Models for Computer Cartography15 Data input to cartographic databases Keyboard entry Manual digitising Scanning Input of existing digital files
Data Models for Computer Cartography16 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 Cartography17 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 Cartography18 Digitiser hardware (cont.)
Data Models for Computer Cartography19 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 Cartography20 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 Cartography21 Scanning hardware Flat-bed optical scanner. The scanner head includes a charge-coupled device (CCD) that is moved over the map being scanned. Drum scanner. The Y axis of the map is given by the rotation of the drum. L: laser; D: detector
Data Models for Computer Cartography22 Scanning operation Scanning Image editing and cleaning Vectorising the scanned image Adding attributes
Data Models for Computer Cartography23 Scanning operation (cont.) Original map Scanning Editing & Cleaning Vectorising Scanned image Resulting vector data
Data Models for Computer Cartography24 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: Geography Network Geography data available in the world: MapWorld National Administration of Surveying, Mapping and Geoinformation of China: China data sets