Copyright, 1998-2013 © Qiming Zhou
GEOG1150. Cartography
Data Models for Computer CartographyData Models for Computer Cartography
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
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The nature of geographical data Geographical position Attributes Spatial relationship Time
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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.
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Relationships
Scale Resolution Detection (precision) (accuracy)
1:1,000,000 500m 1,000m
1:500,000 250m 500m
1:250,000 125m 250m
1:100,000 50m 100m
1:50,000 25m 50m
1:10,000 5m 10m
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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
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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.
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P
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561000
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House
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Eucalypts
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Real World
Raster Representation Vector Representation
The raster and vector models
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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
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The raster coordinates
X (column)
Y (ro
w)
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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.
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The vector coordinates
0 10
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)
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4
11
9
21Map ID Area
(ha)Perimeter
(m)Soil Type
ID
4 435 880 21
9 210 580 25
11 628 1140 21
21 252 650 15
Soil TypeID
Name pH
15 Black soil 6.5
21 Brown soil 6.0
25 Red soil 5.0
… …
Data-map link
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Data input to cartographic databases Keyboard entry Manual digitising Scanning Input of existing digital files
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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
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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.
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Digitiser hardware (cont.)
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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.
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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
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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
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Scanning operation
Scanning Image editing and cleaning Vectorising the scanned image Adding attributes
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Scanning operation (cont.)
Original map
Scanning
Editing & Cleaning
Vectorising
Scannedimage
Resulting vector data
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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 Geoinformation of China: China data setshttp://www.tianditu.cn/map/index.html
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