Transcript Data Models for Computer Cartography

Data Models for
Computer Cartography
Copyright, 1998-2013 © Qiming Zhou
GEOG1150. Cartography
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.

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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
(precision)
Detection
(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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The raster and vector models
Real World
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6575000
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Eucalypts
Pine Forest
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House
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Raster Representation
561000
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Vector Representation
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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
Y (row)
X (column)
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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
Y
(7, 10)
10
(9, 8)
(5, 7)
(1, 7)
(8, 7)
(3, 6)
(7, 5)
(9, 5)
(3, 4)
A
(2, 3)
Y=3
0
(5, 3)
X
X=2
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Data-map link
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Map ID
Area
(ha)
Perimeter
(m)
Soil Type
ID
4
435
880
21
9
210
580
25
11
628
1140
21
21
252
650
15
9
Soil Type
ID
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Name
pH
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Black soil
6.5
21
Brown soil
6.0
25
Red soil
5.0
……
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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 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
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Scanning operation

Scanning
 Image editing and cleaning
 Vectorising the scanned image
 Adding attributes
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Scanning operation (cont.)
Scanning
Scanned
image
Original map
Editing & Cleaning
Vectorising
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 sets
http://www.tianditu.cn/map/index.html
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