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.

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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 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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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