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GI Systems and Science January 11, 2012

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Points to Cover Introduction Maps and spatial data How spatial data is represented in GIS Scale Spatial entities Coordinate system

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Introduction GIS turn spatially referenced data into information GIS creates representations of the real world Follow some logic Simplified Selective The simplified view of the real world is otherwise known as a model Means of getting a grip on complexity of the real world (Hardisty et al., 1993).

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Maps and Spatial Data GIS approach to modeling and presenting spatial data is influenced by cartography Map a representation, normally to scale and on a flat medium, of a selection of material or abstract features on, or in relation to, the surface of the earth Any map is a cartographic abstraction of the real world What is represented on a map and how it is represented depends on the purpose of the map Map is a communication device

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Maps and Spatial Data Figure 2.5 Cold War propaganda map Source: Topfoto / Roger-Viollet

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Types of Maps Topographic Contain diverse set of data on different themes Thematic Show data on particular theme

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Representing Spatial Data Scale Spatial entities Coordinate system Projection

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Scale The ratio of a distance on a map to the corresponding distance on the ground Can be expressed in three ways Large scale versus small scale

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Scale-Related Generalization Figure 2.11 Source: Heywood et al., 2011

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Spatial Entities All features of the real world are represented using three basic two-dimensional models: Points Lines Polygons The model chosen to represent a particular feature depends on the scale

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Figure 2.10 Representing a city at different map scales Source: Reproduced by permission of Ordnance Survey on behalf of HMSO. Crown Copyright 2011. All rights reserved. Ordnance Survey Licence number 100030901

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Coordinate Systems A coordinate system is used to locate features on the Earths surface or representation of this surface Three categories of spatial referencing methods: Geographic coordinate systems Rectangular coordinate systems Universal Transverse Mercator (UTM) grid Non-coordinate systems Provide spatial reference using a descriptive code Postal code system

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Geographic Coordinate System Latitude: describes how far north or south of the equator a feature is located each degree (written: 40) is divided into 60 minutes (written: 35') and each minute into 60 seconds (written: 19"). e.g. 40 35' 19" N Lines of latitude = parallels Image from ArcGIS Desktop Help Copyright Environmental Systems Research Institute, Inc.

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Geographic Coordinate System Longitude: denotes how far east or west of the prime meridian a feature is located e.g. 50 27' 03" E Lines of longitude = meridians Image from ArcGIS Desktop Help Copyright Environmental Systems Research Institute, Inc.

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Representing Earths Surface The figure courtesy of Columbia Earth Institute library website GCS is based on the assumption that Earth is a sphere Shape of the Earth is better represented by a spheroid (ellipsoid) Equatorial bulge Polar flattening

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Representing Earths Surface A list of most common reference spheroids The table courtesy of Wikipedia NameEquatorial axis, mPolar axis, m Inverse flattening(1/f), m Airy 18306 377 563.46 356 256.9299.324 975 3 Clarke 18666 378 206.46 356 583.8294.978 698 2 Bessel 18416 377 397.1556 356 078.965299.152 843 4 International 1924 6 378 3886 356 911.9297 Krasovsky 19406 378 2456 356 863298.299 738 1 GRS 19806 378 1376 356 752.3141298.257 222 101 WGS 19846 378 1376 356 752.3142298.257 223 563 Sphere (6371 km)6 371 000

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Representing Earths Surface The figure courtesy of Columbia Earth Institute library The Earths surface is not smooth and regular Geoid is a model that gives a more accurate representation of the Earths surface Based on the variation in the gravity field of the Earth

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Representing Earths Surface Geoid represent the surface of the Earth at the mean sea level. Commonwealth of Australia 2011 on behalf of ICSM, http://www.icsm.gov.au/mapping/index.htmlhttp://www.icsm.gov.au/mapping/index.html

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Representing Earths Surface A datum is a set of values that defines the position of spheroid relative to the centre of the Earth Combination of an spheroid and a geoiod

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Representing Earths Surface Types of datums Local or regional Spheroid is positioned to fit the geoid in a particular area Geocentric Align with the surface of a spheroid more evenly over the whole Earth Commonwealth of Australia 2011 on behalf of ICSM http://www.icsm.gov.au/mapping/index.html