GI Systems and Science January 11, 2012

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

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).

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

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

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

Representing Spatial Data  Scale  Spatial entities  Coordinate system  Projection

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’

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

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

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

Coordinate Systems  A coordinate system is used to locate features on the Earth’s 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

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.

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.

Representing Earth’s 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

Representing Earth’s Surface  A list of most common reference spheroids The table courtesy of Wikipedia NameEquatorial axis, mPolar axis, m Inverse flattening(1/f), m Airy Clarke Bessel International Krasovsky GRS WGS Sphere (6371 km) ∞

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

Representing Earth’s Surface  Geoid represent the surface of the Earth at the mean sea level. © Commonwealth of Australia 2011 on behalf of ICSM,

Representing Earth’s 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

Representing Earth’s 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