Transcript Geographic Data: Collection, Interpretation, Display

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 2011. All rights reserved. Ordnance Survey Licence number
100030901
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
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
The figure courtesy of Columbia Earth Institute library
website
Representing Earth’s Surface

A list of most common reference spheroids
Name
Equatorial axis, m
Airy 1830
Clarke 1866
Bessel 1841
International
1924
Krasovsky 1940
GRS 1980
WGS 1984
Sphere (6371 km)
6 377 563.4
6 378 206.4
6 377 397.155
6 378 388
6 356 256.9
6 356 583.8
6 356 078.965
6 356 911.9
Inverse
flattening(1/f),
m
299.324 975 3
294.978 698 2
299.152 843 4
297
6 378 245
6 378 137
6 378 137
6 371 000
6 356 863
6 356 752.3141
6 356 752.3142
6 371 000
298.299 738 1
298.257 222 101
298.257 223 563
∞
The table courtesy of Wikipedia
Polar axis, m
Representing Earth’s Surface
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
The figure courtesy of Columbia Earth Institute library
Representing Earth’s 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.html
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
http://www.icsm.gov.au/mapping/index.html