GIS: An IntroductionGIS: An Introduction

Andrew LovettAndrew Lovett

School of Environmental Sciences School of Environmental Sciences University of East Anglia University of East Anglia

a.lovett@uea.ac.uka.lovett@uea.ac.uk

What is a GIS?What is a GIS?

A Geographical Information System (GIS) can be defined in several different ways.

One common approach is to emphasize the functions performed e.g.

“ a system for capturing, storing, checking, integrating, manipulating, analysing and displaying data which are spatially referenced to the earth”

Report of the Chorley Committee to the UK Department of the Environment, 1987.

Layers of InformationLayers of Information

A GIS can display and work with many different sets of information at a time (e.g. population, elevation, satellite imagery) – these are usually called layers or themes.

You could imagine these as transparent maps overlaid on each other – the GIS allows you to see how layers relate, and to make decisions based on multiple data sets.

The spatial reference provides a means of data integration.

Real World

UsersInterpretation

andDecisions

Operational GISAnalysis andPresentation

GIS DataDescription andRepresentation

LineLine

BuildingBuilding

Pump HousePump House HouseHouse

StreetStreet WaterLine

WaterLine

FeatureFeature

PolygonPolygon

Source: Longley et al. (2005) p. 178

A key influence on the success of a GIS project

What is a GIS?What is a GIS?

Types of GIS DataTypes of GIS DataJust as maps do, representing real world features in a GIS database also invariably involves some simplification and abstraction.

There are two broad types of information:

• Continuous data can be measured anywhere (e.g. elevation, temperature) and forms a surface.

• Discrete data show individual objects and where they are located, as point, line or area features.

Some types of real world features can be described using either approach e.g. a lake.

Continuous representation of UEA Broad. Band 4 reflectance on Landsat TM.

Discrete representation of UEA Broad. OS Land-Line map data.

Factors such as spatial scale and the intended use of the data can also influence the approach to conceptualisation.

Types of GIS DataTypes of GIS Data

Continuous and discrete data lend themselves to storage and use in different ways, leading to two common data formats:

Raster format is effectively a grid of information representing a single variable or category, at a defined resolution. It lends itself well to continuous data.

Vector format shows data as made up of point, line and area entities, but boundaries and locations are not confined to a grid. It lends itself to discrete data.

Types of GIS DataTypes of GIS Data

The original map (A) is divided into a regular grid of cells (B).

Each cell is assigned a number (C) representing the feature present. All cells have values so the structure is space filling.

A B

C

Raster DataRaster Data

The accuracy with which shapes are shown depends on the cell resolution (the length of one side of a cell).

Rasterised at 5m resolution

(higher resolution)

Rasterised at 10m resolution

(lower resolution)

Vector DataVector Data

Features are represented as sets of points, lines and polygons (areas). In vector datasets, attribute information is stored separately from spatial data and joined (i.e. linked) through unique feature IDs.

Feature ID for polygon

Questions a GIS can AnswerQuestions a GIS can Answer

Source: Kraak & Ormeling (2003) Cartography: Visualisation of Geospatial Data, 2nd Edition, Prentice Hall, Harlow,p.7.

The Development of GISThe Development of GIS

The global market for geospatial products and services was estimated at $30 billion per annum in 2005.

Personal Digital Assistant (PDA)

Internet GIS, Geobrowsers

1960s-1980s 1990s 2000s