AN INTRODUCTION TO
GEOGRAPHICAL INFORMATION
SYSTEMS
What is GIS?
Defining GIS
A computer system: hardware, software & procedures.
Uses spatially referenced data.
Carries out management and analysis tasks.
Can be used to add value to spatial data.
A spatial decision support system.
Geographic Information Science: the science behind the system → the study of
the fundamental issues arising from the creation, handling, storage and use of
geographic information & impacts of GIS on individuals and society and the
influences of society on GIS.
Components of a GIS
I. Computer system (hardware and operating system).
II. Software, spatial data, data management and analysis procedures.
III. People.
Spatial data: information about position, connections with other features and details
of non-spatial characteristics. Must be simplified → three basic entity types:
Point: represent the locations of features.
Line: represent features that are linear in nature (roads, rivers). It is simply an
ordered set of points & a string of x and y co-ordinates joined together.
Area features: represent geographical zones. It is a closed set of lines. Also
called polygons. A three-dimensional are is a surface.
Island polygons.
Adjacent polygon: boundaries are shared between adjacent areas.
Functions of a GIS
Data input: the process of converting data from its existing form to one that can be
used by the GIS.
Types of data:
I. Graphical data: spatial characteristics of the real-world feature being
modelled.
II. Non-spatial attribute data: what the features represent. They provide extra
information.
,Data management: facilitate the storage, organization and retrieval of data using a
database management system (DBMS).
Transformation of data: the process of changing the representation of a single entity,
or a whole set of data.
Data analysis, types:
Those used for storage and retrieval.
Constrained queries that allow the user to look at patterns in their data.
Modelling procedures for the prediction of what data might be at a diferent
time and place.
Data output: form will depend on cost constraints, the audience and the output
facilities.
Spatial data
Model: a synthesis of data which is used as a means of getting to grips with systems
whose spatial scale or complexity might otherwise put them beyond our mental
grasp.
Data: observations we make from monitoring the real world → raw facts.
Information: data with meaning and context added.
Data sources:
Primary data: collected through first-hand observation.
Secondary data: collected by another individual or organization.
All data have three modes/dimensions:
I. Temporal dimension: when the data were collected.
II. Thematic dimension: the character of the real-world feature to which the
data refer → non-spatial/attribute data.
III. Spatial dimension: the values, character strings or symbols that convey to
the user information about the location of the feature being observed.
Maps and their influence on the character of spatial
data
Thematic maps: show data relating to a particular theme or topic.
Topographic maps: contain a diverse set of data on diferent themes.
Topology: an underlying geometric structure which allows the user to describe the
relationship between features.
Mapping process is of general nature:
I. Establish a purpose the map is to serve: turn data into information that will
be communicated to a third party. The purpose for which a spatial data set
, has been created will influence the quality and spatial detail provided by the
data set.
II. Defne the scale at which the map is to be produced: scale gives an indication
of how much smaller than reality a map is. Small scale map: cover large
areas (1:250.000).
Ratio scale (1:5000)
Verbal scale (1 cm represents 50 m)
Graphical scale.
III. Select the features from the real world which must be portrayed on the map
and chose a method for representation of these features: point, lines and
areas. The method chosen to represent a spatial feature will depend on the
scale used.
IV. Generalize these features for representation in two dimensions: simplification
of detail is necessary in order to maintain clarity. Scale-related
generalization: as the scale increases, the cartographer has greater scope
for including more detail.
V. Adopt a map projection for placing features onto a fat piece of paper: map
projections transfer the spherical Earth onto a two-dimensional surface.
Cylindrical projection (Mercator): the surface of the Earth is projected
onto a cylinder that encompasses the globe. Maintaining scale, shape,
area. The view of the poles is very distorted.
Azimuthal projection (single wall): only a part of the Earth’s surface
is visible, distortion will occur at all four edges, distance for the most
part is preserved.
Conic projection (tepee): area is distorted, distance is very distorted
towards the bottom of the image, scale is for the most part preserved.
VI. Apply a spatial referencing system to locate these features relative to each
other: is used to locate a feature on the Earth’s surface or a two-dimensional
representation of this surface such as a map. Characteristics: stability, ability
to show points, lines and areas, ability to measure length, size and shape.
Geographic co-ordinate system: latitude and longitude (meridians,
from pole to pole). Assumes that the Earth is a perfect sphere → not
correct → The Quaternary Triangular Mesh: deals with irregularities in
the Earth’s surface.
Rectangular co-ordinate system: a map graticule / grid is place on
top of the map → projected the lines of latitude or longitude from our
representation of the world as a globe onto a flat surface using a map
projection.
Non-co-ordinate systems: provide spatial references using a
descriptive code rather that a co-ordinate (e.g. postal codes).
Problems: spatial entities may be mobile and may change, the same object
may be referenced in diferent ways & the large number of diferent spatial
referencing systems in use.
VII. Annotate the map with keys, legends and text to facilitate use of the map.
, Topology: the geometric characteristics of objects which do not change under
transformations such as stretching or bending and are independent of any co-
ordinate system. Consists of three elements:
Adjacency: when areas share a common boundary.
Containment: describes areas features that may be wholly contained within
another area feature.
Connectivity: a geometric property used to describe the linkages between line
features.
Thematic characteristics of spatial data
Attributes: the characteristics of an entity. Give additional information about the
character of the entities.
Scale of measurement: important characteristic of an entity → dictates how the data
can be used.
Nominal scale: numbers are used to establish identity. These numbers
cannot be processed in a mathematical sense.
Ordinal scale: numbers establish order. You cannot obtain information about
relative sizes.
Interval scale: the diference between numbers is meaningful but the scale
does not have a real origin.
Ratio scale: the diference between the numbers is significant.
Other sources of spatial data
Collections of related information
Administrative data: data on births, deaths and marriages or details of those claiming
social security benefits which are collected routinely or on a one-of basis.
Survey data: a national coverage survey → a set of detailed questions on a particular
topic.
Census data: collects demographic and household information about an entire
population.
Aerial photography
= capturing images from a position above the Earth’s surface or without contact with
the object of interest → wide availability, low cost, wide area views, time-freezing
ability, high spectral and spatial resolution & three-dimensional perspective.
Angle: vertical (taken directly below the airplane) and oblique (taken at an angle).
Scale varies across an aerial photograph.
Factors that may influence interpretation may be considered (time of day and
time of year).
