gis

Zimbabwe School of Mines Higher National Diploma Geographic Information System and Remote Sensing

Chapter 1 Introduction

1.0 What is G.I.S?

A GIS is an information system designed to work with data referenced by spatial / geographical

coordinates. In other words, GIS is both a database system with specific capabilities for spatially

referenced data as well as a set of operations for working with the data. It may also be considered

as a higher order map.

GIS technology integrates common database operations such as query and statistical analysis

with the unique visualization and geographic analysis benefits offered by maps. These abilities

distinguish GIS from other information systems and make it valuable to a wide range of public

and private enterprises for explaining events, predicting outcomes, and planning strategies.

(ESRI)

A Geographic Information System is a computer based system which is used to digitally

reproduce and analyse the feature present on earth surface and the events that take place on it. In

the light of the fact that almost 70% of the data has geographical reference as its denominator, it

becomes imperative to underline the importance of a system which can represent the given data

geographically.

A typical GIS can be understood by the help of various definitions given below:-

A geographic information system (GIS) is a computer-based tool for mapping and

analyzing things that exist and events that happen on Earth

Burrough in 1986 defined GIS as, "Set of tools for collecting, storing, retrieving at will,

transforming and displaying spatial data from the real world for a particular set of

purposes"

Arnoff in 1989 defines GIS as, "a computer based system that provides four sets of

capabilities to handle geo-referenced data :

data input

data management (data storage and retrieval)

1


manipulation and analysis

data output. "

Hence GIS is looked upon as a tool to assist in decision-making and management of attributes

that needs to be analysed spatially.

1.1 Objectives and Potentials of G.I.S

GIS Objectives

Maximise the efficiency of planning and decision making

Provide efficiency means of data distribution and handling

Elimination of redundant database-minimize duplication

Capacity to integrate information from many sources

Complex analysis/query involving geographical referenced data to generate new

information

Potentials of G.I.S

Once G.I.S has been implemented the following benefits are expected

Geospatial data are better maintained in a standard format

Revision and updating easier

Geospatial data and information are easier to search, analyze and represent.

More value added product

Geospatial data can be exchanged and shared freely

1.2 COMPONENTS OF A GEOGRAPHIC INFORMATION SYSTEM

A working Geographic Information System seamlessly integrates five key components:

hardware, software, data, people, and methods.

H A R D W A R E

Hardware includes the computer on which a GIS operates, the monitor on which results

are displayed, and a printer for making hard copies of the results. Today, GIS software

runs on a wide range of hardware types, from centralized computer servers to desktop

computers used in stand-alone or networked configurations. The data files used in GIS

are relatively large, so the computer must have a fast processing speed and a large

hard drive capable of saving many files. Because a GIS outputs visual results, a large,

2


high-resolution monitor and a high-quality printer are recommended.

S O F T W A R E

GIS software provides the functions and tools needed to store, analyze, and display geographic

information. Key software components include tools for the input and manipulation of

geographic information, a database management system (DBMS), tools

that support geographic query, analysis, and visualization, and a graphical user interface (GUI)

for easy access to tools. The industry leader is ARC/INFO, produced by Environmental Systems

Research, Inc. The same company produces a more accessible product, ArcView, that is similar

to ARCINFO in many ways.

GIS D A T A

Base Maps- includes streets, highways, boundaries for census, postal and political areas,

rivers and lakes, parks and landmarks; place names.

Environmental maps – include data related to the environment, weather, environmental

risk, satellite imagery, topography, and natural resources

Socio-economic data- include data related to census/demography. Healthcare, real state,

telecommunications, emergency preparedness, crime, business establishments and

transportation.

P E O P L E

GIS users range from technical specialists who design and maintain the system to those

who use it to help them perform their everyday work. The basic techniques of GIS are

simple enough to master that even students in elementary schools are learning to use

GIS. Because the technology is used in so many ways, experienced GIS users have a

tremendous advantage in today’s job market.

M E T H O D S

A successful GIS operates according to:

a well-designed plan

business rules

models and operating practices unique to each organization.

3


1.3 CONTRIBUTING DISCIPLINES AND TECHNOLOGIES

GIS is a convergence of technological fields and traditional disciplines

GIS has been called an "enabling technology" because of the potential it offers for the

wide variety of disciplines which must deal with spatial data

each related field provides some of the techniques which make up GIS

o many of these related fields emphasize data collection - GIS brings them together

by emphasizing integration, modeling and analysis

as the integrating field, GIS often claims to be the science of spatial information

Geography

broadly concerned with understanding the world and man's place in it

long tradition in spatial analysis

provides techniques for conducting spatial analysis and a spatial perspective on research

Cartography

concerned with the display of spatial information

currently the main source of input data for GIS is maps

provides long tradition in the design of maps which is an important form of output from

GIS

computer cartography (also called "digital cartography", "automated cartography")

provides methods for digital

representation and manipulation of cartographic features and methods of visualization

Remote Sensing

images from space and the air are major source of geographical data

remote sensing includes techniques for data acquisition and processing anywhere on the

globe at low cost, consistent update potential

many image analysis systems contain sophisticated analytical functions

interpreted data from a remote sensing system can be merged with other data layers in a

GIS

Photogrammetry

4

http://www.geog.ubc.ca/courses/klink/gis.notes/ncgia/u01.html#OUT1.2.4




http://www.geog.ubc.ca/courses/klink/gis.notes/ncgia/u01.html#OUT1.2


using aerial photographs and techniques for making accurate measurements from them,

photogrammetry is the source of most data on topography (ground surface elevations)

used for input to GIS

Surveying

provides high quality data on positions of land boundaries, buildings, etc.

Geodesy

source of high accuracy positional control for GIS

Statistics

many models built using GIS are statistical in nature, many statistical techniques used for

analysis

statistics is important in understanding issues of error and uncertainty in GIS data

Operations Research

many applications of GIS require use of optimizing techniques for decision-making

Computer Science

computer-aided design (CAD) provides software, techniques for data input, display and

visualization, representation, particularly in 3 dimensions

advances in computer graphics provide hardware, software for handling and displaying

graphic objects, techniques of visualization

database management systems (DBMS) contribute methods for representing data in

digital form, procedures for system design and handling large volumes of data,

particularly access and update

artificial intelligence (AI) uses the computer to make choices based on available data in a

way that is seen to emulate human intelligence and decision-making - computer can act

as an "expert" in such functions as designing maps, generalizing map features

o although GIS has yet to take full advantage of AI, AI already provides methods

and techniques for system design

Mathematics

5








several branches of mathematics, especially geometry and graph theory, are used in GIS

system design and analysis of spatial data

Civil Engineering

GIS has many applications in transportation, urban engineering

6



Chapter 2 GIS FUNCTIONS

2.0 GIS TASKS

General purpose GIS’s perform seven tasks.

Input of data

Map making

Manipulation of data

File management

Query and analysis

Visualization of results

An overview

7


2.1 Input of Data

Before geographic data can be used in a GIS, the data must be converted into a suitable digital

format. The process of converting data from paper maps or aerial photographs into computer

files is called digitizing. Modern GIS technology can automate this process fully for large

projects using scanning technology; smaller jobs may require some manual digitizing which

requires the use of a digitizing table.

Today many types of geographic data already exist in GIS-compatible formats. These data can

be loaded directly into a GIS.

Data input includes three major steps which are:

Data capture ( keyboard entry, manual digitizing, scanning)

Editing and cleaning

Geo-coding

2.1.1 Data sources for GIS

Maps

Aerial photos

Satellite images

Technical descriptions

GPS data

Geographically data contains four integrated components, namely, location, attribute, spatial

relationship and time.

Geographic data include those which are spatially referenced

A GIS operation which support spatial analysis

2.1.2 The three types of GIS Data (spatial, attribute, meta)

1. spatial data

A. vector data

i. Point Data -- layers described by points (or "event") described by X, Y (lat,

long; east, north)

ii. Line/Polyline Data -- layers that are described by X, Y, points (nodes,

events) and lines (arcs) between points (line segments and polylines)

8


iii. Polygon Data -- layers of closed line segments enclosing areas that are

described by attributes

Polygon data can be "multipart" like the islands of the state of Hawaii.

B. raster data (grids of numbers describing e.g., elevation, population, herbicide use,

etc)

C. images or pictures such as remote sensing data or scans of maps or other photos.

This is special "grid" where the number in each cell describes what color to paint or

the spectral character of the image in that cell. (to be used, the "picture" must be

placed on a coordinate system, or "rectified" or "georeferenced")

D. TINs - Triangular Irregular Networks - used to discretize continuous data

2 attribute data are non-spatial characteristics that are connected by tables to points, lines,

events on lines, and polygons (and in some cases GRID cells). They give descriptive

information about specified spatial objects. They don’t have direct information about the

spatial location but can be linked to spatial objects they describe and usually organised in

tables.

A point, vector or raster geologic map might describe a "rock unit" on a map with a

single number, letter or name, but the associated attribute table might have

age

lithology

percent quartz

etc, for each rock type on the map.

9


Most GIS programs can either plot the polygon by the identifier or by one of the attributes

The above examples from the following project show two ways to portray census

data in Virginia. In the top image, each county/city gets a name and unique color,

and in the bottom, the population density per square mile is read from the layer's

attribute table and plotted using a different color for each class of density.

10


3 Metadata

metadata are the most forgotten type

ArcView is very poor at it (writes some stuff to a log file, but that's it)

absolutely necessary if you're going to use data, or if someone is going to use

your data later (or your information)

contains information about

i. scale

ii. accuracy

iii. projection/datum

iv. data source

v. manipulations

vi. how to acquire data

You will be keeping metadata in ArcGIS using ArcCatalog's metadata feature.

2.1.3 Data input identifiers

They enable both spatial and attribute data to be stored separately but accessed together.

Identifiers are:

- unique values- usually integers

- stored as part of the spatial data structure- as numeric value( system generated)

- stored as part of the attribute data structure- as a field in a table

2.1.4 Data model

Conversion of real world geographical variation into discrete objects is done through data

models. It represents the linkage between the real world domain of geographic data and

computer representation of these features

2.1.5 Spatial Data Models

GIS uses two basic data models to represent spatial features: vector and raster. The data model

determines how the data are structured, stored, processed, and analyzed in a GIS. The vector

data model uses points and their x, y coordinates to construct spatial features of points, lines,

and areas. Vector-based features are treated as discrete objects over the space. The raster data

model uses a grid to represent the spatial variation of a feature. Each cell in the grid has a value

that corresponds to the characteristic of the spatial feature at that location. Raster data are well

11


suited to the representation of continuous spatial features like precipitation and elevation. Many

GIS functions are either vector-based or raster-based.

Qualitative or nominal data - discrete (1=basalt, 2=granite, etc for a geological map)

Ordinal or rank data - discrete (low, medium, high; implies a quantity but is in "bins" or

discrete categories)

interval - continuous (example from Theobald, Temperature)

ratio - continuous (hill slope angle, which could be measured/calculated to any precision

and reported in floating point values or integer values)

cyclic - continuous (with a break at one or more points, like compass direction or the

"aspect" of a hill slope)

Choosing the format for continuous vs. discrete data types

Vector storage better for discrete and raster for continuous

Raster Data

- divides the entire study area into regular grid of cells

- each cell contains a single value

- easy space –filling since every location in the study area corresponds to a cell in the raster

- raster data can be imagined as collection of cells organized like matrix

12


Vector Data Model

- represented by lines, points and polygons

- fundamental primitive is a point

- points are stored as X, Y coordinates and represent features as having no dimension

- objects are created by connecting points with straight lines (or arcs)

- areas are defined by set of lines

2.1.6 Comparison of Raster and Vector Data Models

Raster Model

advantages

Vector Model

advantages

It a simple data structure It provides a more compact data structure

Overlay operations are easily and efficiently

implemented

Provides efficient encoding of topology and

more efficient implementation of operations

that require topological information like

network analysis

High spatial variability is efficiently

represented

Is better suited to supporting graphics that

closely approximate hand drawn maps

Is more or less required for efficient

manipulation and enhancement of digital

images

less data storage volume

Disadvantages Disadvantages

Is less compact More complex data structure

Topological relationships are more difficult

to represent

"overlays" rapidly increase complexity and

data storage needs

decreased boundary precision, Representation of spatial variability is

inefficient

higher data storage requirements (8-32 bytes

per cell* rows* columns), but compression

(run length encoding, quad trees) helps

Manipulation and enhancement of digital

images cannot be effectively done

13


RASTER

vector

Real world

14


2.1.7 Representing Spatial Elements

Raster

Stores images as rows and columns of numbers with a Digital Value/Number (DN) for each

cell.

Units are usually represented as square grid cells that are uniform in size.

Data is classified as “continuous” (such as in an image),

or “thematic” (where each cell denotes a feature type.

Numerous data formats (TIFF, GIF, ERDAS.img etc)

Vector

Allows user to specify specific spatial locations

and assumes that geographic space is continuous,

not broken up into discrete grid squares.

We store features as sets of X,Y coordinate pairs

15


Entity Representations

We typically represent objects in space as three distinct spatial elements:

Points - simplest element

Lines (arcs) - set of connected points

Polygons - set of connected lines

We use these three spatial elements to represent real world features and attach locational

information to them

Attributes

In the raster data model, the cell value (Digital Number) is the attribute. Examples:

brightness, landcover code, SST, etc.

For vector data, attribute records are linked to point, line & polygon features. Can store

multiple attributes per feature. Vector features are linked to attributes by a unique feature

number.

2.2 Map Making

Maps have a special place in GIS. The process of making maps with GIS is much more

flexible than are traditional manual or automated cartography approaches. It begins

16


with database creation. Existing paper maps can be digitized and computer-compatible

information can be translated into the GIS. The GIS-based cartographic database can

be both continuous and scale free. Map products can then be created centered on any

location, at any scale, and showing selected information symbolized effectively to

highlight specific characteristics.

The characteristics of atlases and map series can be encoded in computer programs

and compared with the database at final production time. Digital products for use in

other GIS’s can also be derived by simply copying data from the database. In a large

organization, topographic databases can be used as reference frameworks by other

departments.

2.3 Data Management

A collection of non-redundant data which can be shared by different application systems is

known as a database.

Several layers of geographic data covering the same location are considered as database.

When data volumes become large, it is often best to use a database management

system(DBMS) to help store, organize and manage data.

A DBMS is nothing more than a computer software for managing a database. There are many

different designs of DBMSs, but in GIS relational design has been the most useful. In the

relational design, data are stored conceptually as a collection of tables.

A DBMS contains:

Data definition language

Data dictionary

17


Data-entry module

Data update module

Report generator

Query language

Advantages of Database Approach:

● Reduction in data redundancy - the databases are shared rather than independent and this reduces problems of inconsistencies in stored information, e.g. different addresses in different departments for the same customer.

● Maintenance of data integrity and quality

● Data are self-documented or self-descriptive - as information on the meaning or interpretation of the data can be stored in the database, e.g. names of items, metadata.

● Avoidance of inconsistencies - making the data follow prescribed models, rules and standards.

● Reduced cost of software development – as many fundamental operations are taken care of, however DBMS software can be expensive to install and maintain.

● Security restrictions - database includes security tools to control access, particularly for writing.

18


2.4 Data manipulation

GIS data need to undergo transformation before they can be integrated, displayed or

analyzed.

- Same scale, coordinate system, format, etc

A temporary transformation for display purposes or a permanent one required for analysis.

2.5 Spatial Analysis/Modeling

Spatial Operation

Buffering

Overlay

Spatial Statistics

Spatial Data Mining

Proximity Analysis

Buffer: Delineation of a zone around

the feature of interest within a given distance.

For a point feature, it is simply a circle with

its radius equal to the buffer distance

19


Buffer Example

Variable Distance Buffer

Buffer zone can be made variable according to certain attributes. Suppose we have a point

pollution source, such as a power plant. We certainly want to keep our residential area

away a distance from it. However, this distance can be made variable according to the

amount of pollution that a power plant produces.

For small power plant, the distance can be short, while for large power plant that generate

lot of pollutant, we should keep a longer distance from it. As we is shown below

20

Rural district in N.E. Thailand51 study villages:evaluate land use in the district relative to population changeNeed to determine types & quantities of land use surrounding each village:generate 3-km buffers around village centroidsoverlay buffers on land-cover classification generated from satellite imageryuse buffers to “cut out” land near each village and summarize land uses within “cut out” area


Buffers for lines and Polygons

21

Move circle of specified radius along lie(or lines forming polygon)

Draw orthogonal from line to edge of circle

Buffer line is tangent to circle where the orthogonal intersects it.


2.6 Spatial Analysis

Overlay function creates new “layers” to solve spatial problems

Spatial Operation with Multiple Vector Layers

• Overlay analyses

– Operate on spatial entities from two or more maps to determine spatial overlap,

combination, containment, intersection…etc.

– one of the most “fundamental” of GIS operations

– formalized in 1960s by landscape architects who used acetate map overlays

– now a basic part of the GIS toolbox

• Vector overlays-

– combine point, line, and polygon features

– computationally complex

• Raster overlays-

– cell-by-cell comparison, combination, or operation

– computationally less demanding

• Basic idea:

– spatially combine/compare two data layers to:

22


(a) generate new output data layer, or

(b) assign attributes of one data layer to another

– most cases: one of the data layers will contain polygon entities

• Point-in-polygon overlay à

– increasing conceptual and computational complexity

• Point-in-polygon vector overlay

• Overlay point layer (A) with polygon layer (B)

– in which B polygon are A points spatially located?

» assign polygon attributes from B to points in A

Example: comparing soil mineral content at sample borehole locations (points) with

landuse (ploys)

23


Line-in-polygon vector overlay

• Overlay line layer (A) with polygon layer (B)

– in which B polygons are A lines spatially located?

» assign polygon attributes from B to lines in A

Example: assign landuse attributes (polys) to streams (lines)...

Polygon-polygon vector overlay

• Overlay polygon layer (A) with polygon layer (B)

– result: what are the spatial polygon combinations of A and B?

» generate new data layer with combined polygons

• attributes from both polygon layers are included in output

• How are polygons combined?

(i.e. what geometric rules are used for combination?)

– UNION (Boolean OR)

– INTERSECTION (Boolean AND)

24


– IDENTITY

• Polygon overlay will generally result in a significant increase in the number of spatial

entities in the output

– can result in output that is too complex too interpret

Boolean Operations

Some of the fundamental overlay analysis for vector data are UNION, and INTERSECT

corresponding to Boolean operations of OR, AND

UNION

overlay polygons and keep areas from both

layers

INTERSECTION

overlay polygons and keep only areas in the input

layer that fall within the intersection layer

IDENTITY

overlay polygons and keep areas from input

layer

25

A BOR A B

A BAND

UNION

INTERSECTA B


2.7 CONNECTIVITY FUNCTIONS

Contiguity Measures. Contiguity measures evaluate characteristics of spatial units that are

connected. These units share one or more characteristics with adjacent units and form a group.

The term UNBROKEN is the key concept. Different adjacent features may have more than one

attribute but they must all have a COMMON attribute to be considered as reflecting contiguity.

Contiguity is used to measure shortest and longest straight line distances across and area and to

identify areas of terrain with specified size and shape characteristics.

Example. An area of continuous pastureland with an area of no more than 10 acres with no part

of it wider than the sound of the Acme Pig Call can be heard.

Proximity Functions. The simple distance between features, commonly units of length but can be

other units such as how far away the ACME PIG CALL can be heard.

Four parameters are used to measure proximity. 1. target locations. 2. unit of measurement. 3. a

function to calculate proximity. 4. and the area to be analyzed.

A common type of proximity analysis is the buffer zone. Coverage can be quit complicated

involving many layers and mathematically complex such as the decreasing sound levels due to

the inverse proportion law of noise generated by various types of air traffic in the vicinity of a

housing area.

2.8 Network Functions

Definition: A network is a set of interconnected linear features that form a pattern or framework.

They are commonly used for moving resources from one location to another. City Streets, Power

Transmission Lines, and Airline Service Routes are examples.

There are three principal types of GIS Analysis performed by Networking. 1. Prediction of

loading on the network itself (prediction of flood crests), rate optimentation (emergency routing

of ambulances), and resource allocation (zones for servicing rescue areas).

26


Networks analysis entails four components. 1. set of resources (goods to be delivered). 2. one or

more locations where the resources are located (several warehouses where the goods are

located). 3. an objective to deliver the resources to a set of destinations (customer locational data

base). 4. Set of constraints that places limits on how the objective can be met (is it economically

feasible to deliver pizzas to Lincoln from a store in Omaha ?).

2.9 Spread Functions

The Spread Function is simply the "best" way to get from point A to point B. "Best" can be

fastest, it can be most the most economical, or a subjective measurement such as most scenic. It

is an evaluation of phenomena that accumulates with distance.

Imagine a square and you are going to travel from the lower left corner to the upper right corner.

The straight line distance is 1.414 times the side of the square, and the distance across the sides is

2.0 times the length of a side. If this square represented a pasture containing angry buffaloes it

would probably beneficial to walk around the fenced perimeter and go the extra distance.

Output of this particular GIS functions is sometimes referred to as ACCUMULATION

SURFACE or FRICTION SURFACE. These concepts refer to the "effort it takes to get from A

to B, such as the square traversed was knee deep mud (or a lake) across the diagonal but dry at

the perimeter. It would be farther, but easier to again go the extra distance.

2.10 Seek or Stream Functions

Seek and Stream are synonymous and refer to a function that is directed outward in a step by step

manner using a specified decision rule. This procedure is initiated and proceeds until the any

further movement violates the decision rule.

This GIS function, as an example, could be used to evaluate erosion potential. The decision rule

in this case would be elevation. As the process proceeds outward from the source (rainfall), the

decision will always proceed downhill, never uphill. The path of least resistance best describes

this function, Sea level, interior drainage or the edge of the area analyzed causes the function to

terminate.

27


2.11 Intervisibility Functions

This GIS function is typified by the phrase LINE OF SIGHT. It is a graphic depiction of the area

that can be seen from the specified target areas. Areas visible from a scenic lookout, or the

required overlap of microwave transmission towers can be mapped using this procedure.

Intervisibility functions rely on digital elevation data to define the surrounding topography.

Applications such as landscape layouts, military planning, and the obvious communication

utilization are best serviced.

The output of this function is somewhat unique in that it is often displayed in a SIDE VIEW

format. The vertical field of view and maximum viable distance are the component parameters.

It is powerful tool for trial and error analysis in which the placement of objects can constantly be

re-evaluated. Offshoots of this type of procedure can produce graphics that exhibit three

dimensional perspective. SHADED RELIEF IMAGES or SHADED RELIEF MODELS, along

with PERSPECTIVE VIEWS are valuable presentational tools. The process called draping is

used to apply another data set over this shaded depiction to further enhance presentability.

2.12 OUTPUT FUNCTIONS

Map Annotation

Definition: Titles, Legends, Scale Bars, and North Arrows are the simplest forms of depicting

information concerning the map.

The various programs available usually handle this as user input and it is not generated by the

software. Flexibility as to location (position), fonts, symbology, and size are varied as to the

individual programs. Text labels are an important aspect of map viewing and are all different as

to program. Sophistication is increasing and actual hard copy maps can be enhanced with

secondary software applications.

28


Texture Patterns and Line Styles

Texture patterns and line styles are difficult to alter from program guidelines so initial analysis of

the output should be considered when choosing a software.

Graphic Symbols

Graphic Symbols are used to portray the various entities depicted on the map. Some software

packages provide a simple standard symbol set, but do not allow user input, others store them

within the GIS and they can be called to use as needed, others assign a symbol to an attribute and

allow the symbology to be automatically plotted. As before the selection of the software and its

application should be carefully considered as to the output presentation needed.

29


Chapter 3 Map Projections and Coordinate Systems in GIS

3.1 Map Projections

Map projection transforms the spatial relationship of map features on the Earth’s surface to a

flat map.

Map projection enables a map user to work with two-dimensional coordinates, rather than

spherical or three-dimensional coordinates. But the transformation from the Earth’s surface

to a flat surface always involves distortion and no map projection is perfect

A projection is the translation of spherical coordinates onto a planar surface, while a datum

is the ellipsoid, or “figure of the earth” that approximates the actual shape of the earth, and is

used in the transformation equation A datum is the geometric, 3-D “figure of the earth”

which is used as the basis for projecting onto a planar surface. The most common datums we

run across are the North American Datum of 1927 (NAD27), which is tangent to a point on

the surface of the earth (Mead’s Ranch, in Kansas), and the North American Datum of 1983

(NAD83), which is centered on the center of the earth. The World Geodetic System of 1984

(WGS84) is the common datum used by the GPS configuration, and is essentially identical to

the NAD83

How the ellipsoid (datum) is projected onto a planar surface.

30


3.2 Coordinate Systems

A coordinate system is based on a map projection. Once an ellipsoid has been projected onto

a planar surface, a coordinate system must be defined to specify locations on that surface.

The familiar XY coordinate pairs of a typical graph is an example of a coordinate system.

Universal Transverse Mercator (UTM), State Plane Coordinate System (SPCS), and

Longitude and Latitude are commonly used coordinate systems in GIS Plane coordinate

systems are typically used in large-scale mapping such as a scale of 1:24,000 or larger.

Coordinate systems are designed for detailed calculations and positioning. Therefore,

accuracy in a feature’s absolute position and its relative position to other features is more

important than the preserved property of a map projection.

Map projections come with names like Lambert conic conformal or Albers conic equal-area.

Lambert and Albers are names of the cartographers who originally proposed the projections.

The other parts of the name describe the map projection’s preserved property and projection

surface.

Map projections are grouped into four classes by their preserved properties: conformal, equal

area or equivalent, equidistant, and azimuthal or true direction. A conformal projection

preserves local shapes. An equivalent projection represents areas in correct relative size. An

equidistant projection maintains consistency of scale for certain distances. An azimuthal

projection retains certain accurate directions.

Cartographers often use a geometric object to illustrate how a map projection can be

constructed. For example, by placing a cylinder tangent to a lighted globe, a projection can

be made by tracing the lines of longitude and latitude onto the cylinder. The cylinder in this

case is the projection surface, and the globe is called the reference globe. Other common

projection surfaces include a cone and a plane. A map projection is called a cylindrical

projection if it can be constructed using a cylinder, a conic projection using a cone, and an

azimuthal using a plane. Why the understanding of map projections is important in practical

applications of GIS? A basic principle in GIS is that map layers to be used together must be

based on the same coordinate system.

31


Otherwise, map features from different layers will not register spatially in a proper manner.

Increasingly, GIS users download digital maps from the Internet, or acquire them from

governmental agencies and private companies. Some digital maps are measured in longitude

and latitude values, while others are in various coordinate systems different from the one

intended for the GIS project. Invariably, these digital maps must be projected and re-

projected before they can be used together. Typically, projection and re-projection are among

the initial tasks performed in a GIS project.

3.3 Errors in GIS

No map is perfect, even the most accurate maps created by a GIS have some deficiencies.

These deficiencies occur due to “Errors” that may have taken place at different stages of GIS

implementation. These errors reduce the accuracy of the map generated. However by use of

well defined and controlled procedures these errors can be avoided.

There are two types of errors in GIS:

1 Source Errors:

They are the errors that are present in “Source Data” that is given to the GIS. They occur

before the actual implementation of GIS

Instrumental inaccuracies

- Satellite/ air photo/ GPS/ surveying (spatial).

- Inaccuracies in attribute measuring instruments

Human Processing :

- Misinterpretation (e.g. photos), spatial and attribute

- Effects of scale change and generalization

- Effects of classification (nominal / ordinal / interval).

Actual Changes :

- Catastrophic change: fires, floods, landslides

- Gradual 'natural' changes: river courses, glacier recession.

- Seasonal and daily changes: lake/sea/ river levels.

- Man-made: urban development, new roads.

- Attribute change: forest growth (height etc.), discontinued trail / roads, road

surfacing.

32


2 Processing Errors:

They are the errors that occur during the processing of the data i.e. during the

implementation of GIS.

Input:

- Digitizing: human error, the width of a line, spikes, knots, also entering attribute data.

Dangling nodes (connected to only one arc): permissible in arc themes (river

headwaters etc.).

Pseudo-nodes (connected to one or two arcs) - permissible in island arcs, and where

attributes change, e.g. road becomes paved from dirt or vice versa.

- Projection input error

Manipulation

- Interpolation of point data into lines and surfaces

- Overlay of layers, digitized separately, e.g. soils and vegetation.

- The compounding effects of processing and analysis of multiple. layers: for example,

if two layers each have correctness of 90%, the accuracy of the resulting overlay is

around 81%.

- Density of observations

- Inappropriate or inadequate inputs for models

Output:

- Scale changes - detail and scale bars.

- Color palettes: intended colours don't match from screen to Printer

33


3.4 LIFECYCLE OF A GIS (PLANNING GIS)

Successful implementation of GIS requires planning the project before its actual implementation. Planning leads to a better structured and organized system.

Phase 1-Planning

A planning process is the first stage in the life cycle. This phase involves a systematic review of users, their data, and their information needs. Decision makers are told about the costs and benefits of GIS and to include potential users in planning process so that they receive an overview of the technology.

Phase 2-System Design

The design phase matches user needs to GIS functionality. Design includes not only selection of hardware and software, but also the design of the GIS spatial and attribute database. A Relational database is generally used for the GIS. The Database design will include specifications for scale, projection, and coordinate systems. Data is be tracked using a "Data Dictionary." During the design phase an incremental plan is often used for implementation of the technology. Incremental implementation means that users will build a GIS piece-by-piece.

34


In some cases a Prototype’ is developed so that refinements can be made before finalizing the fully implemented system.

Phase 3-Implementation

During the implementation phase, attention to all user needs must be provided through training and education. Hands-on users must be trained to utilize and maintain the system and the database. All types of users should be made cognizant of how the GIS will affect them and their data processing tasks. They must also be made aware of the changes that GIS will introduce in the area of information generation and decision making.

Phase 4-Maintenance

Finally, a GIS application must be maintained and kept current in terms of data and user support. In some cases, a GIS is designed to meet the needs of a specific, finite project. In other instances, GIS is used to support an on-going mission or program. In the former case, the GIS application will terminate once the project is completed and maintenance will probably not be an issue. However, even if the initial GIS application is no longer being utilized, the data generated for the initial project may be useful to other projects or users. In those instances, a current data dictionary will be vital for determining the utility of the existing digital data for other uses

In the case of an on-going GIS effort the system must be kept up-to-date in order to fulfil its design goals. Maintenance includes updating hardware and software, adding new data and updating existing data records, and keeping users current in terms of system functionality

3.5 Examples of Applied GIS

Urban Planning, Management & Policy

Zoning, subdivision planning Land acquisition Economic development Code enforcement Housing renovation programs Emergency response Crime analysis Tax assessment

Environmental Sciences

Monitoring environmental risk Modeling stormwater runoff Management of watersheds, floodplains, wetlands, forests and aquifers Environmental Impact Analysis Hazardous or toxic facility siting

35


Groundwater modeling and contamination tracking

Civil Engineering/Utility

Locating underground facilities Designing alignment for freeways, transit Coordination of infrastructure maintenance

36

gis

Technology

data files

data distribution

spatiallyreferenced

database system

geographical referenced

easier geospatial data

product geospatial data

analysis data output