Arcview Gis Basics

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WHY GIS Problems of understanding phenomena

that

Geographic phenomena Temporal dimension

Objects have different characteristics for

different locations and moment in time

Many organizations/institutions are

investing in GIS

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Why GIS ….. Lowering in price of computers Geography and data describing it form our

everyday life Our decisions are influenced, constrained or

dictated by geography

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Why GIS…… GIS provides a means of Integrating information Understanding phenomena that have a geographic nature Address some of the most pressing problems

Such understanding is the basis for more sensitive and intelligent decision making

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What is GIS It is a computerized system that facilitates Data entry (georeferenced) Analysis (various ways of analysis) Data presentation (maps)

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…GIS phases…. Data preparation and entry 1st where data about the phenomena is collected and prepared for entry

Data analysis 2nd stage of review of collected data and discovering patterns

Data presentation Final results in which results are presented in appropriate way

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Questions GIS can answer… When dealing with data and information we

are trying to represent some part of the real world As it is As it was As we think it will be

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Questions GIS can answer…

Location

what exist at a particular location

Condition identify a location where certain conditions are satisfied

Trends identify changes/differences within an area

Patterns whether occurrence has a pattern or not

Modeling determines what happens to an event if a certain condition behaves in a certain way (Models : are simplified abstractions of reality, representing or describing its most important elements and their interactions ) MZUNI RS/GIS mdt2006

Components of GIS… Mostly GIS is mistakenly thought as a single piece of software BUT…….. It is made up of different component

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…Components of GIS Spatial data - containing representation of

geographic phenomena Attribute data – describe the characteristics/qualities of features Cartographic display Map digitising Database management system Geographic analysis system Image processing Appropriate hardware Trained personnel MZUNI RS/GIS mdt2006

GIS components

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Geographic phenomena Something that 



can be georeferenced (it has a position in space) can be named and described

 it

is / was present during a time span

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Structure of spatial data Earth feature and their attributes are stored in

two systems

Raster format Vector format

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Vector format Spatial features are represented in form of Points Line features Area features/polygons

Mostly obtained by digitising

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Vector representation Points 

eg location, given by xy coordinate pair

Line features 

Series of xy coordinates (points) that best reflect its characteristics. eg roads, rivers etc

Area feature 

Defined by boundary line and a code

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Raster format Spatial data organised in grid cell called

pixels (picture element) Pixels are basic units for recording

information Have same dimensions Each pixel have one value

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Pixel size The dimensions of the grid cells (pixels) are a

crucial aspect of raster data structuring

Pixel size is related to the dimensions of the

minimum area to be spatially represented

A pixel size of one quarter of the dimensions of

the smallest map feature is needed to detect all areas, e.g.: 

If the smallest map unit is 1 ha (100 x 100 m), a maximum pixel size of 25 m should be chosen



However, 100 cells per polygon are required to

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Raster representation 0

1

0

3

4 (3,1) xy

1 2 Rows

3 4 Columns

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2

V e c to r d a ta m o d e l

Raster vs Vector y

y

y

code code x

x

C o lu m n K e y f o r t h e v e c t o r m o d e ls :

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x

A re a

R ow

L in e

R ow

R ow

P o in t R a s te r d a ta m o d e l

code

C o lu m n i n t e m e d i a t e p o in t n ode

C o lu m n

Comparison of Raster and Vector format Raster model

Vector model

Simple data structure Easy and efficient overlaying Compatible with Remote Sensing imagery High spatial variability is efficiently represented Simple for programming by user Same grid cell definition for various attributes

Complex data structure Difficult to perform overlaying Not compatible with RS imagery Inefficient representation of high spatial variability

Inefficient use of computer storage Errors in perimeter and shape Difficult to perform network analysis Inefficient projection transformations Loss of information when using large pixel sizes Less accurate and less appealing map output

Compact data structure Efficient encoding of topology Easy to perform network analysis

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Highly accurate map output

Scale and Resolution Map scale Ratio between distance on map and distance

on the ground

Resolution Commonly associated with raster format Size of the pixels (cell width)

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End of part one

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Data capture/Sources of data The quality of GIS products depends on the

quality of the input data

Data input can account for up to 80% of GIS

operation time

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Methods of data capture/Sources of data Direct acquisition This the primary and sometimes the most ideal way to obtain spatial data. Not always applicable due to cost and availability

Indirect acquisition Problems of accuracy and aggregation of errors Cost effective method

From other sources In most cases at a cost MZUNI RS/GIS mdt2006

Direct spatial data acquisition Land surveying

determination of location by

means of distance and angle measurements GPS surveys (satellite observation and realpositioning) world measurement

ground based approach MZUNI RS/GIS mdt2006

spatial database

Direct spatial data acquisition Remote sensing Aerial photography Satellite imagery

real world

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sensor

image data

observation and measurement

remote sensing based approach

spatial database

Indirect spatial data acquisition Digitizing existing maps manual digitizing screen digitizing of scanned maps (semi-) automatic digitizing Existing digital data from clearinghouses the World Wide Web

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Illustration of indirect data acquisition

Digitising process

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Other sources of data 

Available from elsewhere



Clearing house

InterNet

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CD­ROM

Digitising

Manual digitising

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Digitising On screen

digitising

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On-screen versus manual digitising more comfortable for the operator more accurate ( zooming facilities ) faster (digitising & editing at the same time) up-dating procedure (geometrically corrected

satellite imagery and scanned aerial photos can be overlaid with the old map vector data ) source documents have to be scanned

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Spatial data analysis GIS Spatial-analytical distinguishes it from

other data processing systems

The capabilities use spatial and non-spatial

data to answer questions and solve problems

The principal objective is transform and

combine data from diverse sources into useful information

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GIS Analytical capabilities Measurement functions Computing function distances, area (2D) and volume (3D) Retrieval functions Spatial queries retreating features selectively using user defined logical conditions Classification functions Re-assigning thematic characteristics value to features in a data layer All these features allow exploration of data without making major changes - mostly done on a single layer

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GIS Analytical capabilities Overlay functions Combining data layers to derive new

information Easily done in raster data  These operators are called map algebra or raster calculus  Commonly used overlay functions in vector are the clip-by and overwrite and intersection functions 

Neighborhood functions Evaluates the characteristics of an area

surrounding a feature’s location MZUNI RS/GIS mdt2006 Looks at buffer zones around a feature and

GIS Analytical capabilities Connectivity functions Evaluates how features are connected Useful when dealing with networks of connected features like road networks, water courses etc

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Break

QUESTIONS…!!?

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Spatial referencing… Encompasses the physical/geometric constructs

and the tools required to describe

 the geometry and motion of objects near and on earth’s surface

Some are itemised on the legend of a published

map GIS user need to understand the basic concepts and terminology of spatial referencing

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…Spatial referencing… why? Spatial referencing is important in order to Integrate data from different sources Integrate national and global data sets use of satellite positioning (e.g. GPS) ….

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The shape of the earth and the datum Earth is a rotating body, flattened towards

the poles

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The shape of the earth and the datum The earth surface is continuously changing in

shape due to irregularities in mass distribution inside the earth

Earth as a GEOID

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The shape of the earth and the datum geoid surface is an undulating surface All measurements are taken from a

reference point with the following properties; surface of zero height measurable (to be sensed with instruments) level (i.e. flat

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The geoid as reference surface for Heights A reference surface for heights must be a surface of zero height measurable (to be sensed with instruments) level (i.e. flat)

The geoid is the most obvious choice the geoid approximates all earth’s oceans oceans are only affected by gravity: sea level every point on the geoid has the same zero

height all over the world MZUNI RS/GIS mdt2006

The geoid and the vertical datum The ocean’s water level is registered at

coastal locations over several years. The resulting water level is called the Mean Sea Level (MSL)

Variation of these local vertical datum exist due to tidal differences, ocean currents, winds, water temperature Salinity i.e.. affect sea level at the

measurement location

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Vertical datum Starting from these MSL points, the

heights of points on the Earth can be measured using geodetic leveling techniques

MSL

Levelling benchmarks of known height in a levelling network

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A reference surface for locationschanges in shape Geoid surface continuously due to changes in mass density inside the earth

Geoid is NOT suitable as a reference surface

for the determination of locations

a mathematical reference frame is needed compute positions, distances, directions,

etc

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A reference surface for locations The ELLIPSOID is the most convenient

geometric reference for measuring locations

semi -minor axis

Pole

semi-major axis

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Equatorial plane

Horizontal datum Countries establish a horizontal datum: an ellipsoid with a fixed position, so that the

ellipsoid best fits the surface of the area of interest (the country) topographic maps are produced relative to this horizontal (geodetic) datum the size, shape and position (the datum) define

the horizontal datum

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Horizontal datum local and global datum

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Map projections To produce a map, the curved surface of

the Earth surface (as modeled by an ellipsoid) is mapped onto a flat plane using a MAP PROJECTION

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Classes of Map projections

Azimuthal Projection

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Cylindrical projection (example)

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Conical projection (example)

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Geo-referencing Image is created either by a satellite, airborne

scanner or by and office scanner

the image is stored in row and column geometry

in raster format

There is no relationship between the row/column

and the real world coordinates (UTM, Geographic coordinates or any other reference map projection)

Geo-referencing is the process of relating row and

column numbers and the real world coordinates.

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Azimuthal projection (example)

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END

QUESTIONS…!!!??

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GIS and Maps The hard copy maps are used as As an input for GIS Communication results A tool while working with GIS

Maps can be an input after scanning and import

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Maps and Communication

Maps can be used to communicate when where question is asked. Where were the farmers sampled in Mzimba district for maize yield estimates?

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Maps Maps can provide more information other than location Inform about thematic attributes of

geographical object 

For example: sex, household size, date of planting, date of first fertilizer application ect. 

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GIS and Maps

Maps answers what only in relation to location what is the predominant pattern of farms in south Mzimba?

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GIS and Maps maps can answer question when When did The Netherlands have the longest coast?

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GIS and Maps As such maps are: the best means of transferring spatial information Offer an insight about geographical feature On screen maps are interactive Have a database allowing more complex queries

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What is a map? A representation or abstraction of geographical reality. A tool for representing geographic information in a way that it is visual designed A reduced and simplified representation of (part of earth’s surface on plane

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A map & Reality

Image

Image and a map

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Map

Qualities of a map Ability to offer abstraction of reality A map provides relationship between

effectiveness and map scale Map Scale is the ratio between a distance on

the map and corresponding distance in reality.

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Map scale Map Scale is the ratio between a distance on

the map and corresponding distance in reality A map that shows details on smaller area is

called larger-scale map A map that shows details on larger area is called smaller-scale map

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GIS and Maps GIS based maps are becoming popular today These maps involve combination and

integration of many different data sets

An overly operation is used to combine

datasets The combination is spatial data layer

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GIS and Maps

The map shows

a) polygon b) Roads But displayed in a

view and printed as one map

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END gis & map

QUESTIONS…!!!?? MZUNI RS/GIS mdt2006