GIS presentation g2 Contents: • Representing Geography -Yahya • Geographic data models – Zeferino • Creating and maintaining geographic databases – Nima & Lucia • DHIS & GIS in Mozambique - Leopoldo
Representing Geography • All human activities require knowledge about the earth. - representations help us get that knowledge. - we use it for planning. where to get/keep… • Digital presentation has many advantages over the others. - data are easy to process, transform and analyze, easy to copy and transmits. - maps can change scale, zoom, pan. no sheet boundary limitations!
Representing Geography contd • Concerned with the earth’s surface or near surfaces. - Represent every thing? No. a key issue is what and how. - useful if it is accurate. • Geographic data link place, often time and attributes - “The temperature at Dar es salaam 43E, 12S on December 2, 2002 was 32C”; geographic fact - “Mount Kilimanjaro is 5000m high” ? - “Zanzibar City center is situated at 44E, 1130’S and is 3m above sea level” ! Data used fictitious!
Representing GeographyAttributes • Classified as nominal, ordinal, interval, ratio. • Nominal: identifies one entity from another; e.g. place names; no comparison/relationship can be made. • Ordinal: values have natural order; e.g. soil classes; class1, class2,…. “class2 is between class1 and 3”! • Interval: differences make sense; e.g. temperatures. • Ratio: ratio make sense; e.g weight. What about temperature?!
Representing GeographyConceptual representation • Two ways: Discrete objects and Fields • Object view: the world is empty except where it is occupied by objects with defined boundaries. - e.g a district is littered with PHCs, roads, population - identified by dimension; points, lines, polygons. - information can be expressed in tables; objects with attributes. Consider a terrain, composed of peaks, valleys, ridges, slopes, etc… Do we count them? Do we list them? • Useful to think of terrain as a continuous surface - elevation defined rigorously at every point. Field view!
Representing GeographyConceptual representation • Field view: real world represented as finite numbers of variables, each one defined at every possible position • Differences: objects/fields - Objects distinguished by their dimensions;points, lines, areas. - Fields distinguished by what varies and how smoothly. Field or object? - Population density - Himalaya mountains - Al-Rahma Hospital in Zanzibar.
Representing GeographyDigital representation • Two methods: raster and vector. • Raster: represent the world into arrays of cells and assign attributes to the cells. - laying of tile floor on a flat surface - the earth is not flat. Discussion section 4.7 - sources: remote sensing satellites, imagery - applied: resources, environmental planning - fixed resolution - volume of data depends on cell size - represent more honestly
Representing GeographyDigital representation • Vector: lines are captured as points connected by precisely straight lines. An area is made of vertices connected by straight lines. - volume depends on density of vertices - used for social, economic and administration. - source: social, environmental - variable resolution
Representing GeographyRelationship: Conceptual/Digital In principle In practical there is strong association Code Objects Raster Fields Vector Code Objects Raster Fields Vector
Representing Geography paper maps • Represent the world in two dimensions. - meaningful scale - true maps - static • Challenge: paper maps are useful metaphors for the contents of a geographic database, but lets think more on better ways of geographic representation. Presented by: Yahya El Hamad
Geographic Data Modeling What is a data model? ”Set of construct for describing and representing selected aspects of the real-world in a computer” Operactional GIS GIS Users
Real-world phenomena Reality Levels of data model abstraction Increase abstraction Human-Oriented Conceptual Model Partially model of objects Logical Model Implementation-orientad Computer-Oriented Phisical Model GIS application
Models • 1. CAD data Model • Systems real-world entities are represented symbolically as simple point, line, and polygon vectors. • Tipical use local drawing coordinate instead of real-world coordinate for representing objects. • Individual objects a represented with unique identify.
Models (2) • 2. Raster data Model • Uses an array of cells, or pixels to represent real-world objects • Store multiple atributes for each cell in a type of value attribute table. • The array itself is stored as a compressed file or as a record in a database.
Models (3) • 3.Vector data Model • Each object in the real-world is classified into a generic type: point, line, or polygon. • The coordinates that difine the geometry of each object may have 2, 3, or 4 dimensions.
Models (4) • 4. Object data Model • Real-world is modeled as a collection of geographic objects and its relationship. • Each entity in the real-world to be included in the GIS is an object. • Objects of same type are grouped toghether as object classes. • Each object class is stored in form of a database table.
GEOGRAPHIC DATABASE • Database • Geographical database • Advantage of database approach over file based systems:data sharing, security, data independence, data centralization
DATABASE MANAGEMENT SYSTEM(DBMS) • Database Management System: The software which were created for the purpose of creating database, manage database and control access to the data.
TYPES OF DBMS • RDBMS - Relational Database Management system eg DB2,Informix Dynamic server,Microsoft Access, sql server , oracle universal server etc. • ODBMS - Object oriented DBMS store objects persistently, eg objectstore, gemstone. • ORDBMS - Relational database adapted to handle objects. Example Oracle spatial option,Informix spatial datablade etc.
RDBMS • Problems: - Inability to store complete object directly in the database (object state and behaviour). - Couldn’t process rich data types such geographic objects (maps, shapes etc). - Poor perfomance (speed, complex structure).
The roles of GIS and DBMS Tasks - Data load - Editing - Mapping - Analysing Geographic Information system - Store - Indexing - Security - Query Object Relational Database management system Data
Storing data in DBMS • Layer is a organised collection of data on a particular theme such as roads in certain district in Mozambique, health posts in Gaza Province • Database design- Normalization rule(codd (1970)) Achieving simple tables structure which has disadvantage of poor perfomance and complex structure
SQL • Designed specifically for database • Two types of SQL statements: - Data Definition Language(DDL): used to create, alter and delete DB structure eg CREATE, ALTER,DROP - Data Manipulation Language (DML): Used to manipulate and retrieve data eg Select, update, delete
STRUCTURING GEOGRAPHIC INFORMATION • The aim is to improve efficiency(speed) of querying, analysing and mapping • Two types • Topologic - create a topology for vector datasets using either batch or interactive techniques.This can help to speed up in certain types of queries • Indexing database
Editing and Data Maintenance • Process of making changes to GDB by adding new objects or changing existing objects • In Gis we have tools for creating and editing Geographic objects geometric and attributes eg object coordinates can be added to GDB digitizing tables, survery etc In multiple user environment we have to take control of concurrent access problem.