Data Models and Database Systems for GIS Lecture PDF

Data Models and Database Systems for GIS How to represent features on the Earth Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  GIS and Map The story of GIS b...

Data Models and Database Systems for GIS How to represent features on the Earth Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  GIS and Map The story of GIS begun in the world of maps A map is a simplified visual representation of what we see/features from the real world Maps can model the world in more than one way  Map Types:-  A Topographic map shows the physical surface features (roads, rivers, buildings)  A Contour map shows lines which connect point locations at which a certain property has the same value (height above sea level, isobars, air pressure)  A Choropleth map shows areas characterized by some general common feature (political maps, agric crop types, soil types). Map Features:- Four main symbols depict features on a map Point (towns, stations, buildings, etc) Line (roads, rivers, railways, etc. ) Polygon shape or Area (lakes, boundaries,states, etc.) Textual annotation (names of locations, buildings, oceans)  Everything is a model in a GIS All GIS are computer representations of some aspect of the real world These representations are simplifications Real world is simplified into a number of data sets and these data sets may themselves be simplified So, in a GIS, the real world is conceived as a ‘synthesis of data’, or a model. GIS models include both data and ideas about how these data interact Here we’ll consider GIS models for data – most commonly known as data models Introduction to Geographic Information Systems School of Geography, University of Nottingham  Spatial Data Model: Definition Spatial data modelling is the analysis of spatial objects that are used in a spatial analyses and the identification of the relationships among these objects. Introduction to Geographic Information Systems School of Geography, University of Nottingham  Two main types of GIS Data Model Vector : point, lines, polygons Spaghetti vector-limited due to no topology Arc node vectors-also encode topological relationships Raster : highly simplistic but suffers from issues of:- Resolution Large file sizes Data redundancy Introduction to Geographic Information Systems School of Geography, University of Nottingham  Spatial Data Modelling: Process 1. 2. Identify features of interest in the Choose how to represent them context of the study conceptually 4. 3. Create structure for data model Turn conceptual representation so that it can be physically into a data model coded for use in computer Introduction to Geographic Information Systems School of Geography, University of Nottingham Conceptualising GIS Data Models 4 basic conceptualisations for spatial entities: 1. Points 2. Lines 3. Polygons (areas) 4. Grids (raster) Points, lines, polygons are collectively known as VECTOR data Grid data is known as RASTER Introduction to Geographic Information Systems School of Geography, University of Nottingham  Which Conceptualisation? Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Issues affecting conceptualisation: Scale; Dynamism; Boundaries and End users Scale: at small scales cities are points, at larger scales, they are areas (polygons): Introduction to Geographic Information Systems School of Geography, University of Nottingham  Dynamism: Is a river a line or an area? In flood – area Not in flood – line Settlement- point Settlement- Polygon? Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Boundaries: Is a spatial entity discrete or are its boundaries fuzzy? Think about a soil map… Temperature- fuzzy? Humidity- fuzzy? State – discrete? Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham Modelling and conceptualisation: boundaries Impact of buildings on people’s perceptions of wild land/life interaction: distance from building and perception of impact. Introduction to Geographic Information Systems School of Geography, University of Nottingham  End-user Different GIS users ‘see’ the world in different ways To a driver, a road may best be represented as a line To a highways engineer it may best be represented as an area/polygon Or… Introduction to Geographic Information Systems School of Geography, University of Nottingham Vector spaghetti model: Outlines of feature are drawn in full irrespective of other features around them Polygons that adjoin will have double boundaries No way of knowing about lines that cross one another Spaghetti data model contains no way of encoding the spatial relationship between individual entities Introduction to Geographic Information Systems School of Geography, University of Nottingham  Database When the volume of data becomes very large, or you need to allow many people to access the data at the same time , it becomes preferable to store the information in a database A database is a tool capable of storing large amounts of complex information in structured ways Information in a database is organized into individual records that can be referenced, stored, indexed, linked and queried Introduction to Geographic Information Systems School of Geography, University of Nottingham Database Databases are, in principle more robust , secure and scalable than storing information in flat files In large industrial software systems there will usually be multiple database operating together in a database management system (DBMS) Database technology plays a big part in GIS Introduction to Geographic Information Systems School of Geography, University of Nottingham  GIS Database types Relational Database Management System (RDBMS) Object-oriented database management system (OODBMS) Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham RDBMS Introduction to Geographic Information Systems School of Geography, University of Nottingham  RDBMS A structured query language (SQL) use by almost all RDBMS allows interrogation of the data For the computer to understand the data correctly there can be no duplicate records in a table In a relational Database each records in a table must be uniquely identified for a queries to be meaningful Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  OODBMS OODBMS organised information very differently that RDBMS Rather than spreading the information about an entity across a range of linked tables, it is stored together in discrete lumps called objects Each object is defined within a hierarchy of object classes so that it inherits properties from a parent class Introduction to Geographic Information Systems School of Geography, University of Nottingham  OODBMS: Additional attributes can be defined within the object and they are said to exhibit encapsulation because they can self-describe their own particular set of properties, and therefore the way in which they can be queried. Object-oriented database can make it easier to model real- world phenomena in a logical form Introduction to Geographic Information Systems School of Geography, University of Nottingham  Summary There are two data models for GIS, Vector and Raster data models Conceptualising data models for use in GIS is far from simple Need to think about: Who will use the data and their needs… Is the entity being conceptualised dynamic? At what scale are the entities to be analysed? How should boundaries best be represented? Once you’ve determined the answer to these questions, you can move on to choosing a data model and data structure for use! GIS can work with two types of database systems, RDBMS and OODBMS Introduction to Geographic Information Systems School of Geography, University of Nottingham