1 Chapter 4 Object-Based Vector Data Model. Copyright © The McGraw Hill Companies, Inc. Permission...

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Chapter 4

Object-Based Vector Data Model

Copyright © The McGraw Hill Companies, Inc. Permission required for reproduction or display.

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CHAPTER 4: OBJECT–BASED VECTOR DATA

MODEL• 4.1 Object-based Data Model• 4.2 The Geodatabase Data Model• 4.3 Interface• 4.4 Topology Rules• 4.5 Advantages of the Geodatabase Data

Model


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CHAPTER 4: OBJECT–BASED VECTOR DATA

MODEL• Georelational data model is a split system• Object-based model stores spatial and attribute

data together rather than in a split system• Geometry (spatial data) stored as an attribute

along with other attributes• Eliminates use of split system and need for data

synchronization


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4.1 Object-Based Data Model

• Windows environment– Menus, icons, etc. instead of command line

• Model treats spatial data as objects• Object can represent a spatial feature

(road or lake)• Object can also represent a layer or the

coordinate system on which the layer is based


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Two Differences between Georelational and Object-

Based Models• 1. Stored in single system rather than

split


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Figure 4.1The object-based data model stores each land use polygon in a record. The Shape field stores the spatial data of land use polygons. Other fields store attribute data such as Landuse_ID and Category.

A Land Use Data Set


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A Major Breakthrough

• Using a single system is a major breakthrough because software developers must regularly deal with issues of data storage and data file structure.


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• Second difference between georelational and object-based data models– 2. Allows spatial feature (object) to be

associated with properties and methods• Property - an attribute or characteristic of an

object• Method - a specific action that can be

performed on an object


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4.1.1 Classes

• Set of objects with similar characteristics• Hierarchical structure• Feature class - data set that stores

features of the same geometry type in the data base.


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Figure 4.2The Geometry property of the Feature class can differentiate the object types of point, line, and polygon.


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4.1.2 Relationships Between Classes

• Following grouping objects into classes, we must then sort out the relationships between classes

• Association, aggregation, composition, type inheritance, and instantiation


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Association• How one class may be associated with

another• Streets and signal lights

– Street can be associated with one or more street lights but with only one coordinate system

Figure 4.3 Two examples of class associations


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Aggregation

• Whole-part relationships between classes• One class is a part of another class

– State is an aggregate of counties


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Composition

• Similar to aggregation• The composite owns the parts

– Highway may have from zero to any number of rest areas, and the lifetime of the rest areas are controlled by the existence of the highway


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Type Inheritance

• Defines relationship between a superclass and a subclass

• Subclass is a member of the superclass and inherits the properties and methods of the superclass

• Subclass can have additional properties not common to the rest of the superclass


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Figure 4.4 An example of type inheritance


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Instantiation

• Object of one class may be created from objects of another class

Figure 4.5 An example of instantiation


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Summary

• 4.1.2 Relationships Between Classes – Following grouping objects into classes, we

must then sort out the relationships between classes

– Association, aggregation, composition, type inheritance, and instantiation


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4.2 The Geodatabase Data Model

• Third major ESRI data model following coverage model of 1980s and shapefile model of 1990s

• ArcObjects - collection of thousands of objects, properties, and methods


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4.2.1 Geometric Representation of Spatial Feature

• Uses geometries of point, polyline, and polygon to represent vector-based spatial features– Point - simple feature with a point or multipoint

feature with a set of points

– Polyline - set of line segments which may or may not be connected

– Polygon - Made of one or many rings• Ring - set of connected, closed, nonintersecting line

segments• See Box 4.1, page 65 of text


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Figure 4.6The linear measures (M) of a route are stored with X- and Y-coordinates in a geodatabase. In this example, the M values are in miles, whereas the X- and Y-coordinates are in feet.


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Figure 4.7A route, shown here as a thicker, gray line, is built on a polyline with linear measures in a geodatabase.


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4.2.2 Data Structure• Geodtabase data model distinguishes

between feature classes and feature datasets

• Feature class– Stores spatial data of the same geometry type

• Feature dataset– Stores feature classes that share the same

coordinate system and area extent


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Feature Classes and Feature Datasets

• Feature class is like a shapefile in having simple features

• Feature dataset is similar to a coverage in having multiple datasets based on the same coordinate system and area extent


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Figure 4.8In a geodatabase, feature classes can be standalone feature classes or members of a feature dataset.


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4.3 Interface

• Set of externally visible operations of an object

• Objects have properties and methods, which are hidden

• To use them we work with an interface– Software issue. Users do not deal directly

with ArcObjects, which are accessed through menus, icons, and dialogs (GUI interface)


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Interface

• Encapsulation– Hide properties and methods of an object so that the

object can only be accessed through the predefined interface

• Inheritance– Stipulates that an object can inherit properties and

methods from the class to which it belongs

• Polymorphism– Stipulates that the same method, if applied to

different objects, can produce different effects


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4.4 Topology Rules

• Topology introduced in the coverage model but disappeared in the shapefiel model

• User-chosen relationship rules• See Table 4.1, page 69


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4.5 Advantages of the Geodatabase Data Model

• Take advantage of functionalities from object-oriented technology

• Convenient framework for storing and managing GIS data

• Eliminates complexity of coordinating between spatial and attribute components of database

• Custom objects may be developed

1 Chapter 4 Object-Based Vector Data Model. Copyright © The McGraw Hill Companies, Inc. Permission...

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