Post on 22-Dec-2015
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Chapter 4
Object-Based Vector Data Model
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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