OBJECT-BASED VECTOR DATA MODEL
Advances
in computer technology have made it possible to store spatial data and attribute data together in a
single system:
A new data model called: Object-based Vector Data Model
OBJECT-BASED VECTOR DATA MODEL
A new data model requires new concepts, terms and data formats.
The Windows ® environment is a familiar application of object-oriented technology
Menus, icons, and dialog boxes instead of Command lines
Clicking on a menu shows allowable operations for the object
OBJECT-BASED VECTOR DATA MODEL
This new model uses objects to represent and organize spatial features.
The geometry (spatial data) of a feature is stored as an attribute along with other attributes in a record.
Objects can have properties and methods to capture characteristics of real-world objects.
OBJECT-BASED VECTOR DATA MODEL
Object-based data model treats spatial data as objects.
An object can represent any spatial feature: a road, a timber stand, or a hydrologic unit.
An object can also represent a road layer or the coordinate system that the road layer is based on.
OBJECT-BASED VECTOR DATA MODEL - TERMINOLOGY
Object
Anything that can be seen or touched. Objects have attributes and behaviors.
Properties
Attributes are characteristics that describe objects. They are also called data or properties.
Text.Font = Arial
Operations (actions)
An object’s behaviors are called operations that either the object can perform or can be performed on.
Table.AddRecord
OBJECT-BASED VECTOR DATA MODEL - TERMINOLOGY
Class
A pattern or blueprint for creating an object. It contains all the attributes and behaviors that describe the
object.
Instance
The object you create from a class is called an instance of a class.
Object/Instance vs. Class
Cookie – Cookie-cutter
Car – Blueprint for manufacturing the car
OBJECT-BASED VECTOR DATA MODEL - DIFFERENCES
Main differences between georelational data model and object-based data model:
Object-based data model stores spatial and attribute data of spatial features in a single system
Spatial data are stored in a field using a data type called “BLOB” (binary large object)
Using a single system resolves some of the issues of data storage and data file structure
Object-based data model allows a spatial feature to be associated with a set of properties and methods
A property describes an attribute or characteristic of an object
A method performs a specific action.
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OBJECT-BASED VECTOR DATA MODEL - CLASSES
A class is a set of objects with similar attributes.
A class defines the properties and methods of the objects that a members of that class.
For example: a class called feature can cover point, line and polygon feature objects.
The feature class defines the same properties and same methods for all three types of objects.
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OBJECT-BASED VECTOR DATA MODEL: GENERALIZATION AND SPECIALIZATION
Hierarchical structures required for management of properties and methods in GIS
Generalization and Specialization create the structure in which objects can be grouped into superclasses
and subclasses.
Generalization uses commonality among objects to group them into a higher-order class
For example: three features types of point, line and polygon are grouped into same class through
generalization
Specialization differentiates objects of a given class by an attribute or attributes.
For example: ‘geometry’ can differentiate the three feature types
OBJECT-BASED VECTOR DATA MODEL – RELATIONSHIPS BETWEEN CLASSES
Once objects are grouped into classes – relationships between classes can be defined
Association: how many instances of one class can be associated with another class through multiplicity
expressions.
Common multiplicity expressions are one (default) and one or more (1..*)
For example, an address is associated with one zip code, but can also be associated with one or more
apartments
OBJECT-BASED VECTOR DATA MODELS – MORE RELATIONSHIPS
Aggregation: describes the whole-part relationship between classes
For example – a census tract (whole) is an aggregate of a number of census blocks (parts)
Multiplicity of “whole” typically 1 and multiplicity of “part” is zero or any positive integer.
Composition: similar to aggregation
For example – a highway can have zero or any number of rest areas
OBJECT-BASED VECTOR DATA MODELS - RELATIONSHIPS
Type inheritance: defines relationship between a superclass and a subclass.
A subclass is a member of a superclass, inheriting properties and methods of the superclass.
Subclass can have additional properties and methods separating it from other members of the superclass.
For example: a residential area is a member of ‘built-up’ superclass but is separated from other members of
that class by lot size
OBJECT-BASED VECTOR DATA MODEL - RELATIONSHIPS
Instantiation – an object of a class can be created from an object of another class.
For example – a high-density residential area object can be created from a residential area object.
OBJECT-BASED VECTOR DATA MODELS - GEODATABASE
The Geodatabase model is part of a collection of thousands of objects, properties and methods designed by
ESRI similar to the ‘coverage’ model of ArcINFO and the ‘shapefile’ model of ArcView
OBJECT-BASED VECTOR DATA MODEL – GEOMETRIC REPRESENTATIONS
The Geodatabase Data Model uses the geometry of point, polyline and polygon to represent vector-based
spatial features.
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The Geodatabase is similar to the shapefile in terms of feature geometries.
The Geodatabase is also similar to the coverage in terms of simple features
The difference is in the composite features of regions and routes.
Regions: geometric characteristics of a region subclass are retained (although the region subclass is not
supported itself)
Multipart polygons may have spatially joint or disjoint components and can overlap
Routes: the route subclass from coverages is replaced by polylines with “m” (measure) values.
 No sections or arcs, the geodatabase uses m values for linear measures along a route and stores the m value
directly with the x- and y-coordinates in the geometry field.
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OBJECT-BASED VECTOR DATA MODEL – DATA STRUCTURE
The Geodatabase Data Model distinguishes between “feature class” and “feature dataset”
Feature class stores spatial data of the same geometry type
Like a shapefile, it has simple features
Feature dataset stores feature classes that share the same coordinate system and area extent.
Like a coverage, it has multiple data sets
Unlike a coverage, it can contain different theme layers (coverages only contain different parts of a single
layer)
A feature class does not have to be included in a feature dataset – a ‘stand alone’ feature class
Feature classes included in a feature dataset ‘share’ topological relationships with each other
OBJECT-BASED VECTOR DATA MODEL - INTERFACE
An interface is a set of externally visible operations of an object
Defines the communication boundary between two entities, such as a piece of software, a hardware device,
or a user
Properties and methods of the object may be hidden so an ‘interface’ is necessary to work with the
properties and methods – not necessarily with the object itself.
There are three basic principles underlying the concept of interface:
Encapsulation
Inheritance
Polymorphism
In ArcGIS, both ArcCatalog and ArcMap have embedded Visual Basic Editor, which runs macros using
ArcObjects (the interface).
Using the macros, you can create customized commands, menus, and tools
Most ArcGIS users, however, do not have to deal with ArcObjects!
OBJECT-BASED VECTOR DATA MODEL – ENCAPSULATION
Encapsulation is the mechanism used to hide properties and methods of an object so the object can only be
accessed through predefined interfaces
In object-based programming, information hiding reduces software development risk.
OBJECT-BASED VECTOR DATA MODELS - INHERITANCE
Inheritance means an object inherits properties and methods from the class to which the object belongs.
A subclass also inherits the class properties and methods
For example: residential area is a superclass while low-density and high-density residential areas are
subclasses.
The principle of inheritance directly links the structural aspects and the behaviorial aspects of objects
This saves time since the same properties and methods will not have to defined twice – once for low-density
and once for high-density areas
Inheritance is a way to form new classes using classes that have already been defined.
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The new classes, known as derived classes take over attributes and behavior of the pre-existing classes
OBJECT-BASED VECTOR DATA MODEL - POLYMORPHISM
Polymorphism means the same method can produce different effects, depending on the kind of object to
which the method is being applied.
For example, if AddLayer is a method for adding data to a view, it can be set to add a coverage given a
coverage data source or a shapefile if given a shapefile data source.
Similar to inheritance polymorphism save repeating same methods for different classes.
OBJECT-BASED VECTOR DATA MODELS – TOPOLOGY RULES
The
concept of topology was introduced with the coverage data model, but disappeared with the shapefile.
coverage model enforces three topological relationships with its data structures:
Connectivity
Area definition
Contiguity
The geodatabase model defines topology as relationship rules, allowing the user to choose the ‘rules’ to be
implemented in a feature dataset
.
OBJECT-BASED VECTOR DATA MODEL – TOPOLOGY RULES
The
Feature Type
Polygon
Line
Point
Rule
Must not overlap, must not have gaps with, must be covered by
feature class of, must cover each other, must be covered by,
boundary must be covered by, area boundary must be
covered by boundary of, and contains point
Must not overlap, must not intersect, must not have dangles,
must not have pseudo-nodes, must not intersect or touch
interior, must not overlap with, must be covered by feature
class of, must be covered by boundary of , end point must be
covered by, must not self overlap, must not self intersect, and
must be single part
Must be covered by boundary of, must be properly inside polygon, must be
covered by end point of, and must be covered by line.
OBJECT-BASED VECTOR DATA MODELS – REAL WORLD TOPOLOGY RULES
Here
are a few real-world applications of topology rules:
must not overlap
County must not have gaps
County boundary must not have dangles (i.e., must be closed)
Contour lines must not intersect
Label points must be properly inside polygons
Census tracts and counties must cover each other
Counties
OBJECT-BASED VECTOR DATA MODEL - ADVANTAGES
The main advantages of the Geodatabase topology are:
The topology structure is not explicitly stored.
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The user decides which topology rules are important for his dataset.
The user has to specify the conditions of the dataset.
The user decides what should happen with spatial objects if they do not meet all the conditions.
Topological errors are not solved automatically, but the user has to solve errors manually.
The validation process is relatively short.
It should be noted that with this validation, only the errors are marked, not solved.
After editing some objects in the geodatabase, not the entire dataset has to be validated again.
The edited area is marked as a dirty area and only the dirty areas have to be validated again.
The Geodatabase topology is easy to use.
You don’t need to have special knowledge about topology to create a set of logical conditions for the data.
The wizard to implement the topological rules is very useful and straightforward.
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OBJECT-BASED VECTOR DATA MODELS - DISADVANTAGES
The main disadvantages of the Geodatabase topology are:
There is a lot of redundancy.
all edges shared between neighbors are stored twice.
After validation of the topology, the dataset still does not satisfy the conditions.
Only the errors are marked. So, at least one extra step is necessary.
If there appears an object that does not meet the conditions specified, you have to be aware that this does
not have to mean that there’s a topological error.
It is also possible that the defined condition is not defined well.
The toolkit in ArcGIS to solve the topological errors is not very extensive.
Some errors can be solved, but for many errors a more advanced toolkit is necessary.
Not all the possible topological rules can be implemented.
For instance the rule “POINT_OBJECT must not be in POLYGON” cannot be implemented.
OBJECT-BASED VECTOR DATA MODEL – VISUAL BASIC
The "Basic" part refers to the BASIC (Beginners All-Purpose Symbolic Instruction Code) language.
Visual Basic evolved from the original BASIC
language
Contains several hundred statements, functions, and
keywords,
Relate directly to the Windows GUI
The "Visual" part refers to the method used to create the graphical user interface (GUI).
Pre-built objects are added to the screen
Replacing all the lines of code needed to describe
the appearance and location of interface elements.
OBJECT-BASED VECTOR DATA MODEL – VISUAL BASIC APPLICATIONS
VBA is the application edition of Microsoft's Visual Basic programming language.
VBA can normally only run code from within a host application rather than as a standalone application.
OBJECT-BASED VECTOR DATA MODELS - VBA
VBA is the application edition of Microsoft's Visual Basic programming language.
VBA can normally only run code from within a host application rather than as a standalone application.
It can however be used to control one application from another using OLE (object linking and embedding)
Automation (for example, automatically creating a Word report from Excel data, or embed objects from the
ArcObjects library into a PowerPoint application).
OBJECT-BASED VECTOR DATA MODEL APPLICATIONS USING THE VBA ENVIRONMENT
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MS
Office: Word, Excel, PowerPoint, Access etc.
Visio
AutoCAD
ArcGIS
Coral
Draw
ENVIRONMENT IN ARCGIS
Customize Dialog Box
Visual Basic Editor
VBA
OBJECT-BASED VECTOR DATA MODEL – ARCGIS RESOURCES
http://www.esri.com/software/arcgis/about/literature.html
http://www.esri.com/software/arcgis/about/arcobjects.html
http://edn.esri.com/index.cfm?fa=docLibrary.gateway
OBJECT-BASED VECTOR DATA MODEL – ARC/OBJECTS
ArcObjects are a set of computer objects specially designed for programming with ArcGIS applications
ArcObjects include data frames, layers, features, tables, catographic symbols, points, lines, polygons,
records, fields, and so forth
OBJECT-BASED VECTOR DATA MODEL – ARC/OBJECTS
ArcObjects are platform independent software components, written in C++, that provide services to support
ArcGIS applications either on a desktop or on a Web server and typical client/server deployments.
Supporting a number of unique ArcGIS products with specialized applications, ArcObjects is built for a
multi-use scenario.
OBJECT-BASED VECTOR DATA MODELS - SUMMARY
The geodatabase data model eliminates the complexity of coordinating between the spatial and attribute
components – reducing processing overhead.
By storing spatial and attribute data in tables, geodatabases operate fully within a relational database
environment and bring GIS together with other information technology applications.
The geodatabase data model provides the foundation for the development of different geodatabases for
different disciplines.
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