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What exactly is a geodatabase?

A geodatabase is a relational database that stores geographic data.

At its most basic level, the geodatabase is a container for storing spatial and attribute data

and the relationships that exist among them. In a geodatabase, which is a vector  dataformat, features and their associated attributes can be structured to work together as an

integrated system using rules, relationships, and topological associations. In other words,

the geodatabase allows you to model the real world as simply or complexly as your needs

dictate.Geodatabases are created, edited, and managed using the standard menus and tools in

ArcCatalog™, ArcMap™, and ArcToolbox™.

RDBMS

Relational database management system. A type of database in whichthe data is organized across several tables. Tables are associated with

each other through common fields. Data items can be recombinedfrom different files. In contrast to other database structures, an RDBMSrequires few assumptions about how data is related or how it will beextracted from the database.

Concept

Advantages of the geodatabase

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The quest of every GIS user is to create a more accurate representation of the real world.

For many applications, the geodatabase can take you closer to that goal. The essentialidea behind the geodatabase is to make working with GIS data and software more

intuitive. The graphic below illustrates some of the reasons why your organization may

choose to use the geodatabase.All of the functionality shown below can be implemented using standard ArcGIS

commands—no programming is required.

• Centralized GISdata management 

• Continuous sets

of features

• Advanced feature

geometry

• COGO support

• Feature subtypes

•

Flexible, rule- based topology

• More accuratedata editing

• Feature-linked

annotation• Custom features

• Geometric

networks

• Linear referencing

• Versioning

• Disconnected

editing

• UML and CASE

tool supportThis is an interactive graphic. Pause your 

mouse pointer over each bullet item to seethe related information.

Concept

Types of geodatabases

There are two types of geodatabases: personal and multiuser.

A personal geodatabase has the .mdb file extension (a format used by Microsoft Access)and can be read by multiple people at the same time, but edited by only one person at a

time. A personal geodatabase has a maximum size of 2 gigabytes (GB) and stores vector 

data.

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A personal geodatabase

requires no additionalsoftware and handles

small to moderately

sized datasets.

Multiuser geodatabases (also called ArcSDE® or enterprise geodatabases) are suitable

for large workgroups and enterprise GIS implementations. They can be read and edited by multiple users at the same time, and they can store both vector and raster data.Multiuser geodatabases are comprised of ArcSDE software and a DBMS (database

management system) such as IBM, DB2, Informix, Oracle, or SQL Server. The spatial

data is stored in the DBMS and ArcSDE allows you to view and work with the data fromyour GIS software applications. If you are working with an Oracle 8i or SQL Server 

database management system, you can directly connect to your data from ArcCatalog or 

ArcMap. For other database management systems, ArcSDE manages the connection.

A multiuser geodatabase stores large spatial datasets using

ArcSDE and a commercial database management system.

Some database software allows for a direct connection between the ArcGIS application and the database.

If a project becomes too large for a personal geodatabase, you can easily move the data

stored in a personal geodatabase to a multiuser geodatabase using ArcCatalog.

Table 1. Comparison of multiuser and personalgeodatabases.

  DBMSClient /

Server

Long

Transactions*

Disconnected

EditingEditors Raster Size

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Multiuser

IBM DB2,

Informix,

Microsoft SQL

Server, Oracle

Yes Yes Yes

1 or more

at the same

time

Yes Unlimited

Personal Microsoft Access No No No 1 at a time NoUp to 2

GB*Database transactions spanning multiple edit sessions 

Concept

Scalable functionality

In many organizations, geographic data creation, integration, management, and analysis

is handled by more than one person. For example, the person (or group) responsible for data analysis is usually not the same person who designs the GIS database.

This distribution of functions is reflected in the scalable functionality of the geodatabase.

An ArcView® license provides core geodatabase functionality such as creating ageodatabase, populating it with data, and editing simple features. ArcEditor™ and

ArcInfo™ licenses provide more advanced functionality, including creating and editing

geometric networks and relationship classes.This course primarily covers core geodatabase functionality. However, some topics

discuss more advanced functionality. In the introduction to each of these topics, you will

find a table summarizing the differences in functionality for ArcView, ArcEditor, and

ArcInfo.For a more detailed comparison of the functionality available in the three licenses, refer 

to the ArcGIS Desktop Help topic, The ArcGIS Desktop and the geodatabase. (To see

this topic, in ArcCatalog or ArcMap, click the Help menu and choose ArcGIS Desktop

Help. Click the Contents tab, then double-click Geodatabases and Getting started withgeodatabases).

ArcGIS licenses and this course

If you are using an ArcView license, you will be able to do two-thirds of the exercises in

this course. If you are using an ArcEditor or ArcInfo license, you can complete all courseexercises.

If you're not sure which ArcGIS license you're using, open ArcCatalog or ArcMap. The

licensed product name displays in the title bar of the window.

If you would like to complete all of the course exercises, check with your GIS manager or system administrator to see if your organization has a floating license for ArcEditor or 

ArcInfo that you can use. If a higher level license is available, you can use the ArcGIS

Desktop Administrator to switch to a different license.

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For more information about the ArcGIS Desktop Administrator and licenses, refer to the

ArcGIS Desktop Help topic, The ArcGIS License Manager , found in the Contents tab

under Getting started .

Concept

Feature classes

A feature class is a collection of geographic features with the same geometry type, thesame attributes, and the same spatial reference. Feature classes can also store annotation,

which you'll learn about later in this course.

For example, all the telephone poles in London could be represented as a feature class ina geodatabase. Streets would be another feature class. London districts would be a third

feature class. Each of these feature classes would be represented as a shapefile in

ArcView 3.x or as a feature class in an ArcInfo coverage.Feature classes may exist independently in a geodatabase as stand-alone feature classes or 

you can group them into feature datasets.

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spatial reference

 The coordinate system used to store a spatial dataset. For featureclasses and feature datasets within a geodatabase, the spatialreference also includes the spatial domain.

annotation

1. In cartography, text or graphics on a map that provide information

beyond that given by labels. For example, annotation may describefacts or events associated with a place; or indicate directions of windsand currents; or show elevations or depths at various locations.

2. In ArcGIS, text or graphics on a map that can be individuallyselected, positioned, and modified by the software user. The text mayrepresent either feature attributes or supplementary information.Annotation may be manually entered by the user or generated from

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labels. Annotation is stored either in a map document as text orgraphic elements, or in a geodatabase as a feature class.

See also: label; annotation class; graphic text; text formatting tag;feature-linked annotation

Concept

Feature datasets

A feature dataset is composed of feature classes that have been grouped together so they

can participate in topological relationships with each other. All the feature classes in a

feature dataset must share the same spatial reference; that is, they must have the samecoordinate system and their features must fall within a common geographic area.

Feature classes that participate in a geometric network must be placed in a feature

dataset. For example, a feature dataset named PublicWater might contain a line feature

class representing water lines, a point feature class representing junctions, and another  point feature class representing pipe fittings. All features in the feature dataset are

dependent on each other—water lines can't connect without a junction, and junctions and

fittings can't exist unless they are attached to a water line.

In the CityWater geodatabase, three pointfeature classes and one line

feature class were grouped

into the PublicWater featuredataset to create a

geometric network called

WaterNet.

Within a feature dataset, depending on the relative locations of features in one feature

class to features in other feature classes, edits you make to one feature class may result in

edits being made automatically to some or all of the other feature classes in the feature

dataset. For example, in the PublicWater feature dataset, if you move a water line feature,its junctions and fittings will move as well because features in the three feature classes

share coincident coordinates.Concept

Geodatabase tables

The geodatabase is a relational database. Therefore, a geodatabase is composed entirely

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of tables, but most of these tables are hidden from you when you work with a

geodatabase in ArcCatalog or ArcMap. Your interaction with the tables is managed by

the software. There are only two types of tables that you interact with directly: featureclass tables and nonspatial attribute tables.

Both types of tables are created and managed in ArcCatalog and edited in ArcMap. Both

display in the traditional row-and-column format. The difference is that feature classtables have one or more columns that store feature geometry.

 Nonspatial tables contain only attribute data (no feature geometry) and display in

ArcCatalog with the table icon . They can exist in a geodatabase as stand-alone tables, or they can be related to other tables or feature classes. You'll learn more about tables and

relationships in Module 4 of this course.

The cfcc_desc table in the SantaBarbara geodatabase containsattribute data for the Roads feature class (stored inside the

Roads feature dataset).

Building on the basics

The basic building blocks of a geodatabase are feature classes, feature datasets, and

tables. Using these, you can build more complex objects in your geodatabase. You can

create associations based on spatial relationships (topology) or on attributes (relationship

classes). You can also build geometric networks to model the flow of resources.

Objects and the geodatabase

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In addition to feature classes, feature datasets,and tables, a geodatabase can contain topology

objects, geometric networks, and relationship

classes.

Concept

Topology

In a GIS, spatial relationships among features are defined by topology.

In a geodatabase, you can choose whether to create topology for features. If your analysis

will depend on accurately modeling spatial relationships, creating topology is essential.For example, if you are modeling a wastewater network, all of the features must be

connected or the results of any analysis based on your model will be flawed. Or, suppose

that your analysis involves calculating the total area for different types of land cover.Gaps between land cover polygons will result in inaccurate totals.

There are three types of topology available in the geodatabase: geodatabase topology,

map topology, and the topology created for a geometric network. Each type of topology

is created from feature classes that are stored within a feature dataset. A feature class can participate in only one topology at a time.

Geodatabase topology

A geodatabase topology is, to put it simply, a set of rules and properties that defines the

spatial relationships you want to model and preserve in your data. ArcGIS includes over 20 topology rules from which you can choose.

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Each topology rule applies to a particular 

type of spatial relationship. For example,

Must Not Have Gaps is a topology rulethat defines a spatial relationship between

 polygon boundaries.To create and edit a geodatabase topology, you need an ArcEditor or ArcInfo license.You can learn how to implement geodatabase topology in the Creating and Editing 

Objects and the geodatabase

As you work with a geodatabase and read the documentation for it, you'll encounter terms

like "object" and "object class." If you're familiar with object-oriented programming,

you'll know what these terms mean. If you're not, a little explanation is in order.As you might guess, object-oriented programming is all about objects. An object is a

structure that represents a single entity, including both its information and its behavior.

Another way to put it is that an object is made up of data and code which is treated as asingle "thing" by the software.

An object can be a house, a lake, a customer, or a maintenance record. In the

geodatabase, an object is stored as a row in a table (called an object class). An object hasa set of attributes. Attributes describe an object, and could be its name, a measure, or an

identifier to another object. Objects can also have behavior. For example, an object can

 participate in a relationship or be constrained by limits on attribute values.Object classes that store geometry are called feature classes. Object classes that store

relationships are called relationship classes.

Geodatabase Topology course.

Map topologyA map topology creates temporary topological relationships among features in one or 

more feature classes in a geodatabase during an edit session in ArcMap. The ArcMaptopology editing tools are used to maintain these relationships while editing.

A map topology can be created and edited using ArcView, ArcEditor, or ArcInfo. You'll

learn how to work with map topology in Module 3 of this course.

Geometric network topology

In a geometric network, topological relationships are stored in the geodatabase and

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maintained automatically by the software. In geodatabase and map topologies, spatial

relationships are not stored—they are "discovered" as needed (for example, when a

feature class is edited in ArcMap). In a geometric network topology, topologicalrelationships are established between point and line features only.

To create and edit a geometric network, you need an ArcEditor or ArcInfo license. You

will work with geometric networks in Module 5 of this course.

Concept

Geometric networks

In the real world, examples of networks abound: streams joining together to form larger streams, pipes carrying water to homes and businesses throughout a city, and power lines

carrying electricity.

In a geodatabase, you can model each of these real-world networks with a geometric

network. Starting with simple point and line feature classes, you use ArcCatalog to createa geometric network that will enable you to answer questions such as: Which streams will

 be affected by a proposed dam? Which areas will be affected by a water main repair?What is the quickest route between two points in the network?

A water transmission network is modeled in the

geodatabase using a geometric network.

When you build a geometric network, there are a number of options you can choose from

to make your network model more realistic. For example, you can:

• set the direction that resources will flow through the network 

• assign weights that control the speed of flow through different parts of thenetwork 

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• specify rules that control how each element in the network connects to the others

You'll learn about these and other options, as well as the different analyses you can

 perform with a geometric network, in Module 5 of this course.

Concept

Relationship classes

In a geodatabase, relationship classes provide a way to model real-world relationships

that exist between objects such as parcels and buildings or streams and water sampledata. By using relationship classes, you can make your GIS database more accurately

reflect the real world and facilitate data maintenance.

The relationships stored in a relationship class can

 be between two feature classes (such as buildings

and parcels, top) or between a feature class and anonspatial attribute table (such as streams and water 

quality sampling data, bottom).

When you create a relationship class, you define the properties of the relationships.Recall in the first exercise of this module, when you deleted some lots from the

subdivision, the homes located on the lots were also deleted automatically. This behavior 

 between the related objects was controlled by a relationship property.

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You can also create rules to maintain valid relationships. For example, you could set up

rules that control how many buildings are allowed on parcels in different zoning areas.

Relationship classes are created in ArcCatalog. You will work with relationship classes inModule 4 of this course.

Lesson summary

This module introduced you to some basic features of the geodatabase. A geodatabase is

a relational database that stores geographic data. There are two types of geodatabases:

 personal and multiuser.A personal geodatabase can be read by multiple people, but edited by only one person at

a time. Multiuser geodatabases can be read and edited by multiple users at the same time.

In this course, you will work with personal geodatabases in the exercises.

Geodatabases offer many advantages for GIS users. The range of functionality availableis extensive and includes centralized data storage, support for advanced feature geometry,

and more accurate data entry and editing through the use of subtypes, attribute domains,

and validation rules.

Feature classes, feature datasets, and nonspatial attribute tables are the primarycomponents of a geodatabase. Feature classes are collections of features that share the

same geometry type, attributes, and spatial reference. Feature classes can be grouped intofeature datasets. All feature classes in a feature dataset must share the same spatial

reference.

There are three different types of topology in the geodatabase: geodatabase topology,

map topology, and the topology created for a geometric network. Each topology type iscreated from feature classes within a feature dataset. The nature of your analysis will

determine what type of topology, if any, you will need to create.

 Now that you've had an introduction to geodatabases, you're ready to start exploring themin more depth. In the next module, you learn how to create a personal geodatabase using

ArcCatalog.