Migrating Coverages to Geodatabases

by Colin Childs, Esri Education Services

Editor's note:This article helps users decide if moving existing ArcInfo coverages into a geodatabase will be beneficial and provides an overview of the process. Two companion tutorials with a sample dataset are available. See the links at the end of this article.

In ArcGIS, coverages are fully supported and can be displayed, queried, analyzed, and edited using ArcMap, ArcCatalog, and ArcToolbox. However, geodatabases improve the administration, access, management, and integration of the spatial data. Importing coverages into a geodatabase means converting data from one format. This article will discuss not only the benefits of using a geodatabase but how both the spatial and attribute data types will translate in the move from coverage to geodatabase. Choose to migrate coverages into geodatabases when the benefits of integrating feature behavior and storing all data in a database outweigh the effort of conversion. Think of a geodatabase as a next-generation coverage.

Geodatabase Benefits

Geodatabases come in two varieties--personal and multiuser. Personal database support is built into ArcGIS implemented with Microsoft Jet and is suitable for project-level GIS. Multiuser databases are deployed using ArcSDE and require a DBMS such as IBM DB2, Informix, Oracle, or Microsoft SQL Server. Storing spatial and attribute data directly in a commercial database gives a geodatabase capabilities that are not available, or are more difficult to achieve, with other formats. Some of these benefits are listed below.

• A uniform repository for geographic data. All geographic data is centrally stored and managed in one database.
• Data entry and editing is more efficient. The use of subtypes, domains, and validation rules helps maintain database integrity and reduces database maintenance. For many users, this reason alone justifies adopting the geodatabase model.
• Sets of features are continuous. Geodatabases can accommodate very large sets of features without tiles or other spatial partitions.
• Multiuser editing. ArcSDE geodatabases use a data management framework called versioning that lets multiple users access and edit features simultaneously and reconciles any conflicts.
• Feature-linked annotation. Geodatabase annotation can be linked to the feature that it describes. When the linked feature is moved or deleted, the related label is moved or deleted.
• Users work with more intuitive data objects. A properly designed geodatabase contains data objects that correspond to the user's model of data. Instead of generic points, lines, and areas, users work with objects of interest such as transformers, roads, and lakes.
• Using a geodatabase can be simple and straightforward. Geodatabases can be created, accessed, and managed through the standard menus and tools in ArcCatalog, ArcToolbox, and ArcMap. However, the geodatabase model supports intelligent features, rules, and relationships that advanced users can employ in complex GIS applications. Whether the use of a geodatabase will be simple or complex, moving existing coverages to a geodatabase requires an understanding of both coverage and geodatabase formats.

Coverages and Workspaces

Typically ArcInfo coverages are stored in workspaces that implement the georelational model by storing topology and linking attributes to features. An ArcInfo workspace is a special type of folder that stores attributes for data in INFO tables. All tables are managed through an INFO subfolder. Most types of coverage features are explicitly associated with other features in the coverage. For example, the topological association between a node feature and an arc feature is defined by the ARC#, FNODE#, and TNODE# items.

Use the topology tools while editing coverages in ArcMap. For example, reshaping the boundary of a polygon with the Shared Edit tool will automatically update the shape of the arc and maintain the association between features. Similarly, use ArcCatalog to create, move, and delete items in an ArcInfo workspace to maintain data integrity. Never use Windows Explorer or My Computer to manage coverages, grids, or TINs because either application will break the synchronicity between coverages and the INFO subfolder, and the data will be corrupted.

Feature classes in coverages are homogeneous collections of features. Primary coverages contain points (label points), lines (arcs), polygons, and nodes that have topological associations. For example, arcs form the perimeter of polygons, and nodes form the endpoints of arcs. Secondary coverage features consist of tics, links, sections, and annotation. Coverages also contain composite features such as routes (collections of arcs associated with a measurement system) and regions (collections of polygons that can be adjacent, disjoint, or overlapping).

The Geodatabase

Basically, a geodatabase is a storage mechanism for spatial and attribute data that contains specific storage structures for features, collections of features, attributes, relationships between attributes, and relationships between features. All geodatabases, whether personal or ArcSDE, can store tables, feature classes, feature datasets, and functionality such as rules, relationships, and geometric networks.

Feature classes in geodatabases are collections of features with the same type of geometry. Each feature class uses the same attribute columns to describe the features, and all features share a common spatial reference and common behaviors. Features belonging to the same feature class render in the same way, respond in the same way to queries, and have the same attribute validation rules.

If several feature classes share some spatial coincidence and a common spatial reference, they can be inserted into a feature dataset. Although feature datasets are an attractive organizational structure, there is a cost associated with using them. Updates to a feature class in a feature dataset can potentially ripple to other feature classes within the feature dataset that participate in topological relationships.

If feature classes have no topological association with the other feature classes, don't include them in the same feature dataset. Use feature datasets to group classes that have topological relationships with each other (e.g., geometric networks and planar topology). Don't overload feature datasets with unnecessary feature classes--stand-alone feature classes at the database level are perfectly acceptable.

Item Types vs. Field Types

Conceptually, a geodatabase stores the same data as a coverage with feature classes, feature class collections, topological associations, spatial references, and attribute tables. Feature class and attribute Item types in a coverage are mapped to geometry field types in a geodatabase. Note that more than one feature class type in coverage will map to the same geometry type in a geodatabase. For example, points, tics, and nodes all map to the point geometry type. The relationship between feature class types in coverages and geometry type mapping in a geodatabase is shown in the chart to the left. Annotation in the geodatabase is not a geometry type, but is implemented as a feature type.

INFO table Items are mapped based on a combination of INFO type and Item width. For example, an Item of type I (Integer) can be mapped as a short integer, long integer, or double, depending on its width. The chart on the right lists INFO Item types and summarizes how each Item type in a coverage relates to a geodatabase field type in a geodatabase.

When a coverage feature class is imported using ArcCatalog or ArcToolbox, a geodatabase feature class is created to store the features. This feature class stores both the geometry and attributes from the input data. The feature class is automatically registered with the geodatabase system tables so that it can participate in relationships and geometric networks, have validation rules, and so on. Similarly, when an INFO table is imported, a table is created in the geodatabase and automatically registered with the geodatabase system tables.

Converting Coverages to Geodatabase Feature Datasets and Feature Classes

The tools to migrate coverages to a geodatabase are available in ArcView 8.1, ArcEditor 8.1, and ArcInfo 8.1 in the ArcCatalog and ArcToolbox applications. Converting data from a coverage to geodatabase is a relatively simple procedure in most cases. However, migrating composite coverage features and annotation are special cases that are not covered here. Before beginning the process of converting a coverage, ask the following questions about the coverage:

• What kinds of feature classes are present in the coverage?
• How is the data organized? Is a feature dataset appropriate, or should everything be organized only to the feature class level?
• Are there special relationships between objects that must be maintained?
• What is the coordinate system?
• Should data modification rules be added?

The following sections give an overview of the process of migrating coverages using two methods--creating a personal geodatabase in ArcCatalog that can be populated with data from coverages and using the Import Wizard, available from both ArcToolbox and ArcCatalog.

Creating a Personal Geodatabase in ArcCatalog

1. Creating a container (GDB file) that will hold the feature datasets, feature classes, and tables associated with the coverage is the first step. After starting ArcCatalog, right-click on a drive or directory and choose New > ArcInfo Workspace to create a new workspace. Right-click on the new workspace and choose New > Personal Geodatabase and create a personal geodatabase called MyGDB. This new geodatabase will be populated with the data from a coverage.
2. Right-click on the coverage that will be converted, and choose Properties from the context menu. In the Properties dialog, investigate the coverage feature classes and determine if they are topologically related and should be maintained in a feature dataset.
3. Right-click on MyGDB and choose Import > Coverage to Geodatabase. In the resulting dialog box, select the coverage feature class to convert and specify an output feature class and possibly a feature dataset for the converted data.
4. In the Output Settings dialog box, choose the coordinate system, grid size, and field mapping. Additional coverage feature classes can be included in this conversion process by using the batch option.
5. The resulting feature classes in the geodatabase will be correctly associated and maintain topological relationships.

Importing a Coverage Using a Wizard

Wizards are especially useful for new users. Successive panels prompt, explain, and use smart defaults helping users quickly and correctly complete tasks. This example uses the workspace and geodatabase created in the previous tutorial.

1. To import a coverage to the geodatabase using the Import Wizard, start ArcCatalog and right-click on MyGDB in the workspace previously created and choose Import > Coverage to Geodatabase Wizard from the context menu.
2. Specify the input coverage and choose the feature classes to convert in the Coverage to Geodatabase Wizard. Click on Next.
3. In the next pane, specify a new feature dataset to create or pick an existing feature dataset to contain the converted feature classes. Output properties such as spatial reference and output names for the feature classes can also be specified.
4. If the input coverage has a coordinate system defined by a projection (PRJ) file, the wizard will read and add this information to the geodatabase feature class properties. If the input coverage has no spatial reference information, specify the appropriate coordinate system or accept the default of UNKNOWN.
5. The wizard suggests an approximate value for the spatial grid size based on spatial reference, average feature size, and number of features in the input coverage. Accept this default.
6. The wizard checks the item names in the input table for any characters or words that are illegal and automatically changes them.
7. The next pane identifies the field mapping and suggests corrections to these fields. Fields that are no longer needed may be deleted in this step.
8. A panel displayed at the end of the process summarizes the Input and Output options that can be accepted to complete the conversion process.

Read more about geodatabases in Modeling Our World a manual that is part of the ArcGIS documentation.

More detailed instructions on converting coverages to the geodatabase are available from these two tutorials.
Tutorial 1--"Importing a Coverage"
Tutorial 2--"Using the Import Wizard"
Sample Data Set

Table of Contents for the July–September 2001 issue