What is a coverage
Last modified November 25, 2008
Print all topics in : "Coverages"
A coverage is a georelational data model that stores vector data—it contains both the spatial (location) and attribute (descriptive) data for geographic features. Coverages use a set of feature classes to represent geographic features. Each feature class stores a set of points, lines (arcs), polygons, or annotation (text). Coverages can have topology, which determines the relationships between features.
A coverage is stored as a directory within which each feature class resides as a set of files. For example, a coverage will appear in ArcCatalog with the icons as shown below. In this example, you can see that the streams coverage is a line coverage containing an arc (line) file, annotation for the line, and a tic file. There are also two versions of coverage files.
More than one feature class is often required to define the features in a coverage. For example, line and polygon feature classes both exist in a coverage representing polygon features. Polygon features also have label points, which appear as a separate feature class. Every coverage has a feature class containing tic points, which represent known real-world coordinates.These tic points help define the extent of a coverage; they do not represent any actual data points within the coverage. The figure below shows the common feature classes in a coverage. The other coverage feature classes include section, route, region, and link.
For more information about the coverage data model, see How coverages are stored.
Learn about defining a coverage's coordinate system.
A geographic feature in a coverage is identified by a unique feature number. Spatial and attribute data of a feature is linked by means of this number.
|<cover>#||An internal sequence number (software assigned)|
|<cover>-ID||A feature ID (user assigned) for which <cover> is the coverage name|
Feature attributes are stored in the coverage's .adf files. Other attributes can be stored in INFO tables or tables in an RDBMS, then joined to features with a layer or a relationship class.
When you look in a folder in the Catalog, you see all the coverages and any associated INFO tables it may contain; you don't see the INFO folder itself. Expand a coverage to see its feature classes; each feature class represents both the features and their associated attribute tables. If you select a feature class (such as a polygon), you can preview its features and attributes.
Learn more about previewing geographic data in ArcCatalog.
There are three noteworthy characteristics of the link between the spatial and tabular data:
Coverages often have associated files. To see them in ArcCatalog, add them as file types. For example, to see ARC Macro Language (AML) scripts, you would add the file extension .aml to the File Types list.
Learn more about viewing file types.
PC ARC/INFO coverages are similar to ArcInfo coverages, except that their attributes are stored in dBASE tables. PC ARC/INFO coverages can be previewed in ArcCatalog and you can create metadata for them, but other data management operations, such as copy, paste, and delete, are not available.
Coverages created with ArcInfo before version 7 are unavailable in ArcCatalog. After converting the workspace using ArcInfo Workstation, you can access coverages' contents in ArcCatalog.