Fundamentals about maintaining attributes in a coverage
Coverage feature classes have descriptive information stored as attributes in a feature attribute table.
Understanding attribute definitions
Each column in an attribute table is described in terms of the data types used to store values (for example, text, integer numbers, or decimal numbers), the number of characters or digits required for storage, and display characteristics for reporting attribute values. Where a code is used to represent a value (for example, 1 = highway and 2 = collector), code description tables are defined. In a coverage attribute table, a column in the attribute table is an item (otherwise known as field).
The following are attribute item characteristics:
- Name: The name of an item (that is, column) in a table. An attribute of a feature is referenced by the item name, which can be up to 16 characters in length.
- Type: Refers to how the computer stores, interprets, and formats item values. The type can be any of the following:
INFO format types
Integer (1 byte per digit)
Binary integer (requires less storage than I types)
Character (that is, text)
Floating-point (that is, decimal) number stored as 1 byte per digit
Floating-point binary number, either single or double precision
Date that stores day, month, and year (for example, mm/dd/yyyy)
- Width: The number of units of a type used to store the item values. Here is a summary of supported item widths for the types listed above:
INFO type widths
1 to 16 digits
Either 2 or 4 bytes
1 to 320 characters
Always 4 (single precision) or 8 (double precision) bytes
1 to 16 digits
Always 8 bytes; stored as yyyymmdd
- Decimals: The number of decimal places for real numbers (that is, item types F and N).
- Output: The number of characters used to display an item value. For example, in a 2-byte integer (item type B), values can be as high as 32767, which requires 5 characters for display. Dates can be displayed using 8 (mm/dd/yy) or 10 (mm/dd/yyyy) characters. For international date displays, months and days can be switched (for example, dd/mm/yy).
As shown in the following diagram, tables can be related using a common item. Records in tables are related when the value for the common item is the same. Several kinds of relationships exist, including one-to-many, one-to-one, and many-to-one. The diagram below demonstrates a one-to-many relationship between parcels and owners; one parcel can have many owners. The items holding the parcel number for each lot are contained in two tables: the coverage PARCEL.PAT and another table, OWNER. These act as keys to relate records between the two tables.
This section applies to ArcEditor and ArcInfo only.
On the Properties dialog box for a coverage feature class, you'll see an FID column that contains the unique identifier for each record or feature. A feature class also has a Shape column and may have several pseudo items, such as the angle of rotation for marker symbols or properties of coverage annotation subclasses. Pseudo items are maintained by ArcInfo; their names are preceded by a dollar sign ($), for example, $ANGLE.
On the Properties dialog box for a table or feature class, you can add, update, and delete attributes, as well as create indexes for the values the attributes contain. Only user-defined attributes in the attribute table can be modified or deleted. Coverage items maintained by ArcInfo cannot be modified or deleted by the user. Index an attribute to improve the performance of operations that access its contents, such as selecting features with specific attribute values. If you index the Shape column, a spatial index will be created, which improves the performance of any operation that retrieves features by location. After modifying the values in a column, delete the existing index and add a new one.
In addition to new attributes, you can add redefined attributes. Add a redefined attribute to combine or divide the values in existing attributes. Redefined attributes are useful for customizing how the values in a table are displayed. For example, you might choose to show one redefined attribute that concatenates values rather than showing two separate, adjacent columns.