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ArcGIS provides the means to view a table or feature class using the physical data model perspective. This is called the data dictionary view because it displays the contents of each table or feature class. You can see the name for the class and its type in the header. If it is a feature class, you will also be able to see the type of geometry used and whether it contains measure (M) and elevation (Z) values.

Figure 3.4 Tables and feature classes A geodatabase is seen as a collection of tables and feature classes. These examples show the minimal attributes such classes will contain. You may add many other attributes to any of these classes, but you cannot delete the mandatory attributes.

Below the header is a listing of each field in the class’s table. The first field will always be OBJECTID. If it is a feature class, you will also see a Shape field and, depending on the type of feature, Shape_Length and Shape_Area fields. ArcGIS adds these fields and you cannot delete them or change their parameters.

When you create a field, you must specify a data type, whether null values are allowed, a default value that will be entered if the user makes no specific entry, the name of a domain to control data entries, the precision and scale of numeric fields, and the length of a string field. The user can modify field parameters that are not set to a dark gray background. At minimum, you must specify the name and data type for each field, and the length of any text field. The default value of 0 for numeric precision and scale will produce the standard form of that data type for the selected database management system. Indeed, precision and scale parameters you enter will only be used if the database management system uses ArcSDE. As we show below, your parameters for numeric fields may be only advisory, as ArcGIS can change them to optimize performance in the database management system you are using.

Figure 3.5 The data dictionary view This view reveals the descriptive information stored in the Table and Field classes. Each Field class instance is shown as a row in the data dictionary view, with the attributes of each field shown in columns. The darker gray boxes represent field attributes that cannot be changed by the user because ArcGIS controls the value that goes there or the attribute does not apply to that field. Both the table and feature class versions show the same set of field definitions, except that the feature class has an additional field called Shape. The feature class also has information in the upper right corner that reveals what GeometryDef knows about the class, which is also reflected in the class description and icon in the upper left corner.