Geospatial data is organized as follows:
- Feature: The spatial representation of a real-world entity, such as a specific road or an individual utility pole, that specifies the geometry and other properties of the feature.
- Feature class: A category of features with rules that define the allowable data types, default values, and constraints for its member features. For example, you might have feature classes for a set of roads, utility poles, and so on.
A feature class is equivalent to a table in a data base.
- Schema: A collection of related feature classes.
- Data store: A collection of feature data in a single storage location.
SDF data stores allow only one schema per file, but database stores such as Oracle or SQL Server can have multiple schemas. (SHP files can include only one geometry type per file, but you can be store and copy multiple SHP files in a folder. You can use a configuration file to support multiple schemas when you establish your connection to a SHP file.)
The data hierarchy
Geospatial data is stored in a hierarchical fashion, like a set of tables: each row within a table is an individual feature, and each column is a property of that feature. The entire table (including its name, column names, data types, default values, and constraints) represents a feature class. The set of related tables is called a schema, and the entire collection of data resides in a data store.
For example, you might use a data store such as Oracle, which can encompass multiple schemas. The database might define the utilities for a town, with schemas for different types of utilities, such as electrical and water. The electrical schema would include feature classes for utility poles and boxes, while the water schema would include feature classes for pipes and hydrants.
Properties
Each feature class has properties that define it. The Pole feature class would have properties such as identification number, name, model, material, height, installation date, and so on. The properties that define a feature class can have data types, default values, and constraints. These can help ensure that a feature meets certain criteria in order to be included in a particular feature class. For example, the “Large Roads” feature class can include a field called “Lanes.” The constraint for “Lanes” can specify that its value must be six or higher in order for a road to be included in the “Large Roads” feature class.
Data stores
Features can be stored in a spatial database (such as Oracle, MySQL, PostgreSQL/PostGIS, or SQLServer), or in the ESRI ArcSDE data store, which can use either an Oracle- or SQL Server-based spatial database. Features can be stored in a file (such as SQLite, SHP, or SDF). Related SHP files might be stored in a folder.
Features can be accessed from a web-based service (such as WFS or WMS). AutoCAD Map 3D also supports ODBC (Open Database Connectivity), a standardized interface for accessing a database from a program.
A data store usually contains a spatial context, which describes the spatial metadata or parameters within which geometry for a collection of features resides. The spatial context can specify the coordinate system, extents, and tolerance. A data store can include multiple spatial contexts, for example, one context for ground-based data and another context for schematic data.
For more information about features, feature classes, and schemas (including diagrams that illustrate these concepts), see “What Are Features?” and “What is a Schema?” in Best Practices for Managing Geospatial Data, available from the Help menu in AutoCAD Map 3D.
Working with features in your map
To add a feature to your map, you connect to its data store and select the feature classes to include. After you connect to a data store in AutoCAD Map 3D, that data store is a feature source for the current map. Feature sources are listed by provider (for example, all SDF feature sources are listed together in Data Connect). Each feature class you add becomes a feature layer in your map. You can apply filters and spatial queries to the layers to show only some of the features within that layer. Filters are based on attributes, and queries are based on spatial location.