Understanding Feature Types and Attributes
Feature Type: the source dataset schema or structure.
Attributes: can reference either user attributes or format attributes.
About Feature Types
In FME, a Feature Type refers to the source dataset schema or structure.
Every format used by FME identifies features through a classification scheme. This classification is known in FME as a feature type. Feature types are used to differentiate between different types of features (for example roads, rivers, buildings, contours).
Some examples are:
- In MicroStation Design File (DGN) format, each level is a Feature Type.
- In AutoCAD Drawing File (DWG) format, each layer is a Feature Type.
- In Smallworld format, each class is a Feature Type.
- In Esri Shapefile format, each file basename (*.shp file) is a Feature Type.
A Feature Type is contained within a dataset.
When you create a workspace using a dataset, the Feature Types present in that dataset are shown on the left-hand side of the workspace canvas:
About Attributes
Workbench supports two types of attributes:
- User attributes can have their names and data types adjusted to meet the needs of the user. They correspond to the columns in any attribute tables created during the translation.
- Format attributes are fixed by FME. Descriptions of the attributes available for each reader (input format) and writer (output format) are given in the FME Readers and Writers manual. (Choose FME Readers and Writers Reference from the Workbench > Help menu.) Format attributes can be exposed in a workspace so that they can be set to particular values.
Format Attributes
A format attribute is a built-in, FME-generated attribute that relates to the structure or symbology of a feature for any given format. For example, a format attribute records a feature’s color.
You can use these attributes to carry out certain tasks by making the attributes part of the workspace.
Format attributes are most visible when viewing a dataset with the Inspector (FME Data Inspector). When querying a feature, not only will user attributes be shown, but so too will format attributes. In this case, the attributes reflect the nature of the data in its source form.
More Format Attribute Examples
The color of a feature is the simplest example of a format attribute. However, there are many more, all of which vary in their degree of complexity. Some examples of format attributes include the following.
- Rotation: Many features possess a degree of rotation around the Z axis. This rotation is also stored as a format attribute; for example, fm0_rotation (the rotation of a feature in GeoMedia format).
- Subtype: For a format that supports subtypes (a sort of built-in lookup table), the value corresponding to a subtype code may be placed as a format attribute; for example, geodb_subtype_name (the subtype field for Esri Geodatabase format).
- Raster Pyramid: Raster features are handled in a slightly different way to vector features, with one of these differences being a greater reliance on format attributes. Details of Raster Pyramids are often stored or set in format attributes; for example, oracle_raster_pyramid_type (the type of pyramiding to be applied when writing raster data to an Oracle database).
Mapping Attributes from Reader to Writer
A particular set of Format Attributes has the prefix fme_. These attributes represent the data as it is perceived by FME. They are sometimes known as FME Attributes or Generic FME Attributes.
During a translation, the following occurs:
- FME reads the source data and stores information about its features as format attributes. These format attributes reflect the data that is stored in the original Reader Format.
- FME converts the source data’s format attributes into FME Attributes. These FME attributes reflect the source data as the FME internal format would store it.
- FME writes the destination data by creating a new set of format attributes. These format attributes reflect the destination data as it will be stored in the Writer Format.
FME converts from one format to another without having to separately map the source data's format attributes directly to the destination format's attributes for every format. FME converts everything to an FME standard, and then converts it again to the writer format.
Data Type Mapping
Different formats support different data types. Even when two formats support the same data type, it is often represented by a different name. In this situation, FME must decide how to map attributes between data types.
FME's enhanced data model supports the mapping of data types in a manner that is similar to the mapping of attributes described above. A metafile for the reader format instructs FME how to map its attribute data types to the FME model, and a second metafile instructs FME how to map its model types to the destination schema.
For example, consider the data type mappings for MapInfo and Oracle data:
| MapInfo Data Type | FME Data Type |
|---|---|
| char(width) | fme_char(width) |
| date | fme_date |
| decimal(width,decimal) | fme_decimal(width,decimal) |
| float | fme_real64 |
| integer | fme_int32 |
| logical | fme_boolean |
| smallint | fme_int16 |
| Oracle Data Type | FME Data Type |
|---|---|
| char(width) | fme_char(width) |
| float | fme_real32 |
| number(width,decimal) | fme_decimal(width,decimal) |
| double | fme_real64 |
| integer | fme_int32 |
| logical | fme_boolean |
| smallint | fme_int16 |
These mappings show that a MapInfo decimal data type attribute becomes an fme_decimal in the FME data model, which then becomes a number data type when written to Oracle.