NMEA Automatic Identification System (AIS) Reader Parameters

Use this parameter to expose Format Attributes in FME Workbench when you create a workspace:

• In a dynamic scenario, it means these attributes can be passed to the output dataset at runtime.
• In a non-dynamic scenario, this parameter allows you to expose additional attributes on multiple feature types. Click the browse button to view the available format attributes (which are different for each format) for the reader.

A search envelope (also known as a bounding box) is a rectangular area that defines a geographic area. In FME, the easiest way to define a search envelope is to use search envelope parameters.

Defining a search envelope is the most efficient method of selecting an area of interest because FME will read only the data that is necessary – it does not have to read an entire dataset. Search Envelope parameters apply to both vector and raster datasets and can be particularly efficient if the source format has a spatial index.

Most FME readers have parameters to define the search envelope of data that is being read:

The parameters include the x and y coordinates of the bounding box as well as a parameter that defines the coordinate system.

How to Define the Bounding Box

Using the minimum and maximum x and y parameters, define a bounding box that will be used to filter the input features. Only features that intersect with the bounding box are returned. Note that the bounding box intersection is not a full geometry intersection (based on spatial relationships) that would be returned by a transformer like the SpatialFilter.

Search Envelope Coordinate System

Specifies the coordinate system of the search envelope if it is different than the coordinate system of the data. The coordinate system associated with the data to be read must always be set if this parameter is set.

If this parameter is set, the minimum and maximum points of the search envelope are reprojected from the Search Envelope Coordinate System to the reader’s coordinate system prior to applying the envelope.

The underlying function for Use Search Envelope is an intersection; however, when Clip to Search Envelope is checked, a clipping operation is also performed.

• When checked (set to Yes), this option instructs FME to clip features to the exact envelope boundary. FME removes any portions of imported features being read that are outside the search envelope.
• When left unchecked (set to No), features that overlap the boundary will be included in their full (unclipped) form.

Clip to Search Envelope: No	Clip to Search Envelope: Yes
Any features that cross the search envelope boundary will be read, including the portion that lies outside of the boundary.	Any features that cross the search envelope boundary will be clipped at the boundary, and only the portion that lies inside the boundary will be read.
The search envelope includes the bounding box and the extent of the raster.	The search envelope includes only the area within the bounding box. The raster size will still match the bounding box, but the area without data will be filled with Nodata values to represent the absence of data, if the source raster has them. Raster Nodata may be a single value across all bands, a single value per band, or a separate alpha or transparency band that indicates the lack of data values (this is more common in images than other types of rasters).

In AIS, longer messages are split into multiple parts, which are related by an ID. However, the message IDs used when transmitting are not unique. This leads to the possibility that unrelated message pieces (if they have the same ID) could be combined. To prevent this, partially received multi-part messages are dropped as incomplete if the rest of the message is not found within the next set of incoming messages. The size of this set is determined by this parameter.

The value for this parameter is an integer. The default is 40.