VRT Raster (GDAL Virtual Format) Reader Parameters

Dataset

This parameter allows you to choose different naming schemes, and the number of feature types generated for the reader.

Feature type name choices:

  • From File Name(s): Generates one feature type per source filename.
  • From Format Name: Produces only a single feature type containing the format name.

Use this parameter to get additional metadata from source files referenced in the VRT file. Metadata information will be stored generically as format attributes on the output raster feature and may not be the same as the format attributes output generated from the underlying reader (such as GeoPackage in the following example).

Example

A VRT with the following source (a GeoPackage tiles file):

    <SimpleSource>
      <SourceFilename relativeToVRT="1">my_geopackage.gpkg</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="8" RasterYSize="8" DataType="Byte" BlockXSize="256" BlockYSize="256" />
      <SrcRect xOff="0" yOff="0" xSize="8" ySize="8" />
      <DstRect xOff="0" yOff="0" xSize="8" ySize="8" />
    </SimpleSource>

will expose the GeoPackage metadata as attributes in this form:

vrt_raster_band{0}.source{0}.metadata{0}.name=INTERLEAVE
vrt_raster_band{0}.source{0}.metadata{0}.value=PIXEL
vrt_raster_band{0}.source{0}.metadata{1}.name=IDENTIFIER
vrt_raster_band{0}.source{0}.metadata{1}.value=SOME_IDENTIFIER
vrt_raster_band{0}.source{0}.metadata{2}.name=ZOOM_LEVEL
vrt_raster_band{0}.source{0}.metadata{2}.value=17

Note: VRT files containing other VRT files are supported up to arbitrary depth. For example, if the above example was a VRT pulling in a band from another VRT, which was pulling a band from the above GeoPackage file (VRT <– VRT <– GeoPackage), we would see attributes of the form:

vrt_raster_band{0}.source{0}.band_metadata{0}.source{0}.metadata{0}.name=INTERLEAVE
vrt_raster_band{0}.source{0}.band_metadata{0}.source{0}.metadata{0}.value=PIXEL

By default, the GCP information is preserved as properties on the raster geometry.

If you select this checkbox, the GCP information, including a GCP projection, will be read from the file and applied to the raster data as an affine transformation.

GCP Interpolation Method

Specifies the interpolation method used to apply a GCP projection to the raster data.

  • Nearest Neighbor: Applies a projection based on the value of the pixel closest to the GCP. This method is the least resource-intensive, but also produces the lowest resolution GCP.
  • Bilinear: Applies a projection by averaging the closest 2x2 neighborhood of pixel values surrounding the GCP. This method produces higher-quality results than Nearest Neighbor.
  • Bicubic: Applies a projection by applying a weighted average of the closest 4x4 neighborhood of pixel values surrounding the GCP, with closer pixels weighted higher than farther pixels. Bicubic interpolation requires the most amount of processing time.

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:

Screenshot with blank search envelope min and max x and y parameters

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.

Note  If all four coordinates of the search envelope are left at 0, the search envelope will be disabled even if this option is checked.

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).