# Virtual Format

OGR Virtual Format is a driver that transforms features read from other drivers based on criteria specified in an XML control file. It is primarily used to derive spatial layers from flat tables with spatial information in attribute columns. It can also be used to associate coordinate system information with a datasource, merge layers from different datasources into a single data source, or even just to provide an anchor file for access to non-file oriented datasources.

The virtual files are currently normally prepared by hand.

## Creation Issues

Before GDAL 1.7.0, the OGR VRT driver was read-only.

Since GDAL 1.7.0, the CreateFeature(), SetFeature() and DeleteFeature() operations are supported on a layer of a VRT dataset, if the following conditions are met :

- the VRT dataset is opened in update mode
- the underlying source layer supports those operations
- the SrcLayer element is used (as opposed to the SrcSQL element)
- the FID of the VRT features is the same as the FID of the source features, that is to say, the FID element is not specified

# Virtual File Format

The root element of the XML control file is OGRVRTDataSource. It has an OGRVRTLayer (or OGRVRTWarpedLayer or OGRVRTUnionLayer starting with GDAL 1.10.0) child for each layer in the virtual datasource.

A XML schema of the OGR VRT format is available. Starting with GDAL 1.11, when GDAL is configured with libXML2 support, that schema will be used to validate the VRT documents. Non-conformities will be reported only as warnings. That validation can be disabled by setting the GDAL_XML_VALIDATION configuration option to NO.

A OGRVRTLayer element should have a name attribute with the layer name, and may have the following subelements:

- SrcDataSource (mandatory): The value is the name of the datasource that this layer will be derived from. The element may optionally have a relativeToVRT attribute which defaults to "0", but if "1" indicates that the source datasource should be interpreted as relative to the virtual file. This can be any OGR supported dataset, including ODBC, CSV, etc. The element may also have a shared attribute to control whether the datasource should be opened in shared mode. Defaults to OFF for SrcLayer use and ON for SrcSQL use.
- SrcLayer (optional): The value is the name of the layer on the source data source from which this virtual layer should be derived. If this element isn't provided, then the SrcSQL element must be provided.
- SrcSQL (optional): An SQL statement to execute to generate the desired layer result. This should be provided instead of the SrcLayer for statement derived results. Some limitations may apply for SQL derived layers. Starting with OGR 1.10, an optional dialect attribute can be specified on the SrcSQL element to specify which SQL "dialect" should be used : possible values are currently OGRSQL or SQLITE. If dialect is not specified, the default dialect of the datasource will be used.
- FID (optional): Name of the attribute column from which the FID of features should be derived. If not provided, the FID of the source features will be used directly.
- Style (optional): Name of the attribute column from which the style of features should be derived. If not provided, the style of the source features will be used directly.
- GeometryType (optional): The geometry type to be assigned to the layer. If not provided it will be taken from the source layer. The value should be one of "wkbNone", "wkbUnknown", "wkbPoint", "wkbLineString", "wkbPolygon", "wkbMultiPoint", "wkbMultiLineString", "wkbMultiPolygon", or "wkbGeometryCollection". Optionally "25D" may be appended to mark it as including Z coordinates. Defaults to "wkbUnknown" indicating that any geometry type is allowed.
- LayerSRS (optional): The value of this element is the spatial reference to use for the layer. If not provided, it is inherited from the source layer. The value may be WKT or any other input that is accepted by the OGRSpatialReference::SetUserInput() method. If the value is NULL, then no SRS will be used for the layer.
- GeometryField (optional): This element is used to define how the geometry for features should be derived.
  If not provided the geometry of the source feature is copied directly.
  The type of geometry encoding is indicated with the encoding attribute which may have the value "WKT", "WKB" or "PointFromColumns".
  If the encoding is "WKT" or "WKB" then the field attribute will have the name of the field containing the WKT or WKB geometry.
  If the encoding is "PointFromColumns" then the x, y and z attributes will have the names of the columns to be used for the X, Y and Z coordinates. The z attribute is optional.
  Starting with GDAL 1.7.0, the optional reportSrcColumn attribute can be used to specify whether the source geometry fields (the fields set in the field, x, x or z attributes) should be reported as fields of the VRT layer. It defaults to TRUE. If set to FALSE, the source geometry fields will only be used to build the geometry of the features of the VRT layer.

Starting with OGR 1.11, the GeometryField element can be repeated as many times as necessary to create multiple geometry fields. It accepts a name attribute (recommanded) that will be used to define the VRT geometry field name. When encoding is not specified, the field attribute will be used to determine the corresponding geometry field name in the source layer. If neither encoding nor field are specified, it is assumed that the name of source geometry field is the value of the name attribute.

When several geometry fields are used, the following child elements of GeometryField can be defined to explicitely set the geometry type, SRS, source region, or extent.

- GeometryType (optional) : same syntax as OGRVRTLayer-level GeometryType.
- SRS (optional) : same syntax as OGRVRTLayer-level LayerSRS (note SRS vs LayerSRS)
- SrcRegion (optional) : same syntax as OGRVRTLayer-level SrcRegion
- ExtentXMin, ExtentYMin, ExtentXMax and ExtentXMax (optional) : same syntax as OGRVRTLayer-level elements of same name

If no GeometryField element is specified, all the geometry fields of the source layer will be exposed by the VRT layer. In order not to expose any geometry field of the source layer, you need to specify OGRVRTLayer-level GeometryType element to wkbNone.

- SrcRegion (optionnal, from GDAL 1.7.0) : This element is used to define an initial spatial filter for the source features. This spatial filter will be combined with any spatial filter explicitely set on the VRT layer with the SetSpatialFilter() method. The value of the element must be a valid WKT string defining a polygon. An optional clip attribute can be set to "TRUE" to clip the geometries to the source region, otherwise the source geometries are not modified.
- Field (optional, from GDAL 1.7.0): One or more attribute fields may be defined with Field elements. If no Field elements are defined, the fields of the source layer/sql will be defined on the VRT layer. The Field may have the following attributes:
  - name (required): the name of the field.
  - type: the field type, one of "Integer", "IntegerList", "Real", "RealList", "String", "StringList", "Binary", "Date", "Time", or "DateTime". Defaults to "String".
  - width: the field width. Defaults to unknown.
  - precision: the field width. Defaults to zero.
  - src: the name of the source field to be copied to this one. Defaults to the value of "name".
- FeatureCount (optional, from GDAL 1.10.0) : This element is used to define the feature count of the layer (when no spatial or attribute filter is set). This can be usefull on static data, when getting the feature count from the source layer is slow.
- ExtentXMin, ExtentYMin, ExtentXMax and ExtentXMax (optional, from GDAL 1.10.0) : Those elements are used to define the extent of the layer. This can be usefull on static data, when getting the extent from the source layer is slow.

A OGRVRTWarpedLayer element (GDAL >= 1.10.0) is used to do on-the-fly reprojection of a source layer. It may have the following subelements:

- OGRVRTLayer, OGRVRTWarpedLayer or OGRVRTUnionLayer (mandatory): the source layer to reproject.
- SrcSRS (optional): The value of this element is the spatial reference to use for the layer before reprojection. If not specified, it is deduced from the source layer.
- TargetSRS (mandatory): The value of this element is the spatial reference to use for the layer after reprojection.
- ExtentXMin, ExtentYMin, ExtentXMax and ExtentXMax (optional, from GDAL 1.10.0) : Those elements are used to define the extent of the layer. This can be usefull on static data, when getting the extent from the source layer is slow.
- WarpedGeomFieldName (optional, from GDAL 1.11) : The value of this element is the geometry field name of the source layer to wrap. If not specified, the first geometry field will be used. If there are several geometry fields, only the one matching WarpedGeomFieldName will be warped; the other ones will be untouched.

A OGRVRTUnionLayer element (GDAL >= 1.10.0) is used to concatenate the content of source layers. It should have a name and may have the following subelements:

- OGRVRTLayer, OGRVRTWarpedLayer or OGRVRTUnionLayer (mandatory and may be repeated): a source layer to add in the union.
- PreserveSrcFID (optional) : may be ON or OFF. If set to ON, the FID from the source layer will be used, otherwise a counter will be used. Defaults to OFF.
- SourceLayerFieldName (optional) : if specified, an additional field (named with the value of SourceLayerFieldName) will be added in the layer field definition. For each feature, the value of this field will be set with the name of the layer from which the feature comes from.
- GeometryType (optional) : see above for the syntax. If not specified, the geometry type will be deduced from the geometry type of all source layers.
- LayerSRS (optional) : see above for the syntax. If not specified, the SRS will be the SRS of the first source layer.
- FieldStrategy (optional, exclusive with Field or GeometryField) : may be FirstLayer to use the fields from the first layer found, Union to use a super-set of all the fields from all source layers, or Intersection to use a sub-set of all the common fields from all source layers. Defaults to Union.
- Field (optional, exclusive with FieldStrategy) : see above for the syntax. Note: the src attribute is not supported in the context of a OGRVRTUnionLayer element (field names are assumed to be identical).
- GeometryField (optional, exclusive with FieldStrategy, GDAL >= 1.11) : the name attribute and the following sub-elements GeometryType, SRS and Extent[X|Y][Min|Max] are available.
- FeatureCount (optional) : see above for the syntax
- ExtentXMin, ExtentYMin, ExtentXMax and ExtentXMax (optional) : see above for the syntax

## Example: ODBC Point Layer

In the following example (disease.ovf) the worms table from the ODBC database DISEASE is used to form a spatial layer. The virtual file uses the "x" and "y" columns to get the spatial location. It also marks the layer as a point layer, and as being in the WGS84 coordinate system.

    <OGRVRTDataSource>

        <OGRVRTLayer name="worms">
            <SrcDataSource>ODBC:DISEASE,worms</SrcDataSource>
     	<SrcLayer>worms</SrcLayer>
    	<GeometryType>wkbPoint</GeometryType>
            <LayerSRS>WGS84</LayerSRS>
    	<GeometryField encoding="PointFromColumns" x="x" y="y"/>
        </OGRVRTLayer>

    </OGRVRTDataSource>

## Example: Renaming attributes

It can be usefull in some circumstances to be able to rename the field names from a source layer to other names. This is particularly true when you want to transcode to a format whose schema is fixed, such as GPX (<name>, <desc>, etc.). This can be accomplished using SQL this way:

    <OGRVRTDataSource>
        <OGRVRTLayer name="remapped_layer">
            <SrcDataSource>your_source.shp</SrcDataSource>
            <SrcSQL>SELECT src_field_1 AS name, src_field_2 AS desc FROM your_source_layer_name</SrcSQL>
        </OGRVRTLayer>
    </OGRVRTDataSource>

This can also be accomplished (from GDAL 1.7.0) using explicit field definitions:

    <OGRVRTDataSource>
        <OGRVRTLayer name="remapped_layer">
            <SrcDataSource>your_source.shp</SrcDataSource>
            <SrcLayer>your_source</SrcSQL>
            <Field name="name" src="src_field_1" />
            <Field name="desc" src="src_field_2" type="String" width="45" />
        </OGRVRTLayer>
    </OGRVRTDataSource>

## Example: Transparent spatial filtering (GDAL >= 1.7.0)

The following example will only return features from the source layer that intersect the (0,40)-(10,50) region. Furthermore, returned geometries will be clipped to fit into that region.

    <OGRVRTDataSource>
        <OGRVRTLayer name="source">
            <SrcDataSource>source.shp</SrcDataSource>
            <SrcRegion clip="true">POLYGON((0 40,10 40,10 50,0 50,0 40))</SrcRegion>
        </OGRVRTLayer>
    </OGRVRTDataSource>

## Example: Reprojected layer (GDAL >= 1.10.0)

The following example will return the source.shp layer reprojected to EPSG:4326.

    <OGRVRTDataSource>
        <OGRVRTWarpedLayer>
            <OGRVRTLayer name="source">
                <SrcDataSource>source.shp</SrcDataSource>
            </OGRVRTLayer>
            <TargetSRS>EPSG:4326</TargetSRS>
        </OGRVRTWarpedLayer>
    </OGRVRTDataSource>

## Example: Union layer (GDAL >= 1.10.0)

The following example will return a layer that is the concatenation of source1.shp and source2.shp.

    <OGRVRTDataSource>
        <OGRVRTUnionLayer name="unionLayer">
            <OGRVRTLayer name="source1">
                <SrcDataSource>source1.shp</SrcDataSource>
            </OGRVRTLayer>
            <OGRVRTLayer name="source2">
                <SrcDataSource>source2.shp</SrcDataSource>
            </OGRVRTLayer>
        </OGRVRTUnionLayer>
    </OGRVRTDataSource>

## Example: SQLite/Spatialite SQL dialect (GDAL >=1.10.0)

The following example will return four different layers which are generated in a fly from the same polygon shapefile. The first one is the shapefile layer as it stands. The second layer gives simplified polygons by applying Spatialite function "Simplify" with parameter tolerance=10. In the third layer the original geometries are replaced by their convace hulls. In the fourth layer Spatialite function PointOnSurface is used for replacing the original geometries by points which are inside the corresponding source polygons. Note that for using the last three layers of this VRT file GDAL must be compiled with SQLite and Spatialite.

    <OGRVRTDataSource>
        <OGRVRTLayer name="polygons">
            <SrcDataSource>polygons.shp</SrcDataSource>
            </OGRVRTLayer>
        <OGRVRTLayer name="polygons_as_simplified">
            <SrcDataSource>polygons.shp</SrcDataSource>
            <SrcSQL dialect="sqlite">SELECT Simplify(geometry,10) from polygons</SrcSQL>
        </OGRVRTLayer>
        <OGRVRTLayer name="polygons_as_hulls">
            <SrcDataSource>polygons.shp</SrcDataSource>
            <SrcSQL dialect="sqlite">SELECT ConvexHull(geometry) from polygons</SrcSQL>
        </OGRVRTLayer>
        <OGRVRTLayer name="polygons_as_points">
            <SrcDataSource>polygons.shp</SrcDataSource>
            <SrcSQL dialect="sqlite">SELECT PointOnSurface(geometry) from polygons</SrcSQL>
        </OGRVRTLayer>
    </OGRVRTDataSource>

## Example: Multiple geometry fields (GDAL >= 1.11)

The following example will expose all the attribute and geometry fields of the source layer:

    <OGRVRTDataSource>
        <OGRVRTLayer name="test">
            <SrcDataSource>PG:dbname=testdb</SrcDataSource>
        </OGRVRTLayer>
    </OGRVRTDataSource>

To expose only part (or all!) of the fields:

    <OGRVRTDataSource>
        <OGRVRTLayer name="other_test">
            <SrcDataSource>PG:dbname=testdb</SrcDataSource>
            <SrcLayer>test</SrcLayer>
            <GeometryField name="pg_geom_field_1" />
            <GeometryField name="vrt_geom_field_2" field="pg_geom_field_2">
                <GeometryType>wkbPolygon</GeometryType>
                <SRS>EPSG:4326</SRS>
                <ExtentXMin>-180</ExtentXMin>
                <ExtentYMin>-90</ExtentYMin>
                <ExtentXMax>180</ExtentXMax>
                <ExtentYMax>90</ExtentYMax>
            </GeometryField>
            <Field name="vrt_field_1" src="src_field_1" />
        </OGRVRTLayer>w
    </OGRVRTDataSource>

To reproject the 'pg_geom_field_2' geometry field to EPSG:4326:

    <OGRVRTDataSource>
        <OGRVRTWarpedLayer>
            <OGRVRTLayer name="other_test">
                <SrcDataSource>PG:dbname=testdb</SrcDataSource>
            </OGRVRTLayer>
            <WarpedGeomFieldName>pg_geom_field_2</WarpedGeomFieldName>
            <TargetSRS>EPSG:32631</TargetSRS>
        </OGRVRTWarpedLayer>
    </OGRVRTDataSource>

To make the union of several multi-geometry layers and keep only a few of them:

    <OGRVRTDataSource>
        <OGRVRTUnionLayer name="unionLayer">
            <OGRVRTLayer name="source1">
                <SrcDataSource>PG:dbname=testdb</SrcDataSource>
            </OGRVRTLayer>
            <OGRVRTLayer name="source2">
                <SrcDataSource>PG:dbname=testdb</SrcDataSource>
            </OGRVRTLayer>
            <GeometryField name="pg_geom_field_2">
                <GeometryType>wkbPolygon</GeometryType>
                <SRS>EPSG:4326</SRS>
                <ExtentXMin>-180</ExtentXMin>
                <ExtentYMin>-90</ExtentYMin>
                <ExtentXMax>180</ExtentXMax>
                <ExtentYMax>90</ExtentYMax>
            </GeometryField>
            <GeometryField name="pg_geom_field_3" />
            <Field name="src_field_1" />
        </OGRVRTUnionLayer>
    </OGRVRTDataSource>

## Other Notes

- When the GeometryField is "WKT" spatial filtering is applied after extracting all rows from the source datasource. Essentially that means there is no fast spatial filtering on WKT derived geometries.

- When the GeometryField is "PointFromColumns", and a SrcLayer (as opposed to SrcSQL) is used, and a spatial filter is in effect on the virtual layer then the spatial filter will be internally translated into an attribute filter on the X and Y columns in the SrcLayer. In cases where fast spatial filtering is important it can be helpful to index the X and Y columns in the source datastore, if that is possible. For instance if the source is an RDBMS. You can turn off that feature by setting the useSpatialSubquery attribute of the GeometryField element to FALSE.