# GMLAS - Geography Markup Language (GML) driven by application schemas

Available in GDAL >= 2.2

This driver can read and write XML files of arbitrary structure, included those containing so called Complex Features, provided that they are accompanied by one or several XML schemas that describe the structure of their content. While this driver is generic to any XML schema, the main target is to be able to read and write documents referencing directly or indirectly to the GML namespace.

The driver requires Xerces-C >= 3.1.

The driver can deal with files of arbitrary size with a very modest RAM usage, due to its working in streaming mode.

## Opening syntax

The connection string is GMLAS:/path/to/the.gml. Note the GMLAS: prefix. If this prefix it is omitted, then the GML driver is likely to be used.

It is also possible to only used "GMLAS:" as the connection string, but in that case the schemas must be explicitly provided with the XSD open option.

## Mapping of XML structure to OGR layers and fields

The driver scans the XML schemas referenced by the XML/GML to build the OGR layers and fields. It is strictly required that the schemas, directly or indirectly used, are fully valid. The content of the XML/GML file itself is marginally used, mostly to determine the SRS of geometry columns.

XML elements declared at the top level of a schema will generally be exposed as OGR layers. Their attributes and sub-elements of simple XML types (string, integer, real, ...) will be exposed as OGR fields. For sub-elements of complex type, different cases can happen. If the cardinality of the sub-element is at most one and it is not referenced by other elements, then it is "flattened" into its enclosing element. Otherwise it will be exposed as a OGR layer, with either a link to its "parent" layer if the sub-element is specific to its parent element, or through a junction table if the sub-element is shared by several parents.

By default the driver is robust to documents non strictly conforming to the schemas. Unexpected content in the document will be silently ignored, as well as content required by the schema and absent from the document.

Consult the GMLAS mapping examples page for more details.

By default in the configuration, swe:DataRecord and swe:DataArray elements from the Sensor Web Enablement (SWE) Common Data Model namespace will receive a special processing, so they are mapped more naturally to OGR concepts. The swe:field elements will be mapped as OGR fields, and the swe:values element of a swe:DataArray will be parsed into OGR features in a dedicated layer for each swe:DataArray. Note that those conveniency exposure is for read-only purpose. When using the write side of the driver, only the content of the general mapping mechanisms will be used.

## Metadata layers

Three special layers "_ogr_fields_metadata", "_ogr_layers_metadata", "_ogr_layer_relationships" and "_ogr_other_metadata" add extra information to the basic ones you can get from the OGR data model on OGR layers and fields.

Those layers are exposed if the EXPOSE_METADATA_LAYERS open option is set to YES (or if enabled in the configuration). They can also be individually retrieved by specifying their name in calls to GetLayerByName(), or on as layer names with the ogrinfo and ogr2ogr utility.

Consult the GMLAS metadata layers page for more details.

## Configuration file

A default configuration file gmlasconf.xml file is provided in the data directory of the GDAL installation. Its structure and content is documented in gmlasconf.xsd schema.

This configuration file enables the user to modify the following settings:

- whether remote schemas should be downloaded. Enabled by default.
- whether the local cache of schemas is enabled. Enabled by default.
- the path of the local cache. By default, $HOME/.gdal/gmlas_xsd_cache
- whether validation of the document against the schemas should be enabled. Disabled by default.
- whether validation error should cause dataset opening to fail. Disabled by default.
- whether the metadata layers should be exposed by default. Disabled by default.
- whether a 'ogr_pkid' field should always be generated. Disabled by default. Turning that on can be useful on layers that have a ID attribute whose uniqueness is not guaranteed among various documents. Which could cause issues when appending several documents into a target database table.
- whether layers and fields that are not used in the XML document should be removed. Disable by default.
- whether OGR array data types can be used. Enabled by default.
- whether the XML definition of the GML geometry should be reported as a OGR string field. Disabled by default.
- whether only XML elements that derive from gml:_Feature or gml:AbstractFeature should be considered in the initial pass of the schema building, when at least one element in the schemas derive from them. Enabled by default.
- several rules to configure if and how xlink:href should be resolved.
- a definition of XPaths of elements and attributes that must be ignored, so as to lighten the number of OGR layers and fields.

This file can be adapted and modified versions can be provided to the driver with the CONFIG_FILE open option. None of the elements of the configuration file are required. When they are absent, the default value indicated in the schema documentation is used.

Configuration can also be provided through other open options. Note that some open options have identical names to settings present in the configuration file. When such open option is provided, then its value will override the one of the configuration file (either the default one, or the one provided through the CONFIG_FILE open option).

## Geometry support

XML schemas only indicate the geometry type but do not constraint the spatial reference systems (SRS), so it is theoretically possible to have object instances of the same class having different SRS for the same geometry field. This is not practical to deal with, so when geometry fields are detected, an initial scan of the document is done to find the first geometry of each geometry field that has an explicit srsName set. This one will be used for the whole geometry field. In case other geometries of the same field would have different SRS, they will be reprojected.

By default, only the OGR geometry built from the GML geometry is exposed in the OGR feature. It is possible to change the IncludeGeometryXML setting of the configuration file to true so as to expose a OGR string field with the XML definition of the GML geometry.

## Performance issues with large multi-layer GML files.

Traditionnaly to read a OGR datasource, one iterate over layers with GDALDataset::GetLayer(), and for each layer one iterate over features with OGRLayer::GetNextFeature(). While this approach still works for the GMLAS driver, it may result in very poor performance on big documents or documents using complex schemas that are translated in many OGR layers.

It is thus recommended to use GDALDataset::GetNextFeature() to iterate over features as soon as they appear in the .gml/.xml file. This may return features from non-sequential layers, when the features include nested elements.

## Open options

- XSD=filename(s): to specify an explicit XSD application schema to use (or a list of filenames, provided they are comma separated). "http://" or "https://" URLs can be used. This option is not required when the XML/GML document has a schemaLocation attribute with valid links in its root element.
- CONFIG_FILE=filename or inline XML definition: filename of a XML configuration file conforming to the gmlasconf.xsd schema. It is also possible to provide the XML content directly inlined provided that the very first characters are <Configuration.
- EXPOSE_METADATA_LAYERS=YES/NO: whether the metadata layers "_ogr_fields_metadata", "_ogr_layers_metadata", "_ogr_layer_relationships" and "ogr_other_metadata" should be reported by default. Default is NO.
- VALIDATE=YES/NO: whether the document should be validated against the schemas. Validation is done at dataset opening. Default is NO.
- FAIL_IF_VALIDATION_ERROR=YES/NO: Whether a validation error should cause dataset opening to fail. (only used if VALIDATE=YES) Default is NO.
- REFRESH_CACHE=YES/NO: Whether remote schemas and documents pointed by xlink:href links should be downloaded from the server even if already present in the local cache. If the cache is enabled, it will be refreshed with the newly downloaded resources. Default is NO.
- SWAP_COORDINATES=AUTO/YES/NO: Whether the order of the x/y or long/lat coordinates should be swapped. In AUTO mode, the driver will determine if swapping must be done from the srsName. If the srsName is urn:ogc:def:crs:EPSG::XXXX and that the order of coordinates in the EPSG database for this SRS is lat,long or northing,easting, then the driver will swap them to the GIS friendly order (long,lat or easting,northing). For other forms of SRS (such as EPSG:XXXX), GIS friendly order is assumed and thus no swapping is done. When SWAP_COORDINATES is set to YES, coordinates will be always swapped regarding the order they appear in the GML, and when it set to NO, they will be kept in the same order. The default is AUTO.
- REMOVE_UNUSED_LAYERS=YES/NO: Whether unused layers should be removed from the reported layers. Defaults to NO
- REMOVE_UNUSED_FIELDS=YES/NO: Whether unused fields should be removed from the reported layers. Defaults to NO
- HANDLE_MULTIPLE_IMPORTS=YES/NO: Whether multiple imports with the same namespace but different schema are allowed. Defaults to NO
- SCHEMA_FULL_CHECKING=YES/NO: Whether to be pedantic with XSD checking or to be forgiving e.g. if the invalid part of the schema is not referenced in the main document. Defaults to NO

## Creation support

The GMLAS driver can write XML documents in a schema-driven way by converting a source dataset (contrary to most other drivers that have read support that implement the CreateLayer() and CreateFeature() interfaces). The typical workflow is to use the read side of the GMLAS driver to produce a SQLite/Spatialite/ PostGIS database, potentially modify the features imported and re-export this database as a new XML document.

The driver will identify in the source dataset "top-level" layers, and in those layers will find which features are not referenced by other top-level layers. As the creation of the output XML is schema-driver, the schemas need to be available. There are two possible ways:

- either the result of the processing of the schemas was stored as the 4 _ogr_* metadata tables in the source dataset by using the EXPOSE_METADATA_LAYERS=YES open option when converting the source .xml),
- or the schemas can be specified at creation time with the INPUT_XSD creation option.

By default, the driver will "wrap" the features inside a WFS 2.0 wfs:FeatureCollection / wfs:member element. It is also possible to ask the driver to create instead a custom wrapping .xsd file that declares the ogr_gmlas:FeatureCollection / ogr_gmlas:featureMember XML elements.

Note that while the file resulting from the export should be XML valid, there is no strong guarantee that it will validate against the additional constraints expressed in XML schema(s). This will depend on the content of the features (for example if converting from a GML file that is not conformant to the schemas, the output of the driver will generally be not validating)

If the input layers have geometries stored as GML content in a _xml suffixed field, then the driver will compare the OGR geometry built from that XML content with the OGR geometry stored in the dedicated geometry field of the feature. If both match, then the GML content stored in the _xml suffixed field will be used, such as to preserve particularities of the initial GML content. Otherwise GML will be exported from the OGR geometry.

To increase export performance on very large databases, creating attribute indexes on the fields pointed by the 'layer_pkid_name' attribute in '_ogr_layers_metadata' might help.

### ogr2ogr behaviour

When using ogr2ogr / GDALVectorTranslate() to convert to XML/GML from a source database, there are restrictions to the options that can be used. Only the following options of ogr2ogr are supported:

- dataset creation options (see below)
- layer names
- spatial filter through -spat option.
- attribute filter through -where option

The effect of spatial and attribute filtering will only apply on top-levels layers. Sub-features selected through joins will not be affected by those filters.

### Dataset creation options

The supported dataset creation options are:

- INPUT_XSD=filename(s): to specify an explicit XSD application schema to use (or a list of filenames, provided they are comma separated). "http://" or "https://" URLs can be used. This option is not required when the source dataset has a _ogr_other_metadata with schemas and locations filled.
- CONFIG_FILE=filename or inline XML definition: filename of a XML configuration file conforming to the gmlasconf.xsd schema. It is also possible to provide the XML content directly inlined provided that the very first characters are <Configuration>.
- LAYERS=layers. Comma separated list of layers to export as top-level features. The special value "{SPATIAL_LAYERS}" can also be used to specify all layers that have geometries. When LAYERS is not specified, the driver will identify in the source dataset "top-level" layers, and in those layers will find which features are not referenced by other top-level layers.
- SRSNAME_FORMAT=SHORT/OGC_URN/OGC_URL (Only valid for GML 3 output) Defaults to OGC_URL. If SHORT, then srsName will be in the form AUTHORITY_NAME:AUTHORITY_CODE If OGC_URN, then srsName will be in the form urn:ogc:def:crs:AUTHORITY_NAME::AUTHORITY_CODE If OGC_URL, then srsName will be in the form http://www.opengis.net/def/crs/AUTHORITY_NAME/0/AUTHORITY_CODE For OGC_URN and OGC_URL, in the case the SRS is a SRS without explicit AXIS order, but that the same SRS authority code imported with ImportFromEPSGA() should be treated as lat/long or northing/easting, then the function will take care of coordinate order swapping.
- INDENT_SIZE=[0-8]. Number of spaces for each indentation level. Default is 2.
- COMMENT=string. Comment to add at top of generated XML file as a XML comment.
- LINEFORMAT=CRLF/LF. End-of-line sequence to use. Defaults to CRLF on Windows and LF on other platforms.
- WRAPPING=WFS2_FEATURECOLLECTION/GMLAS_FEATURECOLLECTION. Whether to wrap features in a wfs:FeatureCollection or in a ogr_gmlas:FeatureCollection. Defaults to WFS2_FEATURECOLLECTION.
- TIMESTAMP=XML date time. User-specified XML dateTime value for timestamp to use in wfs:FeatureCollection attribute. If not specified, current date time is used. Only valid for WRAPPING=WFS2_FEATURECOLLECTION.
- WFS20_SCHEMALOCATION=Path or URL to wfs.xsd. Only valid for WRAPPING=WFS2_FEATURECOLLECTION. Default is "http://schemas.opengis.net/wfs/2.0/wfs.xsd"
- GENERATE_XSD=YES/NO. Whether to generate a .xsd file that has the structure of the wrapping ogr_gmlas:FeatureCollection / ogr_gmlas:featureMember elements. Only valid for WRAPPING=GMLAS_FEATURECOLLECTION. Default to YES.
- OUTPUT_XSD_FILENAME=string. Wrapping .xsd filename. If not specified, same basename as output file with .xsd extension. Note that it is possible to use this option even if GENERATE_XSD=NO, so that the wrapping .xsd appear in the schemaLocation attribute of the .xml file. Only valid for WRAPPING=GMLAS_FEATURECOLLECTION

## Examples

Listing content of a data file:

    ogrinfo -ro GMLAS:my.gml

Converting to PostGIS:

    ogr2ogr -f PostgreSQL PG:'host=myserver dbname=warmerda' GMLAS:my.gml -nlt CONVERT_TO_LINEAR

Converting to Spatialite and back to GML

    ogr2ogr -f SQLite tmp.sqlite GMLAS:in.gml -dsco SPATILIATE=YES -nlt CONVERT_TO_LINEAR -oo EXPOSE_METADATA_LAYERS=YES
    ogr2ogr -f GMLAS out.gml tmp.sqlite

## See Also

- GML: general purpose driver not requiring the presence of schemas, but with limited support for complex features
- NAS/ALKIS: specialized GML driver for cadastral data in Germany

## Credits

Initial implementation has been funded by the European Union's Earth observation programme Copernicus, as part of the tasks delegated to the European Environment Agency.

Development of special processing of some Sensor Web Enablement (SWE) Common Data Model swe:DataRecord and swe:DataArray constructs has been funded by Bureau des Recherches Géologiques et Minières (BRGM).