GML - Geography Markup Language

Driver short name

GML

Build dependencies

(read support needs Xerces or libexpat)

OGR has limited support for GML reading and writing. Update of existing files is not supported.

Supported GML flavors :

Read

Write

GML2 and GML3 that can

GML 2.1.2 or GML 3 SF-0

be translated into simple feature model

(GML 3.1.1 Compliance level SF-0)

Starting with GDAL 2.2, another driver, GMLAS, for GML driven by application schemas, is also available. Both GML and GMLAS drivers have their use cases.

Driver capabilities

Supports Create()

This driver supports the GDALDriver::Create() operation

Supports Georeferencing

This driver supports georeferencing

Supports VirtualIO

This driver supports virtual I/O operations (/vsimem/, etc.)

Configuration options

Configuration options can be specified in command-line tools using the syntax --config <NAME>=<VALUE> or using functions such as CPLSetConfigOption() (C) or gdal.config_options (Python). The following configuration options are available:

Parsers

The reading part of the driver only works if OGR is built with Xerces linked in. When Xerces is unavailable, read support also works if OGR is built with Expat linked in. XML validation is disabled by default. GML writing is always supported, even without Xerces or Expat.

Note: if both Xerces and Expat are available at build time, the GML driver will preferentially select at runtime the Expat parser for cases where it is possible (GML file in a compatible encoding), and default back to Xerces parser in other cases. However, the choice of the parser can be overridden by specifying the GML_PARSER configuration option to EXPAT or XERCES.

CRS support

The GML driver has coordinate system support. This is only reported when all the geometries of a layer have a srsName attribute, whose value is the same for all geometries. For srsName such as "urn:ogc:def:crs:EPSG:" (or "http://www.opengis.net/def/crs/EPSG/0/" starting with GDAL 2.1.2), for geographic coordinate systems (as returned by WFS 1.1.0 for example), the axis order should be (latitude, longitude) as required by the standards, but this is unusual and can cause issues with applications unaware of axis order. So by default, the driver will swap the coordinates so that they are in the (longitude, latitude) order and report a SRS without axis order specified. It is possible to get the original (latitude, longitude) order and SRS with axis order by setting the configuration option GML_INVERT_AXIS_ORDER_IF_LAT_LONG to NO.

There also situations where the srsName is of the form "EPSG:XXXX" (whereas "urn:ogc:def:crs:EPSG::XXXX" would have been more explicit on the intent) and the coordinates in the file are in (latitude, longitude) order. By default, OGR will not consider the EPSG axis order and will report the coordinates in (latitude,longitude) order. However, if you set the configuration option GML_CONSIDER_EPSG_AS_URN to YES, the rules explained in the previous paragraph will be applied.

The above also applied for projected coordinate systems whose EPSG preferred axis order is (northing, easting).

Starting with GDAL 2.1.2, the SWAP_COORDINATES open option (or GML_SWAP_COORDINATES configuration option) can be set to AUTO/YES/NO. It controls 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 and value of other options like CONSIDER_EPSG_AS_URN and INVERT_AXIS_ORDER_IF_LAT_LONG. 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.

Schema

In contrast to most GML readers, the OGR GML reader does not require the presence of an XML Schema definition of the feature classes (file with .xsd extension) to be able to read the GML file. If the .xsd file is absent or OGR is not able to parse it, the driver attempts to automatically discover the feature classes and their associated properties by scanning the file and looking for "known" gml objects in the gml namespace to determine the organization. While this approach is error prone, it has the advantage of working for GML files even if the associated schema (.xsd) file has been lost.

It is possible to specify an explicit filename for the XSD schema to use, by using "a_filename.gml,xsd=another_filename.xsd" as a connection string. The XSD can also be specified as the value of the XSD open option.

The first time a GML file is opened, if the associated .xsd is absent or could not been parsed correctly, it is completely scanned in order to determine the set of featuretypes, the attributes associated with each and other dataset level information. This information is stored in a .gfs file with the same basename as the target gml file. Subsequent accesses to the same GML file will use the .gfs file to predefine dataset level information accelerating access. To a limited extent the .gfs file can be manually edited to alter how the GML file will be parsed. Be warned that the .gfs file will be ignored if the associated .gml file has a newer timestamp.

When prescanning the GML file to determine the list of feature types, and fields, the contents of fields are scanned to try and determine the type of the field. In some applications it is easier if all fields are just treated as string fields. This can be accomplished by setting the configuration option GML_FIELDTYPES to the value ALWAYS_STRING.

The GML_ATTRIBUTES_TO_OGR_FIELDS configuration option can be set to YES so that attributes of GML elements are also taken into account to create OGR fields.

You can use GML_GFS_TEMPLATE configuration option (or GFS_TEMPLATE open option) set to a path_to_template.gfs in order to unconditionally use a predefined GFS file. This option is really useful when you are planning to import many distinct GML files in subsequent steps [-append] and you absolutely want to preserve a fully consistent data layout for the whole GML set. Please, pay attention not to use the -lco LAUNDER=yes setting when using GML_GFS_TEMPLATE; this should break the correct recognition of attribute names between subsequent GML import runs.

Particular GML application schemas

Feature attributes in nested GML elements (non-flat attribute hierarchy) that can be found in some GML profiles, such as UK Ordnance Survey MasterMap, are detected. IntegerList, RealList and StringList field types when a GML element has several occurrences are also supported.

A specialized GML driver - the NAS driver - is available to read German AAA GML Exchange Format (NAS/ALKIS).

The GML driver has partial support for reading AIXM or CityGML files.

The GML driver supports reading :

The GML driver supports reading responses to CSW GetRecords queries.

Since OGR 2.2, the GML driver supports reading Japanese FGD GML v4 files.

Geometry reading

When reading a feature, the driver will by default only take into account the last recognized GML geometry found (in case they are multiples) in the XML subtree describing the feature.

But, if the .xsd schema is understood by the XSD parser and declares several geometry fields, or the .gfs file declares several geometry fields, multiple geometry fields will be reported by the GML driver according to RFC 41 : Support for multiple geometry fields in OGR.

In case of multiple geometry occurrences, if a geometry is in a <geometry> element, this will be the one selected. This will make default behavior consistent with Inspire objects.

The user can change the .gfs file to select the appropriate geometry by specifying its path with the <GeometryElementPath> element. See the description of the .gfs syntax below.

GML geometries including TopoCurve, TopoSurface, MultiCurve are also supported. The TopoCurve type GML geometry can be interpreted as either of two types of geometries. The Edge elements in it contain curves and their corresponding nodes. By default only the curves, the main geometries, are reported as OGRMultiLineString. To retrieve the nodes, as OGRMultiPoint, the configuration option GML_GET_SECONDARY_GEOM should be set to the value YES. When this is set only the secondary geometries are reported.

Arc, ArcString, ArcByBulge, ArcByCenterPoint, Circle and CircleByCenterPoints will be returned as circular string OGR geometries. If they are included in other GML elements such as CurveComposite, MultiCurve, Surface, corresponding non-linear OGR geometries will be returned as well. When reading GML3 application schemas, declarations of geometry fields such as CurvePropertyType, SurfacePropertyType, MultiCurvePropertyType or MultiSurfacePropertyType will be also interpreted as being potential non-linear geometries, and corresponding OGR geometry type will be used for the layer geometry type.

TopoSurface interpretation rules [polygons and internal holes]

The GML driver is able to recognize two different interpretation rules for TopoSurface when a polygon contains any internal hole:

  • the previously supported interpretation rule assumed that:

    • each TopoSurface may be represented as a collection of many Faces

    • positive Faces [i.e. declaring orientation="+"] are assumed to represent the Exterior Ring of some Polygon.

    • negative Faces [i.e. declaring orientation="-"] are assumed to represent an Interior Ring (aka hole) belonging to the latest declared Exterior Ring.

    • ordering any Edge used to represent each Ring is important: each Edge is expected to be exactly adjacent to the next one.

  • the new interpretation rule now assumes that:

    • each TopoSurface may be represented as a collection of many Faces

    • the declared orientation for any Face has nothing to deal with Exterior/Interior Rings

    • each Face is now intended to represent a complete Polygon, eventually including any possible Interior Ring (holes)

    • the relative ordering of any Edge composing the same Face is completely not relevant

The newest interpretation seems to fully match GML 3 standard recommendations; so this latest is now assumed to be the default interpretation supported by OGR.

NOTE : Using the newest interpretation requires GDAL/OGR to be built against the GEOS library.

Using the GML_FACE_HOLE_NEGATIVE configuration option you can select the actual interpretation to be applied when parsing GML 3 topologies:

Encoding issues

Expat library supports reading the following built-in encodings :

  • US-ASCII

  • UTF-8

  • UTF-16

  • ISO-8859-1

  • Windows-1252

The content returned by OGR will be encoded in UTF-8, after the conversion from the encoding mentioned in the file header is.

If the GML file is not encoded in one of the previous encodings and the only parser available is Expat, it will not be parsed by the GML driver. You may convert it into one of the supported encodings with the iconv utility for example and change accordingly the encoding parameter value in the XML header.

When writing a GML file, the driver expects UTF-8 content to be passed in.

Note: The .xsd schema files are parsed with an integrated XML parser which does not currently understand XML encodings specified in the XML header. It expects encoding to be always UTF-8. If attribute names in the schema file contains non-ascii characters, it is better to use iconv utility and convert the .xsd file into UTF-8 encoding first.

Feature id (fid / gml:id)

The driver exposes the content of the gml:id attribute as a string field called gml_id, when reading GML WFS documents. When creating a GML3 document, if a field is called gml_id, its content will also be used to write the content of the gml:id attribute of the created feature.

The driver autodetects the presence of a fid (GML2) (resp. gml:id (GML3)) attribute at the beginning of the file, and, if found, exposes it by default as a fid (resp. gml_id) field. The autodetection can be overridden by specifying the GML_EXPOSE_FID or GML_EXPOSE_GML_ID configuration option to YES or NO.

When creating a GML2 document, if a field is called fid, its content will also be used to write the content of the fid attribute of the created feature.

Performance issues with large multi-layer GML files.

There is only one GML parser per GML datasource shared among the various layers. By default, the GML driver will restart reading from the beginning of the file, each time a layer is accessed for the first time, which can lead to poor performance with large GML files.

The GML_READ_MODE configuration option can be set to SEQUENTIAL_LAYERS if all features belonging to the same layer are written sequentially in the file. The reader will then avoid unnecessary resets when layers are read completely one after the other. To get the best performance, the layers must be read in the order they appear in the file.

If no .xsd and .gfs files are found, the parser will detect the layout of layers when building the .gfs file. If the layers are found to be sequential, a <SequentialLayers>true</SequentialLayers> element will be written in the .gfs file, so that the GML_READ_MODE will be automatically initialized to SEQUENTIAL_LAYERS if not explicitly set by the user.

The GML_READ_MODE configuration option can be set to INTERLEAVED_LAYERS to be able to read a GML file whose features from different layers are interleaved. In the case, the semantics of the GetNextFeature() will be slightly altered, in a way where a NULL return does not necessarily mean that all features from the current layer have been read, but it could also mean that there is still a feature to read, but that belongs to another layer. In that case, the file should be read with code similar to the following one :

int nLayerCount = poDS->GetLayerCount();
int bFoundFeature;
do
{
    bFoundFeature = FALSE;
    for( int iLayer = 0; iLayer < nLayerCount; iLayer++ )
    {
        OGRLayer   *poLayer = poDS->GetLayer(iLayer);
        OGRFeature *poFeature;
        while((poFeature = poLayer->GetNextFeature()) != NULL)
        {
            bFoundFeature = TRUE;
            poFeature->DumpReadable(stdout, NULL);
            OGRFeature::DestroyFeature(poFeature);
        }
    }
} while (bInterleaved && bFoundFeature);

Open options

Open options can be specified in command-line tools using the syntax -oo <NAME>=<VALUE> or by providing the appropriate arguments to GDALOpenEx() (C) or gdal.OpenEx (Python). The following open options are supported:

  • XSD=<filename>: to specify an explicit filename for the XSD application schema to use.

  • WRITE_GFS=[AUTO​/​YES​/​NO]: (GDAL >= 3.1) whether to write a .gfs file. In AUTO mode, the .gfs file is only written if there is no recognized .xsd file, no existing .gfs file and for non-network file systems. This option can be set to YES for force .gfs file writing in situations where AUTO would not attempt to do it. Or it can be set to NO to disable .gfs file writing.

  • GFS_TEMPLATE=<filename>: to unconditionally use a predefined GFS file. This option is really useful when you are planning to import many distinct GML files in subsequent steps [-append] and you absolutely want to preserve a fully consistent data layout for the whole GML set. Please, pay attention not to use the -lco LAUNDER=yes setting when this option; this should break the correct recognition of attribute names between subsequent GML import runs.

  • FORCE_SRS_DETECTION=[YES​/​NO]: Defaults to NO. Force a full scan to detect the SRS of layers. This option may be needed in the case where the .gml file is accompanied with a .xsd. Normally in that situation, OGR would not detect the SRS, because this requires to do a full scan of the file.

  • EMPTY_AS_NULL=[YES​/​NO]: Defaults to YES. If YES, fields with empty content will be reported as being NULL, instead of being an empty string. This is the historic behavior. However this will prevent such fields to be declared as not-nullable if the application schema declared them as mandatory. So this option can be set to NO to have both empty strings being report as such, and mandatory fields being reported as not nullable.

  • GML_ATTRIBUTES_TO_OGR_FIELDS=[YES​/​NO]: Defaults to NO. Whether GML attributes should be reported as OGR fields. Note that this option has only an effect the first time a GML file is opened (before the .gfs file is created), and if it has no valid associated .xsd.

  • INVERT_AXIS_ORDER_IF_LAT_LONG=[YES​/​NO]: Defaults to YES. Whether to present SRS and coordinate ordering in traditional GIS order.

  • CONSIDER_EPSG_AS_URN=[YES​/​NO​/​AUTO]: Defaults to AUTO.

    Whether to consider srsName like EPSG:XXXX as respecting EPSG axis order.

  • SWAP_COORDINATES=[AUTO​/​YES​/​NO]: Defaults to AUTO. 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 and value of other options like CONSIDER_EPSG_AS_URN and INVERT_AXIS_ORDER_IF_LAT_LONG. 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.

  • READ_MODE=[AUTO​/​STANDARD​/​SEQUENTIAL_LAYERS​/​INTERLEAVED_LAYERS]: Defaults to AUTO. Read mode.

  • EXPOSE_GML_ID=[YES​/​NO​/​AUTO]: Defaults to AUTO. Whether to make feature gml:id as a gml_id attribute.

  • EXPOSE_FID=[YES​/​NO​/​AUTO]: Defaults to AUTO. Whether to make feature fid as a fid attribute.

  • DOWNLOAD_SCHEMA=[YES​/​NO]: Defaults to YES. Whether to download the remote application schema if needed (only if the document looks like a WFS response currently).

  • REGISTRY=<filename>: Defaults to {GDAL_DATA}/gml_registry.xml.. Filename of the registry with application schemas.

  • USE_BBOX=[YES​/​NO]: (GDAL >= 3.7.1) Defaults to NO. Whether to use gml:boundedBy at feature level as feature geometry, if there are no other geometry.

  • USE_SCHEMA_IMPORT=[YES​/​NO]: Defaults to NO. Whether to use schema imports in XSD files so that the feature types corresponding to imported schema can be detected.

注釈

When changing the value of most of the above options, it is recommended to delete the .gfs file if it pre-exists, otherwise mis-behavior might be observed.

Creation Issues

On export all layers are written to a single GML file all in a single feature collection. Each layer's name is used as the element name for objects from that layer. Geometries are always written as the ogr:geometryProperty element on the feature.

Dataset creation options

Dataset creation options can be specified in command-line tools using the syntax -dsco <NAME>=<VALUE> or by providing the appropriate arguments to GDALCreate() (C) or Driver.Create (Python). The following dataset creation options are supported:

  • XSISCHEMAURI=value: If provided, this URI will be inserted as the schema location. Note that the schema file isn't actually accessed by OGR, so it is up to the user to ensure it will match the schema of the OGR produced GML data file.

  • XSISCHEMA=[EXTERNAL​/​INTERNAL​/​OFF]: Defaults to EXTERNAL. If EXTERNAL. This writes a GML application schema file to a corresponding .xsd file (with the same basename). If INTERNAL is used the schema is written within the GML file, but this is experimental and almost certainly not valid XML. OFF disables schema generation (and is implicit if XSISCHEMAURI is used).

  • PREFIX=value: Defaults to ogr. This is the prefix for the application target namespace.

  • STRIP_PREFIX=[TRUE​/​FALSE]: Defaults to FALSE. Can be set to TRUE to avoid writing the prefix of the application target namespace in the GML file.

  • TARGET_NAMESPACE=value: Defaults to http://ogr.maptools.org/. This is the application target namespace.

  • FORMAT=[GML2​/​GML3​/​GML3Deegree​/​GMl3.2]: Select from the following formats:

    • GML2 in order to write GML files that follow GML 2.1.2 (Default before GDAL 3.4)

    • GML3 in order to write GML files that follow GML 3.1.1 SF-0 profile.

    • GML3Deegree in order to produce a GML 3.1.1 .XSD schema, with a few variations with respect to what is recommended by GML3 SF-0 profile, but that will be better accepted by some software (such as Deegree 3).

    • GML3.2in order to write GML files that follow GML 3.2.1 SF-0 profile. (Default since GDAL 3.4)

    Non-linear geometries can be written. This is only compatible with selecting on of that above GML3 format variant. Otherwise, such geometries will be approximating into their closest matching linear geometry. Note: fields of type StringList, RealList or IntegerList can be written. This will cause to advertise the SF-1 profile in the .XSD schema (such types are not supported by SF-0).

  • GML_FEATURE_COLLECTION=[YES​/​NO]:

    Whether to use the gml:FeatureCollection, instead of creating a dedicated container element in the target namespace. Only valid for FORMAT=GML3/GML3.2. Note that gml:FeatureCollection has been deprecated in GML 3.2, and is not allowed by the OGC 06-049r1 "Geography Markup Language (GML) simple features profile" (for GML 3.1.1) and OGC 10-100r3 "Geography Markup Language (GML) simple features profile (with Corrigendum)" (for GML 3.2) specifications.

  • GML3_LONGSRS=[YES​/​NO]: (only valid when FORMAT=GML3/GML3Degree/GML3.2) Deprecated by SRSNAME_FORMAT in GDAL 2.2. Default to YES. If YES, SRS with EPSG authority will be written with the "urn:ogc:def:crs:EPSG::" prefix. 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. If set to NO, SRS with EPSG authority will be written with the "EPSG:" prefix, even if they are in lat/long order.

  • SRSNAME_FORMAT=[SHORT​/​OGC_URN​/​OGC_URL]: Defaults to OGC_URN. (Only valid for FORMAT=GML3/GML3Degree/GML3.2, GDAL >= 2.2). 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.

  • SRSDIMENSION_LOC=[POSLIST​/​GEOMETRY​/​GEOMETRY,POSLIST]: (Only valid for FORMAT=GML3/GML3Degree/GML3.2) Default to POSLIST. For 2.5D geometries, define the location where to attach the srsDimension attribute. There are diverging implementations. Some put in on the <gml:posList> element, other on the top geometry element.

  • WRITE_FEATURE_BOUNDED_BY=[YES​/​NO]: Defaults to YES. (only valid when FORMAT=GML3/GML3Degree/GML3.2) If set to NO, the <gml:boundedBy> element will not be written for each feature.

  • SPACE_INDENTATION=[YES​/​NO]: Defaults to YES. If YES, the output will be indented with spaces for more readability, but at the expense of file size.

  • GML_ID=value: Defaults to aFeatureCollection. (Only valid for GML 3.2) Value of feature collection gml:id.

  • NAME=value: Content of GML name element. Can also be set as the NAME metadata item on the dataset.

  • DESCRIPTION=value: Content of GML description element. Can also be set as the DESCRIPTION metadata item on the dataset.

VSI Virtual File System API support

The driver supports reading and writing to files managed by VSI Virtual File System API, which include "regular" files, as well as files in the /vsizip/ (read-write) , /vsigzip/ (read-write) , /vsicurl/ (read-only) domains.

Writing to /dev/stdout or /vsistdout/ is also supported. Note that in that case, only the content of the GML file will be written to the standard output (and not the .xsd). The <boundedBy> element will not be written. This is also the case if writing in /vsigzip/

Syntax of .gfs files

A XML Schema for .gfs files can be found at https://raw.githubusercontent.com/OSGeo/gdal/master/ogr/ogrsf_frmts/gml/data/gfs.xsd .

Let's consider the following test.gml file :

<?xml version="1.0" encoding="UTF-8"?>
<gml:FeatureCollection xmlns:gml="http://www.opengis.net/gml">
  <gml:featureMember>
    <LAYER>
      <attrib1>attrib1_value</attrib1>
      <attrib2container>
        <attrib2>attrib2_value</attrib2>
      </attrib2container>
      <location1container>
        <location1>
            <gml:Point><gml:coordinates>3,50</gml:coordinates></gml:Point>
        </location1>
      </location1container>
      <location2>
        <gml:Point><gml:coordinates>2,49</gml:coordinates></gml:Point>
      </location2>
    </LAYER>
  </gml:featureMember>
</gml:FeatureCollection>

and the following associated .gfs file.

<GMLFeatureClassList>
  <GMLFeatureClass>
    <Name>LAYER</Name>
    <ElementPath>LAYER</ElementPath>
    <GeometryElementPath>location1container|location1</GeometryElementPath>
    <PropertyDefn>
      <Name>attrib1</Name>
      <ElementPath>attrib1</ElementPath>
      <Type>String</Type>
      <Width>13</Width>
    </PropertyDefn>
    <PropertyDefn>
      <Name>attrib2</Name>
      <ElementPath>attrib2container|attrib2</ElementPath>
      <Type>String</Type>
      <Width>13</Width>
    </PropertyDefn>
  </GMLFeatureClass>
</GMLFeatureClassList>

Note the presence of the '|' character in the <ElementPath> and <GeometryElementPath> elements to specify the wished field/geometry element that is a nested XML element. Nested field elements are supported, as well as specifying <GeometryElementPath> If GeometryElementPath is not specified, the GML driver will use the last recognized geometry element.

The <GeometryType> element can be specified to force the geometry type. Accepted values are : 0 (any geometry type), 1 (point), 2 (linestring), 3 (polygon), 4 (multipoint), 5 (multilinestring), 6 (multipolygon), 7 (geometrycollection).

The <GeometryElementPath> and <GeometryType> can be specified as many times as there are geometry fields in the GML file. Another possibility is to define a <GeomPropertyDefn>element as many times as necessary:

<GMLFeatureClassList>
  <GMLFeatureClass>
    <Name>LAYER</Name>
    <ElementPath>LAYER</ElementPath>
    <GeomPropertyDefn>
        <Name>geometry</Name> <!-- OGR geometry name -->
        <ElementPath>geometry</ElementPath> <!-- XML element name possibly with '|' to specify the path -->
        <Type>MultiPolygon</Type>
    </GeomPropertyDefn>
    <GeomPropertyDefn>
        <Name>referencePoint</Name>
        <ElementPath>referencePoint</ElementPath>
        <Type>Point</Type>
    </GeomPropertyDefn>
  </GMLFeatureClass>
</GMLFeatureClassList>

The output of ogrinfo test.gml -ro -al is:

Layer name: LAYER
Geometry: Unknown (any)
Feature Count: 1
Extent: (3.000000, 50.000000) - (3.000000, 50.000000)
Layer SRS WKT:
(unknown)
Geometry Column = location1container|location1
attrib1: String (13.0)
attrib2: String (13.0)
OGRFeature(LAYER):0
  attrib1 (String) = attrib1_value
  attrib2 (String) = attrib2_value
  POINT (3 50)

Advanced .gfs syntax

Specifying ElementPath to find objects embedded into top level objects

Let's consider the following test.gml file :

<?xml version="1.0" encoding="utf-8"?>
<gml:FeatureCollection xmlns:xlink="http://www.w3.org/1999/xlink"
                       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                       gml:id="foo" xmlns:gml="http://www.opengis.net/gml/3.2">
  <gml:featureMember>
    <TopLevelObject gml:id="TopLevelObject.1">
      <content>
        <Object gml:id="Object.1">
          <geometry>
            <gml:Polygon gml:id="Object.1.Geometry" srsName="urn:ogc:def:crs:EPSG::4326">
              <gml:exterior>
                <gml:LinearRing>
                  <gml:posList srsDimension="2">48 2 49 2 49 3 48 3 48 2</gml:posList>
                </gml:LinearRing>
              </gml:exterior>
            </gml:Polygon>
          </geometry>
          <foo>bar</foo>
        </Object>
      </content>
      <content>
        <Object gml:id="Object.2">
          <geometry>
            <gml:Polygon gml:id="Object.2.Geometry" srsName="urn:ogc:def:crs:EPSG::4326">
              <gml:exterior>
                <gml:LinearRing>
                  <gml:posList srsDimension="2">-48 2 -49 2 -49 3 -48 3 -48 2</gml:posList>
                </gml:LinearRing>
              </gml:exterior>
            </gml:Polygon>
          </geometry>
          <foo>baz</foo>
        </Object>
      </content>
    </TopLevelObject>
  </gml:featureMember>
</gml:FeatureCollection>

By default, only the TopLevelObject object would be reported and it would only use the second geometry. This is not the desired behavior in that instance. You can edit the generated .gfs and modify it like the following in order to specify a full path to the element (top level XML element being omitted) :

<GMLFeatureClassList>
  <GMLFeatureClass>
    <Name>Object</Name>
    <ElementPath>featureMember|TopLevelObject|content|Object</ElementPath>
    <GeometryType>3</GeometryType>
    <PropertyDefn>
      <Name>foo</Name>
      <ElementPath>foo</ElementPath>
      <Type>String</Type>
    </PropertyDefn>
  </GMLFeatureClass>
</GMLFeatureClassList>

Getting XML attributes as OGR fields

The element@attribute syntax can be used in the <ElementPath> to specify that the value of attribute 'attribute' of element 'element' must be fetched.

Let's consider the following test.gml file :

<?xml version="1.0" encoding="UTF-8"?>
<gml:FeatureCollection xmlns:gml="http://www.opengis.net/gml">
  <gml:featureMember>
    <LAYER>
      <length unit="m">5</length>
    </LAYER>
  </gml:featureMember>
</gml:FeatureCollection>

and the following associated .gfs file.

<GMLFeatureClassList>
  <GMLFeatureClass>
    <Name>LAYER</Name>
    <ElementPath>LAYER</ElementPath>
    <GeometryType>100</GeometryType> <!-- no geometry -->
    <PropertyDefn>
      <Name>length</Name>
      <ElementPath>length</ElementPath>
      <Type>Real</Type>
    </PropertyDefn>
    <PropertyDefn>
      <Name>length_unit</Name>
      <ElementPath>length@unit</ElementPath>
      <Type>String</Type>
    </PropertyDefn>
  </GMLFeatureClass>
</GMLFeatureClassList>

The output of ogrinfo test.gml -ro -al is:

Layer name: LAYER
Geometry: None
Feature Count: 1
Layer SRS WKT:
(unknown)
gml_id: String (0.0)
length: Real (0.0)
length_unit: String (0.0)
OGRFeature(LAYER):0
  gml_id (String) = (null)
  length (Real) = 5
  length_unit (String) = m

Using conditions on XML attributes

A <Condition> element can be specified as a child element of a <PropertyDefn>. The content of the Condition follows a minimalistic XPath syntax. It must be of the form @attrname[=|!=]'attrvalue' [and|or other_cond]*. Note that 'and' and 'or' operators cannot be mixed (their precedence is not taken into account).

Several <PropertyDefn> can be defined with the same <ElementPath>, but with <Condition> that must be mutually exclusive.

Let's consider the following testcondition.gml file :

<?xml version="1.0" encoding="utf-8" ?>
<ogr:FeatureCollection
     xmlns:ogr="http://ogr.maptools.org/"
     xmlns:gml="http://www.opengis.net/gml">
  <gml:featureMember>
    <ogr:testcondition fid="testcondition.0">
      <ogr:name lang="en">English name</ogr:name>
      <ogr:name lang="fr">Nom francais</ogr:name>
      <ogr:name lang="de">Deutsche name</ogr:name>
    </ogr:testcondition>
  </gml:featureMember>
</ogr:FeatureCollection>

and the following associated .gfs file.

<GMLFeatureClassList>
  <GMLFeatureClass>
    <Name>testcondition</Name>
    <ElementPath>testcondition</ElementPath>
    <GeometryType>100</GeometryType>
    <PropertyDefn>
      <Name>name_en</Name>
      <ElementPath>name</ElementPath>
      <Condition>@lang='en'</Condition>
      <Type>String</Type>
    </PropertyDefn>
    <PropertyDefn>
      <Name>name_fr</Name>
      <ElementPath>name</ElementPath>
      <Condition>@lang='fr'</Condition>
      <Type>String</Type>
    </PropertyDefn>
    <PropertyDefn>
      <Name>name_others_lang</Name>
      <ElementPath>name@lang</ElementPath>
      <Condition>@lang!='en' and @lang!='fr'</Condition>
      <Type>StringList</Type>
    </PropertyDefn>
    <PropertyDefn>
      <Name>name_others</Name>
      <ElementPath>name</ElementPath>
      <Condition>@lang!='en' and @lang!='fr'</Condition>
      <Type>StringList</Type>
    </PropertyDefn>
  </GMLFeatureClass>
</GMLFeatureClassList>

The output of ogrinfo testcondition.gml -ro -al is:

Layer name: testcondition
Geometry: None
Feature Count: 1
Layer SRS WKT:
(unknown)
fid: String (0.0)
name_en: String (0.0)
name_fr: String (0.0)
name_others_lang: StringList (0.0)
name_others: StringList (0.0)
OGRFeature(testcondition):0
  fid (String) = testcondition.0
  name_en (String) = English name
  name_fr (String) = Nom francais
  name_others_lang (StringList) = (1:de)
  name_others (StringList) = (1:Deutsche name)

Registry for GML application schemas

The "data" directory of the GDAL installation contains a "gml_registry.xml" file that links feature types of GML application schemas to .xsd or .gfs files that contain their definition. This is used in case no valid .gfs or .xsd file is found next to the GML file.

An alternate location for the registry file can be defined by setting its full pathname to the GML_REGISTRY configuration option.

An example of such a file is :

<gml_registry>
    <!-- Finnish National Land Survey cadastral data -->
    <namespace prefix="ktjkiiwfs" uri="http://xml.nls.fi/ktjkiiwfs/2010/02" useGlobalSRSName="true">
        <featureType elementName="KiinteistorajanSijaintitiedot"
                 schemaLocation="http://xml.nls.fi/XML/Schema/sovellus/ktjkii/modules/kiinteistotietojen_kyselypalvelu_WFS/Asiakasdokumentaatio/ktjkiiwfs/2010/02/KiinteistorajanSijaintitiedot.xsd"/>
        <featureType elementName="PalstanTunnuspisteenSijaintitiedot"
                 schemaLocation="http://xml.nls.fi/XML/Schema/sovellus/ktjkii/modules/kiinteistotietojen_kyselypalvelu_WFS/Asiakasdokumentaatio/ktjkiiwfs/2010/02/palstanTunnuspisteenSijaintitiedot.xsd"/>
        <featureType elementName="RekisteriyksikonTietoja"
                 schemaLocation="http://xml.nls.fi/XML/Schema/sovellus/ktjkii/modules/kiinteistotietojen_kyselypalvelu_WFS/Asiakasdokumentaatio/ktjkiiwfs/2010/02/RekisteriyksikonTietoja.xsd"/>
        <featureType elementName="PalstanTietoja"
                 schemaLocation="http://xml.nls.fi/XML/Schema/sovellus/ktjkii/modules/kiinteistotietojen_kyselypalvelu_WFS/Asiakasdokumentaatio/ktjkiiwfs/2010/02/PalstanTietoja.xsd"/>
    </namespace>

    <!-- Inspire CadastralParcels schema -->
    <namespace prefix="cp" uri="urn:x-inspire:specification:gmlas:CadastralParcels:3.0" useGlobalSRSName="true">
        <featureType elementName="BasicPropertyUnit"
                     gfsSchemaLocation="inspire_cp_BasicPropertyUnit.gfs"/>
        <featureType elementName="CadastralBoundary"
                     gfsSchemaLocation="inspire_cp_CadastralBoundary.gfs"/>
        <featureType elementName="CadastralParcel"
                     gfsSchemaLocation="inspire_cp_CadastralParcel.gfs"/>
        <featureType elementName="CadastralZoning"
                     gfsSchemaLocation="inspire_cp_CadastralZoning.gfs"/>
    </namespace>

    <!-- Czech RUIAN (VFR) schema (v1) -->
    <namespace prefix="vf"
               uri="urn:cz:isvs:ruian:schemas:VymennyFormatTypy:v1 ../ruian/xsd/vymenny_format/VymennyFormatTypy.xsd"
               useGlobalSRSName="true">
        <featureType elementName="TypSouboru"
                     elementValue="OB"
                     gfsSchemaLocation="ruian_vf_ob_v1.gfs"/>
        <featureType elementName="TypSouboru"
                     elementValue="ST"
                     gfsSchemaLocation="ruian_vf_st_v1.gfs"/>
    </namespace>
</gml_registry>

XML schema definition (.xsd) files are pointed by the schemaLocation attribute, whereas OGR .gfs files are pointed by the gfsSchemaLocation attribute. In both cases, the filename can be a URL (http://, https://), an absolute filename, or a relative filename (relative to the location of gml_registry.xml).

The schema is used if and only if the namespace prefix and URI are found in the first bytes of the GML file (e.g. xmlns:ktjkiiwfs="http://xml.nls.fi/ktjkiiwfs/2010/02"), and that the feature type is also detected in the first bytes of the GML file (e.g. ktjkiiwfs:KiinteistorajanSijaintitiedot). If the element value is defined than the schema is used only if the feature type together with the value is found in the first bytes of the GML file (e.g. vf:TypSouboru>OB_UKSH).

Building junction tables

The ogr_build_junction_table.py script can be used to build a junction table from OGR layers that contain "XXXX_href" fields. Let's considering the following output of a GML file with links to other features :

OGRFeature(myFeature):1
  gml_id (String) = myFeature.1
  [...]
  otherFeature_href (StringList) = (2:#otherFeature.10,#otherFeature.20)

OGRFeature(myFeature):2
  gml_id (String) = myFeature.2
  [...]
  otherFeature_href (StringList) = (2:#otherFeature.30,#otherFeature.10)

After running

ogr2ogr -f PG PG:dbname=mydb my.gml

to import it into PostGIS and

python3 ogr_build_junction_table.py PG:dbname=mydb

, a myfeature_otherfeature table will be created and will contain the following content :

myfeature_gml_id

otherfeature_gml_id

myFeature.1

otherFeature.10

myFeature.1

otherFeature.20

myFeature.2

otherFeature.30

myFeature.2

otherFeature.10

Reading datasets resulting from a WFS 2.0 join queries

The GML driver can read datasets resulting from a WFS 2.0 join queries.

Such datasets typically look like:

<wfs:FeatureCollection xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:app="http://app.com"
    xmlns:wfs="http://www.opengis.net/wfs/2.0"
    xmlns:gml="http://www.opengis.net/gml/3.2"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    numberMatched="unknown" numberReturned="2" timeStamp="2015-01-01T00:00:00.000Z"
    xsi:schemaLocation="http://www.opengis.net/gml/3.2 http://schemas.opengis.net/gml/3.2.1/gml.xsd
                        http://www.opengis.net/wfs/2.0 http://schemas.opengis.net/wfs/2.0/wfs.xsd">
  <wfs:member>
    <wfs:Tuple>
      <wfs:member>
        <app:table1 gml:id="table1-1">
          <app:foo>1</app:foo>
        </app:table1>
      </wfs:member>
      <wfs:member>
        <app:table2 gml:id="table2-1">
          <app:bar>2</app:bar>
          <app:baz>foo</app:baz>
          <app:geometry><gml:Point gml:id="table2-2.geom.0"><gml:pos>2 49</gml:pos></gml:Point></app:geometry>
        </app:table2>
      </wfs:member>
    </wfs:Tuple>
  </wfs:member>
  <wfs:member>
    <wfs:Tuple>
      <wfs:member>
        <app:table1 gml:id="table1-2">
          <app:bar>2</app:bar>
          <app:geometry><gml:Point gml:id="table1-1.geom.0"><gml:pos>3 50</gml:pos></gml:Point></app:geometry>
        </app:table1>
      </wfs:member>
      <wfs:member>
        <app:table2 gml:id="table2-2">
          <app:bar>2</app:bar>
          <app:baz>bar</app:baz>
          <app:geometry><gml:Point gml:id="table2-2.geom.0"><gml:pos>2 50</gml:pos></gml:Point></app:geometry>
        </app:table2>
      </wfs:member>
    </wfs:Tuple>
  </wfs:member>
</wfs:FeatureCollection>

OGR will group together the attributes from the layers participating to the join and will prefix them with the layer name. So the above example will be read as the following:

OGRFeature(join_table1_table2):0
  table1.gml_id (String) = table1-1
  table1.foo (Integer) = 1
  table1.bar (Integer) = (null)
  table2.gml_id (String) = table2-1
  table2.bar (Integer) = 2
  table2.baz (String) = foo
  table2.geometry = POINT (2 49)

OGRFeature(join_table1_table2):1
  table1.gml_id (String) = table1-2
  table1.foo (Integer) = (null)
  table1.bar (Integer) = 2
  table2.gml_id (String) = table2-2
  table2.bar (Integer) = 2
  table2.baz (String) = bar
  table1.geometry = POINT (3 50)
  table2.geometry = POINT (2 50)

Geometry coordinate precision

Added in version GDAL: 3.9

The GML driver supports reading and writing the geometry coordinate precision, using the OGRGeomCoordinatePrecision settings of the OGRGeomFieldDefn. Those settings are used to round the coordinates of the geometry of the features to an appropriate decimal precision.

Implementation details: the coordinate precision is stored in the XML schema as xs:annotation/xs:appinfo[source="http://ogr.maptools.org/"]/ogr:xy_coordinate_resolution and xs:annotation/xs:appinfo[source="http://ogr.maptools.org/"]/ogr:z_coordinate_resolution optional elements in the declaration of the geometry column. Their numeric value is expressed in the units of the SRS.

Example:

<xs:element name="my_geom" type="gml:SurfacePropertyType" nillable="true" minOccurs="0" maxOccurs="1">
  <xs:annotation>
    <xs:appinfo source="http://ogr.maptools.org/">
      <ogr:xy_coordinate_resolution>8.9e-8</ogr:xy_coordinate_resolution>
      <ogr:z_coordinate_resolution>1e-3</ogr:z_coordinate_resolution>
    </xs:appinfo>
  </xs:annotation>
</xs:element>

Examples

The ogr2ogr utility can be used to dump the results of a Oracle query to GML:

ogr2ogr -f GML output.gml OCI:usr/pwd@db my_feature -where "id = 0"

The ogr2ogr utility can be used to dump the results of a PostGIS query to GML:

ogr2ogr -f GML output.gml PG:'host=myserver dbname=warmerda' -sql "SELECT pop_1994 from canada where province_name = 'Alberta'"

See Also

Credits

  • Implementation for GML_SKIP_RESOLVE_ELEMS=HUGE was contributed by A.Furieri, with funding from Regione Toscana

  • Support for cadastral data in Finnish National Land Survey GML and Inspire GML was funded by The Information Centre of the Ministry of Agriculture and Forestry (Tike)