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:
GML_PARSER=[EXPAT/XERCES]: Select the XML parser. See Parsers.
GML_INVERT_AXIS_ORDER_IF_LAT_LONG=[YES/NO]: See CRS support. Equivalent of
INVERT_AXIS_ORDER_IF_LAT_LONG
open option.GML_CONSIDER_EPSG_AS_URN=[YES/NO]: See CRS support. Equivalent of
CONSIDER_EPSG_AS_URN
open option.GML_SWAP_COORDINATES=[AUTO/YES/NO]: Defaults to
AUTO
. See CRS support. Equivalent ofSWAP_COORDINATES
open option.GML_FIELDTYPES=ALWAYS_STRING: If set to
ALWAYS_STRING
, treat all fields as strings instead of scanning values to detect field types. See Schema.GML_ATTRIBUTES_TO_OGR_FIELDS=[YES/NO]: If
YES
, create fields from attributes of GML elements. See Schema.GML_GFS_TEMPLATE=<filename>: Equivalent of
GFS_TEMPLATE
. See Schema.GML_GET_SECONDARY_GEOM=[YES/NO]: Retrieve node elements of TopoCurve geometries instead of edges. See Geometry reading.
GML_SKIP_RESOLVE_ELEMS=[NONE/ALL/HUGE/<list>]: Defaults to
ALL
. Control the gml:xlink resolving. See gml:xlink resolving.GML_SAVE_RESOLVED_TO=[SAME/<filename>]: Control where file resolved by gml:xlink is saved. See gml:xlink resolving.
GML_FACE_HOLE_NEGATIVE=[YES/NO]: Defaults to
NO
. Select interpretation rule for TopoSurfaces. See TopoSurface interpretation rules [polygons and internal holes].GML_EXPOSE_FID=[YES/NO]: Equivalent of
EXPOSE_FID
.GML_EXPOSE_GML_ID=[YES/NO]: Equivalent of
EXPOSE_GML_ID
.GML_READ_MODE=[AUTO/STANDARD/SEQUENTIAL_LAYERS/INTERLEAVED_LAYERS]: Equivalent of
READ_MODE
. See Performance issues with large multi-layer GML files..GML_DOWNLOAD_SCHEMA=[YES/NO]: (GDAL >= 3.10) Equivalent of
DOWNLOAD_SCHEMA
.GML_USE_SCHEMA_IMPORT=[YES/NO]: Equivalent of
USE_SCHEMA_IMPORT
.
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.
gml:xlink resolving
gml:xlink resolving is supported. When the resolver finds
an element containing the tag xlink:href, it tries to find the
corresponding element with the gml:id in the same gml file, other gml
file in the file system or on the web using cURL. Set the configuration
option GML_SKIP_RESOLVE_ELEMS
to NONE to enable resolution.
By default the resolved file will be saved in the same directory as the
original file with the extension ".resolved.gml", if it doesn't exist
already. This behavior can be changed using the configuration option
GML_SAVE_RESOLVED_TO
. Set it to SAME to overwrite the original
file. Set it to a filename ending with .gml to save it to that
location. Any other values are ignored. If the resolver cannot write to
the file for any reason, it will try to save it to a temporary file
generated using CPLGenerateTempFilename("ResolvedGML"); if it cannot,
resolution fails.
Note that the resolution algorithm is not optimized for large files. For
files with more than a couple of thousand xlink:href tags, the process
can go beyond a few minutes. A rough progress is displayed through
CPLDebug() for every 256 links. It can be seen by setting the
environment variable CPL_DEBUG
. The resolution time can be reduced if
you know any elements that will not be needed. Mention a comma separated
list of names of such elements with the configuration option
GML_SKIP_RESOLVE_ELEMS
. Set it to ALL to skip
resolving altogether (default action). Set it to NONE to resolve all
the xlinks.
An alternative resolution method is available.
This alternative method will be activated using the configuration option
GML_SKIP_RESOLVE_ELEMS=HUGE
. In this case any
gml:xlink will be resolved using a temporary SQLite DB so to identify any corresponding
gml:id relation. At the end of this SQL-based process, a resolved file
will be generated exactly as in the NONE case but without their
limits. The main advantages in using an external (temporary) DBMS so to
resolve gml:xlink and gml:id relations are the following:
no memory size constraints. The NONE method stores the whole GML node-tree in-memory; and this practically means that no GML file bigger than 1 GB can be processed at all using a 32-bit platform, due to memory allocation limits. Using a file-system based DBMS avoids at all this issue.
by far better efficiency, most notably when huge GML files containing many thousands (or even millions) of xlink:href / gml:id relational pairs.
using the
GML_SKIP_RESOLVE_ELEMS=HUGE
method realistically allows to successfully resolve some really huge GML file (3GB+) containing many millions xlink:href / gml:id in a reasonable time (about an hour or so on).The
GML_SKIP_RESOLVE_ELEMS=HUGE
method supports the following further configuration option:
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:
setting
GML_FACE_HOLE_NEGATIVE=NO
(default option) will activate the newest interpretation rulebut explicitly setting
GML_FACE_HOLE_NEGATIVE=YES
still enables to activate the old interpretation rule
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 likeCONSIDER_EPSG_AS_URN
andINVERT_AXIS_ORDER_IF_LAT_LONG
. WhenSWAP_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.
Note
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 ifXSISCHEMAURI
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 bySRSNAME_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 forFORMAT=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 whenFORMAT=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 ToscanaSupport 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)