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GML Format

Description​

GML (Geography Markup Language) is an XML-based standard for encoding geographic information. It's an OGC (Open Geospatial Consortium) standard used for exchanging geographic data. GML supports complex geographic features with multiple geometry types.

File Extensions​

  • .gml - GML files

Implementation Details​

Reading​

The GML implementation:

  • Parses GML FeatureCollection elements
  • Extracts featureMember elements
  • Supports GML 2.0 and 3.2 namespaces
  • Extracts geometry (Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon)
  • Extracts feature properties
  • Converts to GeoJSON-like feature format
  • Uses lxml library for XML parsing

Writing​

Writing support:

  • Writes GML FeatureCollection structure
  • Converts GeoJSON-like features to GML
  • Supports Point, LineString, Polygon geometries
  • Uses GML 3.2 namespace
  • Writes properties as feature attributes

Key Features​

  • OGC standard: Official OGC standard for geographic data
  • Multiple geometries: Supports Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon
  • Feature properties: Extracts and writes feature attributes
  • Namespace support: Handles GML 2.0 and 3.2 namespaces
  • Complex structures: Supports nested geographic structures

Usage​

from iterable import open_iterable

# Basic reading
with open_iterable('data.gml') as source:
    for feature in source:
        print(feature)  # Each feature is a dict with geometry and properties
        # feature['type'] == 'Feature'
        # feature['geometry'] contains geometry data
        # feature['properties'] contains attributes

# Writing
with open_iterable('output.gml', mode='w') as dest:
    feature = {
        'type': 'Feature',
        'geometry': {'type': 'Point', 'coordinates': [102.0, 0.5]},
        'properties': {
            'name': 'Location',
            'id': '1'
        }
    }
    dest.write(feature)

Parameters​

  • prefix_strip (bool): Strip XML namespace prefixes (default: True)
  • feature_member (str): Name of feature member element (default: featureMember)
  • encoding (str): File encoding (default: utf8)

Limitations​

  1. lxml dependency: Requires lxml package for XML parsing
  2. Memory usage: All features are loaded into memory during reading
  3. GML complexity: Full GML standard is very complex; this is a simplified implementation
  4. Coordinate systems: Does not handle CRS (Coordinate Reference System) transformations

Compression Support​

GML files can be compressed with all supported codecs:

  • GZip (.gml.gz)
  • BZip2 (.gml.bz2)
  • LZMA (.gml.xz)
  • LZ4 (.gml.lz4)
  • ZIP (.gml.zip)
  • Brotli (.gml.br)
  • ZStandard (.gml.zst)

Use Cases​

  • OGC workflows: Working with OGC-compliant geographic data
  • GIS systems: Exchanging data between GIS systems
  • Spatial data: Processing spatial data in standard format
  • Government data: Many government agencies use GML

Error Handling​

  • Missing dependency: optional libraries raise ImportError with an install hint (pip install 'iterabledata[<extra>]' when an extra exists).
  • Write mode: read-only formats raise WriteNotSupportedError or ValueError when opened with mode="w".
  • Bad or unsupported input: may raise ValueError, OSError, or library-specific errors.
  • See Troubleshooting for decoding, detection, and engine issues.
  • KML - Keyhole Markup Language
  • GeoJSON - JSON-based geographic format
  • Shapefile - ESRI Shapefile format
  • GeoPackage - SQLite-based geospatial format
  • XML - Base XML format