RFC 17: Configuration of Supported Output Formats and CRSes

Author:Stephan Krauses, Martin Pačes
Last Edit:$Date$

This RFC proposes modifications of the EOxServer allowing configuration of

  • the supported output formats for WMS and WCS
  • the supported CRSes for WMS and WCS

The RFC presents the rationale and proposes data model changes and new global configuration options.


The reason for preparation of this RFC is the need to change the way how the supported (file) formats and CRSes (CRS - Coordinate Reference Systems) for raster data are handled by the EOxServer’s WCS and WMS services to assure compliance to OGC standards, interoperability and configurability of the services.

In case of WMS, the formats and CRSes shall be listed in the WMS Capabilities.

In case of WCS, the supported formats and CRSes shall be reported by the WCS Capabilities (per service parameters) and in the Coverage Descriptions (per coverage parameters). Compatibility with the WCS 2.0.1 corrigendum and the upcoming WCS 2.0 CRS Extension document shall be assured.

Currently, only the native CRS of a dataset is reported in the metadata and only a small hard-coded set output file format is announced as supported (JPEG2000, HDF4, netCDF and GeoTIFF for WCS). Hence, there is no way to configure these parameters.

Furthermore, the underlying MapServer implementation does not return proper OWS exceptions if an CRS not advertised in the service capabilities or coverage descriptions is requested.

Supported CRSes and Output Formats in OGC Web Services

The table below gives an overview over the support for reporting CRS and output format metadata in different standards implemented by EOxServer.

Support for CRS and output format metadata
Service and Version Supported CRS Supported Formats
WMS 1.1.0 per layer per service
WMS 1.1.1 per layer per service
WMS 1.3.0 per layer per service
WCS 1.1.2 per coverage per coverage
WCS 2.0.0 n/a n/a
WCS 2.0.1 per service per service

All services but the WCS 2.0 CRS extension (listed under WCS 2.0.1) allow for reporting CRSes for each coverage / layer individually; the CRS extension could still be amended, though.

On the other hand, only WCS 1.1.2 allows output format specification on a per coverage basis whereas all others standards allow to report supported formats in the global service metadata only.

The WCS 2.0.1 corrigendum introduces the concept of native CRSes and formats which are reported in the coverage description. The native CRS is the one the domain set uses.

Counterintuitively, the WCS 2.0.1 native file format is not necessarily the same as the file format of the stored data. Since not all source file formats are supported as the output file format (e.g. ENVISAT N1), it is rather the default format delivered when there no specific file format is requested (omitting the FORMAT parameter in GetCoverage requests).

Supported Output Formats and WCS 2.0.1 Native Format

As most services (all but WCS 1.1.2, see the table above) allow output format configuration only per service instance, we propose that the list of supported formats shall be kept in the global configuration. This can be most easily done by adding new items to the global configuration file conf/eoxserver.conf.

Due to the nature of the data transmitted by these services the configuration should be separate for WMS and WCS.

The EOxServer implementation for WCS 2.0 and EO-WCS requires three parameters to be defined for each supported format:

  • the MIME type
  • the name of the GDAL driver
  • the default file extension

The possible format choices are restricted by the capabilities of the underlying SW components (MapServer and GDAL). The list of allowed formats can be fount at http://www.gdal.org/formats_list.html.

Although the source format (i.e. the actual format of the stored data) could be determined for each coverage individually at runtime it is preferable to store this information in the database for performance reasons.

The actual native format announced by the WCS 2.0.1 compliant coverage description can differ from the source format as not every source format can be used as output format.

The implementation of the native format reporting for WCS 2.0.1 requires that EOxServer knows the mapping from the source to WCS 2.0.1 native format. As this mapping varies depending on the GDAL version, available external libraries or simply on the preference of the instance administrator the actual mapping shall be configurable, i.e., it shall be a configuration item in conf/eoxserver.conf.

For all the proposed configuration items reasonable default shall be provided.

Supported CRSes

All services but WCS 2.0.1 support per-coverage or per-layer reporting of CRSes. The WCS 2.0 CRS extension is not yet finished and it is suggested that it, too, should allow for CRS metadata being reported in the coverage description, although this provision is not included in the current draft of the document.

Currently, the EOxServer implementation of WMS and WCS sets the ows_srs MapServer parameter to the original CRS of a coverage. Thus the currently only announced CRS is the native CRS of the dataset.

This RFC proposes to introduce global configuration items for WCS and WMS, respectively, allowing definition of CRSes to be reported in addition to the native CRS. These CRSes shall also be used for EO-WMS layers corresponding to DatasetSeries.

In order to report a native CRS for Referenceable Grid Coverages the data model needs to be changed to include the SRID of the GCP projection of ReferenceableDatasets.

Proposed Implementation

Changes to the Data Model

For implementing the native format reporting in WCS 2.0.1, an additional field gdal_driver_name on the LocalDataPackage and RemoteDataPackage model shall be added. For the RasdamanDataPackage model, a dedicated database field is not necessary as the GDAL driver is already known because of the nature of the data package. The driver name should be provided by the DataPackageWrapper implementation.

In order to report the native CRS of Referenceable Datasets, a srid field shall be added to the ReferenceableDatasetRecord model.

Changes to the Configuration Files

The following new configuration settings are needed for output format handling:

  • a list of GDAL formats with MIME types and a flag indicating if the format is writable or read-only
  • a list of MIME types to be reported as supported formats in WMS
  • a list of MIME types to be reported as supported formats in WCS
  • a default format MIME type to be used for native format reporting in WCS 2.0.1
  • an optional mapping of source format to for native format reporting in WCS 2.0.1

The list of GDAL formats shall be configured in a CSV-like separate configuration file in conf/formats.conf. Each line in the file shall correspond to a given format. The syntax is as follows:

<GDAL driver name>,<MIME type>,<either "rw" for writable or "ro" for read-only formats>,<default file extension>



Empty lines shall be ignored as well as any comments started by single # (hash) character and ended by the end of the line.

A default configuration (default_formats.conf) and a template (TEMPLATE_formats.conf) shall be included in the eoxserver/conf directory. The default configuration shall only be used as a fall-back if no formats.conf file is available in the instance conf directory.

The other configuration settings shall be defined in conf/eoxserver.conf:

supported_formats=<MIME type>[,<MIME type>,...]

supported_formats=<MIME type>[,<MIME type>,...]

default_native_format=<MIME type>
source_to_native_format_map=[<src MIME type>,<dst MIME type>[,<src MIME type>,<dst MIME type>,...]

The following new configuration settings are needed for CRS handling:

  • a list of supported CRS IDs (SRIDs) for WMS layers
  • a list of supported CRS IDs (SRIDs) for WCS coverages

The respective entries in conf/eoxserver.conf:



Default settings shall be defined in eoxserver/conf/default.conf.

Module eoxserver.resources.coverages.formats

In order to support output format handling a dedicated module shall be implemented that

  • reads the list of GDAL formats from the configuration files
  • map GDAL driver names to MIME types and vice versa
  • map MIME type (i.e., format) to default file extensions
  • map source format to WCS 2.0.1 native format

Changes to the Service Implementations

The WMS and WCS modules need to be altered to use the new global settings in the service and layer / coverage configuration.

The hard-coded format settings in WCS 2.0 (eoxserver.services.ows.wcs.wcs20.getcov module) shall be removed.

The GDAL driver name obtained from the DataPackageWrapper implementation shall be translated at runtime to the respective MIME type using the functionality provided by eoxserver.resources.coverages.formats module (inluding the translation from the source MIME type to the WCS 2.0.1 native MIME type).

Changes to the Administration Tools

The create_instance command shall copy the template format configuration file to the conf directory of the instance.

The Coverage Managers shall store the GDAL driver name of the native format in the database.

Voting History

Motion:To accept RFC 17
Voting Start:2012-05-11
Voting End:2012-05-17
Result:+5 for ACCEPTED