RFC 16: Referenceable Grid Coverages

Authors:Stephan Krause, Stephan Meissl, Fabian Schindler
Last Edit:$Date$

This RFC proposes an implementation for Referenceable Grid Coverages as well as for the WCS 2.0 operations working on them.

The implementation is available in the SVN under http://eoxserver.org/svn/sandbox/sandbox_ref.


Referenceable Grid Coverages are coverages whose internal grid structure can be mapped to a coordinate reference system by some general transformation. They differ from rectified grid coverages in that the coordinate transformation is not necessarily affine.

In the context of Earth Observation, raw satellite data can be seen as referenceable grid coverages. They are typically delivered as image files but do not have an affine transformation from the image geometry to a georeferenced coordinate system. Depending on the desired geocoding precision, the referencing transformation can be very complex involving additional data (DEMs) and orbit metadata.

EOxServer shall be able to deliver (subsets of) Earth Observation raw data in its original (referenceable grid) geometry using WCS 2.0 and EO-WCS. Furthermore, it shall implement easily computable approximate referencing algorithms based on ground control points (GCPs) in order to enable coordinate transformations and rectified previews of the original data using WMS.

For the time being, the implementation will focus on SAR image data collected by the ENVISAT-ASAR sensor made available by ESA.


The main requirement source for Referenceable Grid Coverage implementation in EOxServer is the ESA O3S project. In the course of this project EOxServer shall be installed in front of a small archive of ENVISAT-ASAR data. In a first step, we will focus on covering the requirements of this use case, adding more generic referenceable grid support in future iterations.

The ENVISAT-ASAR data are available in ENVISAT .N1 original format.

Delivery of the original referenceable grid data shall be supported using WCS 2.0 and EO-WCS. Subsetting shall be supported in pixel coordinates (imageCRS) and in a coordinate reference system. The CRS subsets shall be mapped to pixel coordinates using a simple coordinate transformation based on GCPs.

No support for resampling (size and resolution) or reprojection (outputcrs) parameters is required as these are not applicable to referenceable grid coverages.

At least GeoTIFF shall be supported as output format. GCP and metadata information contained in the .N1 original file shall be preserved.

In order to support (rectified) WMS previews, a simple georeferencing algorithm based on GCPs shall be implemented. This shall be reused to provide rectified versions of referenceable grid coverages using WCS 2.0.

Implementation Details

Input Formats

The implementation for referenceable grid coverages relies on GDAL for input data and metadata (georeferencing information, GCPs). Any format that supports storage of GCPs with the dataset can be used. The two most important formats are the ENVISAT .N1 format and GeoTiff.

Referencing Algorithm and Subsetting

WCS 2.0 allows to define subsets either in the image CRS, i.e. pixel coordinates, or in some geographic or projected coordinate system. For rectified grid coverages geographic coordinates can be easily transformed to pixel coordinates in a straightforward way. This is not the case for referenceable grid coverages, though.

For referenceable grid coverages produced by Earth Observation missions, the “correct” referencing transformation is not known in general. Instead, there are many different algorithms some of them relying on different additional data and metadata (DEMs, orbit information).

For the purposes of the EOxServer Referenceable Grid Coverage implementation, a simple first order interpolation algorithm based on GCPs is used. This algorithm does not use any additional data or metadata. The rationale for this decision is that there is no way to advertise the actual referencing algorithm in WCS or WMS, and therefore the most simple and straightfoward algorithm was used.

Subsets given in georeferenced coordinates are transformed to the image CRS using the inverse transformation algorithm based on GCPs. The implementation uses not only the corner coordinates of the subsetting rectangle but also intermediary points to calculate an envelope and thus to guarantee that the requested extent be included in the result.

Genuine Referenceable Grid Coverage Support in WCS 2.0

Referenceable Grid Coverages in their original geometry are available using the EO-WCS extension of WCS 2.0.

The current implementation supports the subset parameter and transforms the given subsets as indicated in the previous subsection. The size and resolution parameters are not supported as they do only apply to rectified grid coverages.

The format parameter options are implemented in the same way as for rectified grid coverages.

The rangesubset parameter is foreseen for implementation.

In order to be able to serve referenceable grid data, the original WCS20GetCoverageHandler was split up into WCS20GetReferenceableCoverageHandler and WCS20GetRectifiedCoverageHandler. While the latter one still relies on MapServer, the one for referenceable grid data uses the vanilla GDAL Python bindings as well as additional GDAL-based extensions written for the EOxServer project.

Metadata is read from the original dataset and tagged onto the result dataset using the capabilities of the respective GDAL format drivers. Depending on the driver implementation, the way the metadata is stored may be specific to GDAL.

Coverage Metadata Tayloring

The WCS 2.0 standard specifies that the complete referencing transformation be described in the metadata of a referenceable grid coverage. This is a major problem for Earth Observation data as in general there is no predefined transformation; rather there are several different possible algorithms of varying complexity that can be used for georeferencing the image, possibly involving Earth Observation metadata such as orbit information, GCPs and additional data such as DEMs.

Furthermore there is no way to define an algorithm and describe its parameters (e.g. the GCPs) in GML, but only the outcome of the algorithm, i.e. a pixel-by-pixel mapping to geographic coordinates. This would produce a tremendous amount of mostly useless metadata and blow up the XML descriptions of coverage metadata to hundreds of megabytes for typical Earth Observation products.

Therefore the current EOxServer implementation does not deliver any of the gml:AbstractReferenceableGrid extensions in its metadata. Instead a non-standard ReferenceableGrid element is returned that contains all the elements inherited from gml:Grid but no further information. This is only a provisional solution that will be changed as soon as an appropriate way to describe referencing metadata is defined by the WCS 2.0 standard or any of its successors.

Support for Rectified Data in WMS and WCS 2.0

The implementation of the WCS 2.0 (EO-WCS) GetCoverage request as well as the WMS implementation is based on MapServer which supports rectified grid coverages only. It is not possible to use any kind of GCP based referencing algorithm in MapServer directly.

GDAL provides a mechanism to create so-called virtual raster datasets (VRT). These consist of an XML file describing the parameters for transformation, warping and other possible operations on raster data. They can be generated using the GDAL C API and are readable by MapServer (which relies on GDAL as well).

In order to provide referenced versions of referenceable data, EOxServer creates such VRTs on the fly using the EOxServer GDAL extension. The VRT files are deleted after each request.

GDAL Extension

The EOxServer GDAL extension provides a Python binding to some C functions using the GDAL C API that implement utilities for handling referenceable grid coverages. At the moment the Python bindings are implemented using the Python ctypes module.

The eoxserver.processing.gdal.reftools module contains functions for

  • computing the pixel coordinate envelope from a georeferenced subset
  • computing the footprint of a referenceable grid coverage
  • creating a rectified GDAL VRT from referenceable grid data

All functions use a simple GCP-based referencing algorithm as indicated above.

The GDAL Extension was made necessary because the standard GDAL Python bindings do not support GCP based coordinate transformations.

Voting History

Motion:Adopted on 2012-01-03 with +1 from Arndt Bonitz, Martin Paces, Fabian Schindler, Stephan Meißl and +0 from Milan Novacek