What is a coverage?

A coverage is a unified paradigm for modelling and encoding raster data of all kind, from 1D sensor timeseries, over 2D raster images, up to spatio-temporal data cubes. 
It describes spatio-temporal regular and irregular grids (i.e., multi-dimensional data cubes), point clouds, and general meshes.

In plain words coverage are modelled like a function, composed by the following elements:

• Domain set, answering “at what coordinates can I find values?” (the domain of the function). 
• Range set, determining “what are the values?” (the image set of the function). 
• Range type, informing "which are the basic characteristics of the values?". It is utilised to capture the full semantics of the values in the coverage. 
• Optional metadata bucket, carrying any additional information that may be also relevant for users.

The range type of a coverage is also a standardized description based on the SWE (Sensor Web Enablement) Common (http://www.opengeospatial.org/standards/swe) concepts.  

This allows sensor data to be transformed into coverages without information loss, thereby enabling seamless service chains from upstream data acquisition (e.g., through OGC SOS) to downstream analysis-ready user services (such as OGC WMS, WCS, and WCPS). 

Such coverages can be represented in a variety of shapes – including tilings, coordinate/value pair lists – and formats - such as GML, JSON, RDF, a variety of binary encodings, as well as “containers” with mixed encodings. Hence, tools can request coverages in their favourite format from a server.

Coverage standards

Coverages are fully standardized by the OGC (Open Geospatial Consortium):

• At abstract, conceptual level, in OGC Abstract Topic 6 which is identical to ISO 19123. 
• At concrete, interoperable level in the OGC Coverage Implementation Schema (CIS), which is adopted by ISO as 19123–2.

Several versions of the OGC Coverage Implementation Schema (CIS) exist:

CIS 1.0, formerly known as “GML 3.2.1 Application Schema – Coverages” (GMLCOV).

CIS 1.1, which provides a comprehensive, more consistent schema for grid coverages. It is backwards compatible with its predecessor, CIS 1.0.

Find here more details on coverage standardisation.

Use of coverages in INSPIRE

Coverages are widely used in different thematic domains in the scope of the INSPIRE Directive:

• Annex II: Elevation (EL), Land cover (LC), Orthoimagery (OI), Geology (GE);
• Annex III: Soil (SO), Land use (LU), Natural risk zones (NZ), Environmental Monitoring Facilities (EF), Atmospheric conditions (AC), Meteorological geographical features (MF), Oceanographic geographical features (OF), Energy resources (ER), Species Distribution (SD);

However, two different approaches are used by INSPIRE themes to serve coverage data to the user:

1. WCS view: Coverages are considered as features 

INSPIRE Feature Types in these themes are based on Coverage Classes.

Regular or irregular grids are used by the WCS view INSPIRE themes.

Default coverage encoding for the mentioned themes is based on CIS 1.0.

2. SOS view: Coverages are considered as observation results

Regular or irregular grids are used by the SOS view INSPIRE themes.

Which is the main issue about INSPIRE coverages?

The INSPIRE data models from the technical guidelines of the different themes were drafted at conceptual level. For this reason, it was necessary to specify concrete rules for the delivery and encoding of data, either as part of the own data specifications or in separate INSPIRE guidance documents (e.g. INSPIRE D2.7 – Guidelines for the encoding of spatial data v3.3).

This approach was also applied to the INSPIRE application schemas which serve to model coverage data across different themes, by selecting CIS 1.0 (nicknamed GMLCOV in the past) as default encoding rule for coverage data. 

Despite of it the rules specified were not complete enough to assure the interoperability of data (coverages) and related services due to:

• Lack of clarity of the standards for implementing coverages existing at that point of time (GML 3.2.1, GML 3.3, CIS 1.0 - See coverage standardisation).
• Possible misinterpretation (not ample experience on them).
• Immaturity and inconsistencies of the standards in place.

As a result, a number of INSPIRE-defined properties were identified by the Thematic Working Groups (TWGs) to be necessary or appropriate in the context of the European spatial data infrastructure. They were appended to each of the theme-specific coverage feature types in the conceptual data models. 

When deriving the corresponding INSPIRE application schemas from the conceptual data models, such properties were also appended as part of the INSPIRE coverage elements to be encoded, adding contents not foreseen in the OGC CIS 1.0 implementation standard. 

All these theme-specific properties constitute a deviation from the OGC CIS 1.0 standard, because they neither map to any existing GML nor CIS properties. 

From now on we refer to such additional contents as INSPIRE extensions. 

Such extension shall be removed from the INSPIRE application schemas for coverage data.

See graphically the Good Practice Proposal in the next figure.

To know other minor issues identified with INSPIRE coverages, please refer to the article at the end of this page.

Good Practice: Proposal for an interoperable solution

A functional solution to align INSPIRE coverages to OGC standards, is amending in a simple way the INSPIRE XSD coverage schemas by adding the INSPIRE extensions as coverage metadata, included within an OGC compliant CIS 1.0 coverage. 

In other words, INSPIRE extensions may be easily provided within the coverage metadata bucket, being one of the components of an OGC coverage as described earlier - See below.

A coverage service for various (raster data) INSPIRE themes has been setup to overcome these issues as a result of the related activities performed in the scope of the INSPIRE Community Forum (Thematic Clusters) during the last years, particularly based on the results from the Workshop “Feel the power of INSPIRE WCS / WCPS in your hands” organised in the INSPIRE Helsinki 2019 Technical Conference:

“INSPIRE Coverages Demystified”: https://inspire-wcs.eu 

The demo service allows the user to play with different service requests to get INSPIRE coverage data and metadata, analyse and combine different data layers (cross-theme fusion), download and present the final results. 

It is specially designed to demonstrate use of coverages and WCS/WCPS for several INSPIRE themes: Elevation, Orthoimagery and Land cover - The service is powered by the rasdaman WCS reference implementation and offers sample data from several data providers (Institut Cartogràfic i Geològic de Catalunya, ICGC; Landesamt für Vermessung und Geoinformation Schleswig-Holstein (SH), Germany; Finnish Environment Institute (SYKE), Finland).

The objectives are to:

• Experience the potential of these technologies.
• Fully-understand the main benefits of integrating them within their daily geospatial businesses and analytical tasks.

...but mainly:

• Get feedback from the community on this functional good practice to align INSPIRE coverages to OGC standards, by amending in a simple, OGC compliant way the INSPIRE XSD coverage schemas (as INSPIRE coverage metadata).

INSPIRE Good Practice candidate document

A good practice document explaining this INSPIRE Good Practice, its outcomes, benefits and limitations have been prepared in order to present it as a candidate INSPIRE Good Practice.

The good practice was presented in the 63rd MIG-T Meeting (2020-10-13), where INSPIRE MIG-T group voted positively for its endorsement as a candidate INSPIRE Good Practice according the established procedure.

The document is available in the MIG-T Platform:

https:// webgate.ec.europa.eu/fpfis/wikis/download/attachments/559350289/GP3. %20Coverage%20data%20and%20service%20implementation.pdf? version=1&modificationDate=1602060517975&api=v2 

Discussion document

A discussion document explaining this INSPIRE Good Practice was initially prepared in order to spread the word and promote the use of the amended schemas as an INSPIRE Good Practice.

The discussion document was presented during the 62nd MIG-T Meeting, where it was the convenience of presenting it as a INSPIRE Good Practice candidate was highlighted.

The document is available here:

https:// inspire.ec.europa.eu/forum/file/view/266799/discussion-document-good- practice-for-providing-raster-inspire-coverage-data-and-services

Provide your feedback

A related discussion thread has been open in the Software & Tools group of the INSPIRE Community Forum for this purpose:

https://inspire.ec.europa.eu/forum/discussion/view/265095/inspire-coverages-demo-service

Find details about preliminary analysis

• Article “INSPIRE coverages: an analysis and some suggestions”:

https://link.springer.com/article/10.1186/s40965-019-0059-x