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									                      Too Large to be Served?
Open , Interoperable Standards for High-Volume Public Geo Services
                               Peter Baumann1, Stephan Meissl2
                            1
                              Jacobs University, Bremen, Germany
                            1
                              rasdaman GmbH, Bremen, Germany
                                 2
                                   EOX GmbH, Vienna, Austria


1.        Introduction
For navigation, access, aggregation, and analysis of large-scale coverage archives OGC offers the
Web Coverage Service (WCS) Standards suite, consisting of a simple core [2] and a set of mix-
and-match extensions. WCS 2.0, adopted in August 2010, offers several advantages over previous
versions, such as: support for general n-D raster data; support for non-raster coverage types like
point clouds, trajectories, (iso) surfaces, and meshes; crisp, modular, and easy to understand;
flexible and adaptive; harmonized with GML and Sensor Web Enablement (SWE); improved
testability; and allows for efficient, scalable implementations.
For the subsequent use of the terms feature and coverage we rely on the OGC (Open Geospatial
Consortium) and ISO provided definitions. A feature, in OGC and ISO nomenclature, is defined
as an abstraction of some spatio-temporal real-world phenomenon whereas a coverage means a
space-time varying phenomenon; as such, a coverage is a special type of a feature. Raster data are
common representatives of coverages, but irregular grids, point clouds, and meshes likewise
constitute coverages.
The OGC Web Coverage Service (WCS) specification defines open interfaces for accessing and
server-side processing of coverages. Currently, consultations between INSPIRE TWG and OGC
SWG members are running with the goal of maximising reuse of existing specifications and, at
the same time, feeding back requirements spotted by INSPIRE teams into the OGC standards
maintenance process. In our contribution, we present WCS concepts, vistas, and usefulness for
INSPIRE Annex II and III.
2.         The OGC Coverage Model
The coverage model is laid out in the “GML Application Schema for Coverages” specification
[3]. It is based on GML 3.2.1 [6] which in turn relies on OGC Abstract Topic 6 [5] which is iden-
tical to ISO 19123. According to the latter, a coverage is a function mapping locations in space-
time, the coverage’s domain, to values of some data set, its range, described through its range
type. Format encoding extensions specify alternatives to a pure GML delivery, such as GeoTIFF,
NetCDF, and JPEG2000.
This model tentatively has been crafted to be independent from the service. This allows coverages
to be used by all services, they are not tied to WCS and, hence, servers and clients do not need to
be aware of the WCS definitions. In practice, this allows an SOS to generate a coverage, feed it
into a database through WCS-T, and then serve it via WCS and WPS, for example.
3.       The OGC Web Coverage Service
WCS 2.0 is a completely overhauled version. It is no longer a single standard but a modular,
structured suite of specifications fitting into an extensible overall concept. The Core defines
properties that any WCS implementation must offer in order to be conformant with WCS overall;
its main functionality consists of coverage retrieval, trimming and slicing. Extensions define furth-
er functionality which an implementation can add. A minimal complete WCS implementation
must support at least one format encoding and one protocol extension.
The WCS Earth Observation Application Profile (in short: EO-WCS) [4] specializes the generic
WCS 2.0 for use on Earth Observation data. An Application Profile bundles several specifications
and possibly adds additional requirements on an implementation. Extra requirements can be
additions (for example, Dataset Series are introduced by this specification) or constraints (for
example, coverages offered are restricted to 2-D rasters). EO-WCS centers around the notion of an
EO coverage which augments coverages with EO Metadata (based on O&M) and lineage. For the
grouping of massive numbers of coverages EO-WCS provides hierarchical groupings. An
additional request type accomplishes spatio-temporal search across such coverage hierarchies.
4.       Implementation
Currently two WCS implementations are known to us. Open-source MapServer has been
extended to support WCS 2.0 on 2D horizontal grid coverages; on top of it, an EO-WCS interface
is available. The second implementation, also open-source, is rasdaman which supports large nD
grid coverage access through WCS, WCS-T, WCPS, and WPS. These two systems act as WCS
2.0 reference implementations. An online demo of WCS 2.0 is publicly accessible at EarthLook.
5.      Conclusion
OGC’s unified coverage model offers a way to exchange nD coverages across domains using
GML and/or binary formats; its recent extension from nD rasters to irregular grids, point clouds,
meshes, etc. underlines its multi-domain support. The WCS 2.0 suite augments this by providing a
rich, modular framework for serving coverages. Both standards have been adopted in August
2010 and are available from OGC for free.
In parallel to specification writing, its reference implementations have been pursued to verify
viability of the concepts. Also, the OGC WCS conformance testing suite is in an advanced state;
both reference implementations have been tested successfully against it. Remarkably, tests for the
first time drill down to the granularity of pixels, resulting in a substantially increased level of
interoperability.
EO-WCS has been established after manifold consultation with EO data users and super-scale
data centers, including several review cycles. Hence, we believe that it forms a feasible platform
for, e.g., ortho image services in INSPIRE; furthermore, it gives a practical example for the
coupling of O&M metadata and coverages, a task common to many Annex II and III themes.
Acknowledgement
This work has been funded by ESA as part of the HMA-FO project.
References
[1] Baumann, P. (ed.): Web Coverage processing Service (WCPS) Interface Standard.
    OGC 08-068r2
[2] Baumann, P. (ed.): WCS 2.0 Core Interface Standard. OGC 09-110r3
[3] Baumann, P. (ed.): GML Application Schema for Coverages. OGC 09-146
[4] Baumann, P. and Meissl, S. (eds.): WCS 2.0 Earth Observation Application Profile.
    OGC 10-140
[5] OGC (ed.): Abstract Specification Topic 6: Schema for coverage geometry and
    functions. OGC document 07-111 (identical to ISO 19123)
[6] Portele, C. (ed.): Geography Markup Language (GML) Encoding Standard 3.2.1.
    OGC 07-036
[7] Robin, A. (ed.): SWE Common 2.0. OGC 08-094
Summary
INSPIRE Annexes II and III heavily deal with coverage data, that is: “spatio-temporally varying
phenomena” according to ISO 19123. Examples include orthoimagery, elevation, land cover,
climate, and ocean data. Currently, the individual Thematic Working Groups (TWGs) are drafting
the corresponding data specifications.
For navigation, access, aggregation, and analysis of large-scale coverage archives OGC offers the
Web Coverage Service (WCS) Standards suite, consisting of a simple core and a set of mix-and-
match extensions. WCS 2.0, adopted in August 2010, offers several advantages over previous
versions, such as: support for general n-D raster data; support for non-raster coverage types like
point clouds, trajectories, (iso) surfaces, and meshes; crisp, modular, and easy to understand;
flexible and adaptive; harmonized with GML and Sensor Web Enablement (SWE); improved
testability; and allows for efficient and scalable implementations.
We propose to use the generic coverage data and service model of OGC WCS 2.0 in combination
with the metadata model of the OGC Observations and Measurements (O&M) standard. In
particular, the WCS Earth Observation Application Profile, an OGC candidate standard
integrating coverage data and metadata handling, appears to fit the specific needs of flexible
services on ortho image, gridded elevation data, land cover, and further spatiotemporal coverages.

								
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