© crown copyright met office wcsplus making the web coverage service work for the fluid earth...

© Crown copyright Met Office

WCSplusMaking the Web Coverage Service work for the Fluid Earth Sciences Community

Bruce Wright (Senior IT Architect), 7th GO-ESSP Community Workshop, 17-19 September 2008


Collaborators

Ben Domenico, Ethan Davis, Jon Caron (Unidata)

Stefano Nativi, Paolo Mazzetti (University of Florence)

Andrew Woolf, Dominic Lowe(Science and Technology Facilities Council, RAL)

Jon Blower(Reading e-Science Centre)

Jeremy Tandy(Met Office)


Contents

This presentation covers the following areas

• OGC Standards

• Web Coverage Service

• WCSplus

• Example Comparison

• The Future

• Discussion


OGC Standards(Open Geospatial Consortium)

http://www.opengeospatial.org/


Open Geospatial Consortium (OGC)• International industry consortium of 366

companies, government agencies and universities

• Consensus process to develop publicly available interface specifications to support interoperable solutions that "geo-enable" the Web, wireless and location-based services, and mainstream IT

• Specifications that empower technology developers to make complex spatial information and services accessible and useful with all kinds of applications

• ‘Partner’ with ISO/TC 211 Geographic information / Geomatics – many OGC standards are ISO191xx standards


OGC Standards

XML grammar and file format for modelling and storing geographic features

XML grammar and file format for modelling and storing geographic features for display – submitted by Google to be evolved within the OGC consensus process

HTTP interface for retrieving geographic features across the Web using platform-independent calls – GML is the default payload encoding for transporting the geographic features

HTTP interface for retrieving geo-registered map images across the Web using platform-independent calls – a request defines the geographic layer(s) and area of interest to be processed and the payload is usually a map image format (e.g. PNG, JPEG)

HTTP interface for retrieving geospatial "coverages" (gridded data) across the web using platform-independent calls – the default payload is one of a number of standard binary file formats

Geography Markup Language

Keyhole Markup Language

Web Feature Service

Web Map Service

Web Coverage Service

GML

KML

WFS

WMS

WCS


WCS(Web Coverage Service)

http://www.opengeospatial.org/standards/wcs


WCS Request

• HTTP GET with KVP (or POST with either XML or SOAP….or RESTful?):

• URL of service: http://host[:port]/path?

• Standard request parameters: SERVICE=WCS&VERSION=1.0.0

• 3 types of request (REQUEST=):

• GetCapabilities – Service metadata (XML doc)

• DescribeCoverage – Full description of one or more coverage (XML doc)

• GetCoverage – One coverage (known format)

• Other standard request parameters


WCS 1.0

Only a single coverage can be retrieved (homogeneous range set)

2D only, request & response can be different

Can be 2D or 3D

Only a single time dimension

Number of gridpoints

Grid-spacing must be regular

Interpolation (5 options)

Single data file returned – easy to parse (should support at least one GeoTIFF, HDF-EOS, DTED, NITF, GML)

Additional “range set parameters” – extends capability, but leads to different approaches

Limited set of other GetCoverage request parameters:

COVERAGE

CRS

BBOX

TIME

WIDTH / HEIGHT / DEPTH

RESX / RESY / RESZ

INTERPOLATION

FORMAT

“PARAMETER” (e.g. ELEVATION)


WCS 1.1

• More bulky and complex than WCS 1.0– attempt to satisfy all use cases in one standard

• Changes to request parameters (e.g. COVERAGE - > IDENTIFIER)• Reduced ISO 19123 conformance• Still only get one ‘coverage’ – but can support coverage hierarchies,

each composed of multiple (non-homogeneous) fields • 0,1,2 or 3D – but still regularly spaced• Still 1 time dimension• Adds far more extensive CRSs (including vertical) – but still not straight

forward to handle non-height-based vertical coordinate • Scalar or vector fields• Any format now acceptable (depreciated ‘blessed 5’), but should have

WCS encoding profile• GetCoverage response always encoded as ‘Coverage XML doc’, which

references 1 or more data files, which are either:• Bundled in MIME multi-part message, or:• Accessible via separate URL (STORE) – not full asynchronous

capability


WCSplushttp://www.unidata.ucar.edu/mailing_lists/archives/wcsplus/

(Also: http://galeon-wcs.jot.com/WikiHome/GALEON%20Phase2%20Main%20Page)


WCSplus – History

• OGC Technical Committee meeting (Boulder, Sep 2007); Unidata hosted an Interoperability Day

• Addressing use of standard interfaces (inc. WCS) for providing access to data currently served via THREDDS, OPeNDAP, CF-netCDF, etc for the Fluid Earth Sciences (FES) community

• Subsequent discussions resulted in the WCSplus (originally WCS 1.0+) initiative

• WCSplus mailing list established

• Really a subset of the OGC GALEON (Geo-interface for Air, Land, Environment, Ocean NetCDF) IE (Interoperability Experiment)


WCSplus – Philosophy

• Start with WCS 1.0

• Relatively simple

• Already a substantial number of working implementations

• Develop practical & concrete ideas to meet FES community requirements

• Trade-off between generality/powerfulness and simplicity/feasibility -> minimal changes -> relatively easy to implement & test

• Validate through the implementation and experimentation with specific additional features (GALEON 2)

• Feed back into the WCS standard via the WCS Standards Group (WCS 1.2)


WCSplus – ‘Ingredients’ (1)

• CF-netCDF binary encoding – Ben Domenico & Stefano Nativi developing a “WCS 1.1 extension for CF-netCDF 3.0 encoding” (Oct 2008)

• Asynchronous response handling – Discussion paper from Ethan Davis, Stefano Nativi & Paolo Mazzetti (Nov 2007), drawing GADS-WCS Interface Spec from Jon Blower (Mar 2006)

• Multiple heterogeneous fields within a coverage – change ‘range set’ model & AxisDescription

• Support for irregular grids - Use GML with CR that includes CV_ReferencableGrid (see OGC 07-112 proposal for ISO 19123 irregular grids)


WCSplus – ‘Ingredients’ (2)

• Allow multiple times

• Allow non-spatial vertical coordinate

• Specification fully based on ISO 19123 coverage specification

• Sort inconsistencies in KVP and XML schema documents


WCSplus – Implementation

Suggested:

• Be more explicit about HTTP response codes

• Use HTTP GET with KVP only


Example Comparisonhttp://galeon-wcs.jot.com/WikiHome/GALEON%20Phase2%20Main%20Page/GALEON%202%20WCS%20Servers


Visual Weather vs. THREDDS

Service Data source (e.g. all Global model runs)

Single instance of data source (e.g. a single Global model run)

COVERAGE Met parameter (e.g. temperature) but no standard names

Met parameter (e.g. temperature) but no standard names

Spatial domain 2D BBOX + ‘ELEVATION’ range parameter

3D BBOX + ‘VERTICAL’ range parameter

Time domain TIME for validity time + ‘DIM_RUN’ & ‘DIM_FORECAST’ range parameters for analysis time and forecast period

TIME for validity time(Analysis time is covered by service)

Ensembles ‘DIM_ENSEMBLE’ range parameter (not implemented yet)

Not implemented?

FORMAT NetCDF3, GeoTIFF, GeoTIFFfloat, GRIB1, KML

NetCDF3, GeoTIFF, GeoTIFFfloat

VW WCS THREDDS WCS (1.0.0)


The Future


WCS 1.2

• Different Approach:

• Rather limited ‘core’ – possibly only 2D

• Set of extensions

• WCSplus capability realised through:

• FES Application Profile

• Number of relevant programmatic extensions – e.g. extension for CF-netCDF 3.0 encoding

+ Workable standard for our community

– Issues with inter-community interoperability


Forthcoming Activities

• September '08 OGC Technical Committee Meeting, Atlanta Georgia, 15-19 Sept 2008

http://www.opengeospatial.org/event/0809tc

• Workshop on the use of GIS/OGC standards in meteorology, ECMWF, Reading, UK, 24-26 Nov 2008

http://www.ecmwf.int/newsevents/meetings/workshops/2008/OGC_workshop/index.html

• December '08 OGC Technical Committee Meeting, Valencia, Spain, 1-5 Dec 2008







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