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OGC Web Services Initiative, Phase 9 (OWS-9):

Innovations Thread - OPeNDAP

James Gallagher and Nathan Potter, OPeNDAP

© 2012 Open Geospatial Consortium

Goals

• Compare WCS 2.0 and DAP 2.0• Investigate using OPeNDAP’s Hyrax data server

as a backend for WCS 2.0 by…• evaluate the effort in upgrading an existing

server from WCS 1.x to 2.0• evaluating the complexity of supporting

several of the formats commonly used with GIS systems.

WCS and DAP: data models

• DAP (Data Access Protocol) supports a domain-neutral data model, both for data and metadata

• This enables it to serve as a transport for many kinds of information and…

• data that use many different metadata standards/conventions.

• Subsetting takes place using common programming operations like choosing array slices, structure fields, and so on

… data models

• WCS uses a domain-aware data model and interface

• It provides an integrated way to describe (metadata) coverages (data) and…

• subset them in the geospatial and temporal domains

• It also provides range subsetting operations

Hyrax (DAP) as a backend for WCS• A DAP server (e.g., Hyrax, TDS) makes a good backend for WCS• It provides a subsetting interface that abstracts many different ad hoc

data stores• The WCS service can focus on mapping between the WCS data model and

the DAP data model• The DAP server can provide uniform access to many different data

formats

Does the DAP data model provide useful abstractions for a WCS service?

• Adding support for new response formats was straightforward.

• Reading new data (source) formats was similarly easy

• Since each format was accessed uniformly using DAP, the WCS service required no change for the new source formats and minimal changes for the new response formats.

Modular BES Processing Plans

What was coded in the WCS Service?

• The metadata information in the data sources is not abstracted by DAP, instead it is adapted to a common representation but the content is not altered at all.

• The WCS service had to be coded for the specific metadata standards and conventions used by the underlying data sources

WCS 1.x 2.0

• Substantially different XML meant that we had to rework the software that (automatically) builds WCS XML

• The WCS 2.0 XML schema set is much easier to use than the 1.x XML; it validates and is better organized

• Heavy reliance on GML does not reduce the number of distinct specifications that must be read, however!

Issues in serving science data

• Phrases like ‘Science data’ oversimplify… • However, many scientific data sets for multi-

instrument platforms use complex hierarchical formats like HDF5 or NetCDF4

• Files in these formats often hold tens or hundreds of variables

• Meanwhile, widely-supported WCS response formats are typically limited to a single or small number of ‘variables’ (aka ‘fields’)

…and not all science data is georeferenced and projected

• This is true even for ‘Earth science data’• In OWS-9 NASA provided a collection of data

sets that were in ‘satellite’ coordinates and had to be projected by the WCS.

• Note that WCS supports coverages that could be used to represent these data, but clients would be hard pressed to make use of them.

Summary

• WCS 2.0 and DAP 2.0 differ in some key ways that make them complementary

• DAP servers provide good interfaces to data for WCS service implementations

• In this project, support for new data formats was abstracted almost completely by the DAP server

• However, metadata standards/conventions, however, were not abstracted by DAP

• Some response formats for WCS require complex transformations for some kinds of data