THREDDS Data Server OGC WCS CRS and CF

THREDDS Data Server, OGC WCS, CRS, and CF Ethan Davis UCAR Unidata 2008 GO-ESSP, Seattle

THREDDS Data Server • Uses net. CDF-Java to read data • Recognize coordinate systems and tries to classify as a CDM scientific data type • Data access Services – – – HTTP net. CDF Subsetting Service OPe. NDAP OGC WCS OGC WMS (coming soon, see Jon Blower’s talk)

OGC WCS • Describes and supports subset retrieval of gridded, geolocated data. • Subsetting and subsampling can be specified in geographic coordinates. • Based on ISO Coverage specification (ISO 19123) – a coverage represents a set of fields that vary over the same domain (space/time).

OGC WCS Limitations • Currently only georectified grids are supported. The grid can be described with an origin point and offset vectors. – Affects: WCS 1. 0 and 1. 1 – Except: An “image CRS” can be returned – basically index space. Though it should include a description of a transformation to a geographic CRS. – Solution: Proposed addition to GML of non-rectified grid (CV_Referenceable. Grid). [Proposed by Andrew Woolf, Ron Lake, and David Burggraf , OGC CR 07112]. Once in GML can be adopted by WCS.

OGC WCS Limitations • Doesn’t support non-length based vertical coordinates (e. g. , pressure, sigma). – Affects: WCS 1. 0 and 1. 1 – Except: Can handle vertical as a “range” dimension (list not coordinate). Not satisfactory, especially if have array fields. – Solution: Proposed ISO 19111 -2 for parametric coordinates … GML? … WCS

OGC WCS Limitations • Time: handled inconsistently. Described as list of points and periods. Requested as points OR periods. OR in WCS 1. 1 can be handled as part of CRS. – Affects: WCS 1. 0 and 1. 1 – Another alternate: Can represent each time step as a separate coverage. – Solution: WCS 1. 1 allows time in CRS. Start using CRS with time instead of non-CRS time.

OGC WCS Limitations • Limit on the fields that a single coverage can contain. They all must have the same units. – Affects: WCS 1. 0 – Solution: Fixed in 1. 1

OGC WCS Limitations • Not fully aligned with ISO 19123 and GML. Introduces constructs that already exist in ISO 19123 and GML. – Affects: WCS 1. 1 – Solution: Not changed in next draft version (1. 1. 2/1. 2)

CF-net. CDF Extension for WCS • CF-net. CDF not a “blessed” encoding format in WCS 1. 0 • WCS 1. 1 no longer has “blessed” formats instead formats need Extension document. • CF-net. CDF encoding Extension document currently draft – Describes mapping between CF-net. CDF and ISO 19123 Coverage models – Describes CF-net. CDF encoding

WCS and CRS • WCS expects detailed CRS (Coordinate Reference System) – … at least in the horizontal. – Vertical OK but with limitations. – Temporal … maybe. • Coming “soon”: – ISO 19111 -1 general coordinates: allows CRS to contain nonspatial/temporal coordinates and multiple temporal coordinates – ISO 19111 -2 allows parametric coordinates • Possible encodings – OGC WKT – GML/XML – OGC CRS URNs

Coordinate Reference Systems • CF coordinate systems – In many ways, much more general. – But currently not as detailed. • Horizontal CRS, grid mapping has improved. • Vertical CRS … • Temporal CRS ? ? ?

Horizontal CRS • New grid mapping attributes for detailed information on reference ellipsoid. • Is this enough?

Temporal CRS • Need to look at how represented in GML. • Also ISO 19108 – Temporal schema

Vertical CRS • CF already deals with vertical coordinates. Much more flexible than current GML and WCS • Missing details of vertical datum – Standard names already reference (explicitly and implicitly) surface, ellipsoid, geoid. – Need to define which surface, which ellipsoid, which geoid – Also needed for non-coordinate variables.

CF Vertical Datum Issues • Standard names plus new attributes vs extension of grid mapping. • Group ellipsoid and geoid as vertical datum • Allow transformation between vertical CRS (as grid mapping transforms between two horizontal CRS) – Instead of parameterized transformation allow for array of offset values.

TDS, OGC WCS, CRS, CF • Questions?

Vertical CRS • Actual surface: topography/ bathymetry; sea surface • Reference ellipsoid • Geoid: equipotential surface of the earths gravity field that most closely approximates MSL

Coordinate Reference Systems COMPD_CS["OSGB 36 / British National Grid + ODN", PROJCS["OSGB 1936 / British National Grid", GEOGCS["OSGB 1936", DATUM["OSGB_1936", SPHEROID["Airy 1830", 6377563. 396, 299. 32496 TOWGS 84[375, -111, 431, 0, 0], AUTHORITY[["EPSG", "6277"]], PRIMEM["Greenwich", 0, AUTHORITY["EPSG", "8901"] UNIT["DMSH", 0. 0174532925199433, AUTHORITY["EPS AXIS["Lat", NORTH], AXIS["Long", EAST], AUTHORITY[["EPSG", "4277"]], . . .

Coordinate Reference Systems. . . PROJECTION["Transverse_Mercator"], PARAMETER["latitude_of_origin", 49], PARAMETER["central_meridian", -2], PARAMETER["scale_factor", 0. 999601272], PARAMETER["false_easting", 400000], PARAMETER["false_northing", -100000], UNIT["metre", 1, AUTHORITY["EPSG", "9001"]], AXIS["E", EAST], AXIS["N", NORTH], AUTHORITY[["EPSG", "27700"]], . . . AUTHORITY[["EPSG", "7405"]]

Coordinate Reference Systems. . . VERT_CS["Newlyn", VERT_DATUM["Ordnance Datum Newlyn", 2005, AUTHORITY[ UNIT["metre", 1, AUTHORITY["EPSG", "9001"]], AXIS["Up", UP], AUTHORITY[["EPSG", "5701"]], . . .