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

THREDDS Data Server Uses netCDF-Java to read data Recognize coordinate systems and tries to classify as a CDM scientific data type Data access Services –HTTP –netCDF Subsetting Service –OPeNDAP –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_ReferenceableGrid). [Proposed by Andrew Woolf, Ron Lake, and David Burggraf, OGC CR ]. 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 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 and GML. Introduces constructs that already exist in ISO and GML. –Affects: WCS 1.1 –Solution: Not changed in next draft version (1.1.2/1.2)

CF-netCDF Extension for WCS CF-netCDF not a “blessed” encoding format in WCS 1.0 WCS 1.1 no longer has “blessed” formats instead formats need Extension document. CF-netCDF encoding Extension document currently draft –Describes mapping between CF-netCDF and ISO Coverage models –Describes CF-netCDF 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 general coordinates: allows CRS to contain non- spatial/temporal coordinates and multiple temporal coordinates –ISO 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 – 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["OSGB36 / British National Grid + ODN", PROJCS["OSGB 1936 / British National Grid", GEOGCS["OSGB 1936", DATUM["OSGB_1936", SPHEROID["Airy 1830", , ,AUTHORITY["EPSG","7001"]], TOWGS84[375,-111,431,0,0,0,0], AUTHORITY[["EPSG","6277"]], PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]], UNIT["DMSH", ,AUTHORITY["EPSG","9108"]], 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", ], PARAMETER["false_easting",400000], PARAMETER["false_northing", ], 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["EPSG","5101"]], UNIT["metre",1,AUTHORITY["EPSG","9001"]], AXIS["Up",UP], AUTHORITY[["EPSG","5701"]],...