Transcript THREDDS Data Server, OGC WCS, CRS, and CF Ethan Davis UCAR Unidata

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
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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 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-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 19123 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 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["OSGB36 / British National Grid + ODN",
PROJCS["OSGB 1936 / British National Grid",
GEOGCS["OSGB 1936",
DATUM["OSGB_1936",
SPHEROID["Airy 1830",6377563.396,299.32496
TOWGS84[375,-111,431,0,0,0,0],
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"]],
...