# Presentation: 903258

## 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)

**Notes:**
See Jon’s talk on WMS service.

## 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).

**Notes:**
Describes and supports subset retrieveal of gridded data. How is this different from OPeNDAP? Subsetting and subsampling can be specified in geog coords (as opposed to index space in OPeNDAP);.

## 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 07-112]. 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 non-spatial/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",6377563.396,299.3249646,AUTHORITY["EPSG","7001"]],
-                 TOWGS84[375,-111,431,0,0,0,0],
-                 AUTHORITY[["EPSG","6277"]],
-              PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
-              UNIT["DMSH",0.0174532925199433,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",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["EPSG","5101"]],
-         UNIT["metre",1,AUTHORITY["EPSG","9001"]],
-         AXIS["Up",UP],
-         AUTHORITY[["EPSG","5701"]],
- ...