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#107: CF Data Model 1.7 -----------------------------+---------------------------------------------- Reporter: markh | Owner: [email protected] Type: task | Status: new Priority: medium | Milestone: Component: cf-conventions | Version: Resolution: | Keywords: -----------------------------+---------------------------------------------- Comment (by biard): Hi. While you guys are discussing offline, I'll pitch in some thoughts. The contents of the variable referenced by the grid_mapping attribute are a declaration of a Coordinate Reference System (CRS). A CRS is a set of equations, parameters, and measurements that define a 2-D or 3-D coordinate system relative to the body of the Earth. In the simplest case, it is a definition of the ellipsoid to be used to approximate the shape of the Earth and of the (0,0,0) (lon,lat,elevation) point. In more complicated cases, it is a definition of a geoid surface (usually specified with a grid of points) relative to an ellipsoid. In still more complicated cases, it is a definition of a "warped cartesian" map projection coordinate system relative to a geoid or ellipsoid. Transforms may be constructed using the contents of a CRS to convert XYZ coordinates in a map projection to longitude, latitude, and Z (either relative to the geoid or the ellipsoid). It's interesting to note that the longitude and latitude of a point with a large Z value will be different depending on whether the Z is relative to a geoid or an ellipsoid. Transforms can also (usually) be constructed between two different CRSs. This is actually a overly simple statement. There are map projection CRSs that declare a longitude,latitude anchor point, but don't connect the region they were designed for to any global vertical datum, which means you can't fully transform points measured in that CRS to any other CRS. This is all due to the history of CRSs, where they started as means to connect survey measurements in different regions to one another, and grew incrementally from there to the current state where we have GPS and a global scope. Jon, regarding your aside above, I don't think the phrase "depth in metres below sea level" defines a vertical datum (the surface that defines your zero point relative to the body of the Earth in a CRS). It may define the Z axis of a coordinate system, but it is not a CRS. Grace and peace, Jim -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/107#comment:41> CF Metadata <http://cf-pcmdi.llnl.gov/> CF Metadata This message came from the CF Trac system. To unsubscribe, without unsubscribing to the regular cf-metadata list, send a message to "[email protected]" with "unsubscribe cf-metadata" in the body of your message.
