**Title: Specify Axis Order for CRS-WKT grid mappings** **Moderator:** @user
**Moderator Status Review [last updated: YY/MM/DD]:** Brief comment on current status, update periodically **Requirement Summary:** There is a requirement for using Coordinate Reference System Well Known Text to define coordinate reference systems that coordinate value sets be presented as ordered tuples, where each tuple represents a location with respect to that coordinate reference system. Currently CF does not provide a mechanism to specify axis order when linking a data variable to a CRS-WKT instance via a grid mapping. **Technical Proposal Summary:** **Benefits:** Who or what will benefit from this proposal? Users of CRS-WKT will benefit from this proposal as it will ensure that data sets comply with the OGC mandate to provide coordinate tuple value ordering as part of the reference between Whilst axis order can sometimes be inferred from deep inspection of the crs_wkt string and comparison to standard names of variables, this is a challenging inference to make and prone to significant problems, especially in automation scenarios. I think that treating the WKT string as a parcel for parsing by other systems is much safer. **Status Quo:** CF has adopted CRS-WKT from OGC but has not provided a mechanism to state how the variables containing coordinate values need to be structured in order to comply with the OGC mandates on implementation of Referencing by Coordinates in general, of which CRS-WKT is an implementation. **Detailed Proposal:** Provide an explicit mechanism for defining axis order, to enable individual coordinate variables to be unambiguously expanded into coordinate tuple collections. The expanded form of the grid_mapping attribute is > a blank-separated list of words "grid_mapping_variable: coordinate_variable > [coordinate_variable …] [grid_mapping_variable: …]", which identifies one > or more grid mapping variables, and with each grid mapping associates one or > more coordinate_variables, i.e. coordinate variables or auxiliary coordinate > variables. This in situ feature provides a useful point to provide this capability extension. As the extended form already explicitly references individual variables to use as coordinate values, the order of these values may be used to explicitly state the axis order of the coordinate values defined with respect to the coordinate reference system. Explicitly, the following text may be added as following paragraphs. > Where an extended "grid_mapping_variable: coordinate_variable > [coordinate_variable …]" entity is defined, then the order of the coordinate > variable definitions provides an explicit order for these coordinate value > variables to be combined into coordinate tuples. > The order of the coordinate_variable instances defines the order of elements > within a coordinate value tuple. This enable an application reading the data > from a file to construct an array of coordinate value tuples, where each > tuple is ordered to match the specification of the coordinate reference > system being used. > This explicit 'axis order' is important when the `grid_mapping_variable` > contains an attribute `crs_wkt`. It is mandated by the OGC CRS-WKT standard > that coordinate tuples with correct axis order are provided as part of the > reference to a Coordinate Reference System. I would also suggest including text within the later `Use of the CRS Well-known Text Format` section: > Where `crs_wkt` is added to a grid_mapping, the extended syntax for the > `grid_mapping` attribute enables the axis order for the coordinates being > referenced to be explicitly stated. The explicit definition of axis order is > expected by the OGC standards for referencing by coordinates. -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/223 This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
