Changes between Version 1 and Version 2 of markhDataModelNotesCRS
 Timestamp:
 04/07/14 10:11:23 (5 years ago)
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markhDataModelNotesCRS
v1 v2 59 59 }}} 60 60 61 The grid is defined by a set of values, quantities with respect to an ISO19111 CRS. In CFNetCDF terminology, there are two coordinate variables defined with respect to a grid_mapping variable. 61 The grid is defined by a set of values, quantities with respect to an ISO19111 CRS. In CFNetCDF terminology, there are two coordinate variables defined with respect to a grid_mapping variable. 62 63 This definition process is all about providing explicit details about the meaning of the individual values of a coordinate to allow them to be interpreted properly. 62 64 63 65 64 66 65 The functionality provided by ''formula terms'' in a CF NetCDF file enables a set of coordinate values to be inferred from other information in the file. This is a hugely flexible tool, which has so far been used to define parameterised vertical coordinates, common in atmosphere models, but has potentially many further uses. One, well defined transformation is detailed, enabling a set of coordinate values to be derived following the defined process.67 The functionality provided by ''formula terms'' in a CF NetCDF file enables a set of coordinate values to be inferred from other information in the file. One, well defined transformation is detailed, enabling a set of coordinate values to be derived following the defined process. This is a hugely flexible tool, which has so far been used to define parameterised vertical coordinates, common in atmosphere models; it has the potential for many further uses not connected to spatial coordinates. 66 68 67 The result is a coordinate, just like a coordinate variable or auxiliary coordinate variable. In many cases, it will be useful to be able to define this derived coordinate with respect to some well specified coordinate reference system; there has been significant recent discussion on this topic on the mailing list.69 The result of this defined process is conceptually a coordinate, just like a coordinate variable or auxiliary coordinate variable. 68 70 69 The derived nature is related to the coordinate values. The spatial referencing is an explicitly separate concept. I think we will store up a large amount of confusion and complexity for the future if we attempt to conflate these two concepts within the CF data model. 71 In many cases, it will be useful to be able to define this derived coordinate with respect to some well specified coordinate reference system; there has been significant recent discussion on this topic on the mailing list. This process of defining a relationship to a coordinate reference system is a second, separate step, not connected to the derived nature of the coordinate at all. The derived nature is related to the coordinate values. The spatial referencing is an explicitly separate concept. 70 72 71 If we keep them separate, we may be able to follow the ISO terminology and describe Coordinate Reference System instances within our model in ways which are consistent with the ISO definitions, which I would find particularly useful. 73 I think we will store up a large amount of confusion and complexity for the future if we attempt to conflate these two concepts within the CF data model. 74 75 If we keep these separate, we may keep the model simpler and perhaps be able to follow the ISO terminology and describe Coordinate Reference System instances within our model in ways which are consistent with the ISO definitions, which I would find particularly useful. 72 76 73 77