wiki:MultipleProjectionsJohn

Version 4 (modified by jonathan, 13 years ago) (diff)

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I prefer this proposal, because it represents each coordinate system explicitly as something that can be referred to by name. That makes it possible for a coordinate system to be represented even if there is no variable that uses it (yet). Disadvantages include the length of the representation and the arbitrariness of the type chosen to represent the coordinate system variables Dutch_CoordSystem and German_CoordSystem. These variables only exist to hang attributes on, so both their type and value are irrelevant. Nevertheless, this representation seems to capture the intent cleanly, and the extra name required for the coordinate system provides an additional opportunity for making the coordinate systems clear. -- Russ

This proposal introduces a coordinate system variable which is separate from the data variable. The coordinate system variable points to the grid mapping and the projection coordinates. The data variable points to one or more coordinate system variables. The advantage of this is that it directly associates the grid mapping and the projection coordinates, and it saves having to make the same association on every data variable. I would say that this is an attractive idea, and it is quite natural to introduce this abstraction as a way of interpreting the file. However, I think it would be inconsistent to separate the coordinate system from the data variable only in this rather specialised case, while we do not do it in the commoner situations where the coordinate system is defined by the 1D coordinate variables, or by 2D lat-lon auxiliary coordinate variables. Also, this abstraction may make the file harder to understand and to use by simple programs or inexpert users. Users are accustomed to seeing the coordinates attached to the data variable, which seems an obvious approach. We have so far not been requested to introduce a way to define a coordinate system which doesn't apply to an existing data variable. Jonathan