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Hi Michael, Michael Kay wrote: > > > Thus, distance is poor interchange data... > > > > Good interchange data is position. > > The only way I know of to identify spatial position is as a distance > from some agreed origin. Yes, but the point is that <distance to="destination airport">590</distance> Has an implied referent: from "me" to the destination airport. That makes the utility of the data very limited (only to clients the understand the implied referent, which is mostly just "me", the sender). With position, on the other hand, all referents are explicit. Consequently, the data is useful not just to one client, but to any client. Practically speaking, what this means is that with position data clients can do things like calculate the aircraft heading, compute time to fly-over, insert the data into a map, etc. Position is "high value" data. The issue that Tony and Tom have raised is that, despite position being more generally useful to clients, sometimes it's necessary to send the calculated data (distance) rather than the fundamental data (position) because the recipient may take the position data and generate a distance that is not quite consistent (due to rounding errors, for example) with the distance that the sender believes to be the case. This is the issue we are wrestling with now. > All data interchange relies on the sender and recipient having > some shared knowledge. There is no question there must be shared knowledge (metadata). For example: If the data being interchanged is position data then the metadata is the coordinate reference system. I went on to argue that there should be precisely one, unambiguous coordinate reference system. It becomes the "lingua franca" coordinate reference system. If the data being interchanged is distance data then the metadata may be, say, the units (meters, miles, etc). /Roger
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