Re: Identifying Data for Interchange [was: XML Components]
----- Original Message ----- From: "Michael Kay" <michael.h.kay@n...> To: "'Roger L. Costello'" <costello@m...>; <xml-dev@l...> Sent: Sunday, January 05, 2003 10:51 AM Subject: RE: Identifying Data for Interchange [was: XML Components] > > 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. This is incorrect. Position is a vector, measured with respect to some origin. Distance from such an origin is a scalar, and requires an additional direction relative to some reference line to convey as much information. From knowledge of the position and the origin from which it is measured, you can determine anything that a distance would give you, but it may be less convenient for the logic you're trying to apply. For instance, if you're trying to invoke some rule that says "when within 10 miles of JFK airport, switch cockpit radio to channel 3", having a distance from JFK sirport is more convenient than having a position (even with respect to JFK as origin). If all you have is a distance from Heathrow airport, you cannot tell whether you are anywhere near New York, since the distance only specifies a spherical surface centered on Heathrow. If you have just the position, you need to compute the distance to determine the applicability of the rule. In real life, the air traffic control region of a given airport is probably not a sphere (I am ignorant of any real-life details). Probably you would be better off knowing the position, and computing whether it lies within the boundaries of the air traffice control region of an airport. Of course, the other issues raised in this thread, such as different ways of computing distance from a position (does the altitude take part in the distance computation, the origin of the altitude computation, relative to an ellipsoid, or actual ground level, or sea level, the units of measure and reference locations, all are important. The auxiliary information that specifies the reference locations and coordinate systems must be either passed along with the position data or be shared knowledge between sender and receiver. Jeff
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