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----- Original Message -----
From: "Michael Kay" <michael.h.kay@ntlworld.com>
To: "'Roger L. Costello'" <costello@mitre.org>; <xml-dev@lists.xml.org>
Sent: Sunday, January 05, 2003 10:51 AM
Subject: RE: [xml-dev] 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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