Thanks for the comments. Some thoughts inline:
> However, marking alone isn't very interesting or helpful, except to
> recognize that all you can do is discard the information received.
Exactly. That's why i was pushing for an agreed default that is well
known and understood to any author and consumer.
> (1) Either the sender re-codes the datum to WGS-84, if that is
> possible without losing information;
That's exactly the idea. If people are unhappy with that option, there's
always GML to express nearly arbitrarly complex geometries in thousands
of CRSes. GML documents are not simple and concise, though.
> (2) Or it negotiates what datum to use, hoping to find common
> ground -
> only applies in certain circumstances and would point to extensions
> of, say, HTTP;
As you noted, negotiation is not possible in many situations, because it
requires a protocol that can negotiate - therefore i don't think it's an
option.
> (3) Or it provides multiple versions, in the hope that one of them
> will work for the receiver.
Would be an option, although not an efficient one - especially
considering that there are thousands of CRSes out there.
Alex
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