[dhcwg] Re: Unit of Measurement...?
Greg Troxel <gdt@ir.bbn.com> Thu, 10 July 2003 17:36 UTC
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To: Henning Schulzrinne <hgs@cs.columbia.edu>
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From: Greg Troxel <gdt@ir.bbn.com>
Date: Wed, 09 Jul 2003 14:36:21 -0400
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Subject: [dhcwg] Re: Unit of Measurement...?
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I gather the datum for long/lat does not uniquely define the datum for altitude. Exactly. Older datums (pre navigation satellite) are either horizontal only or vertical only. There is a newer group (WGS84, ITRF) which are three dimensional. The reason for the split is that classical geodetic methods for determining horizontal coordinates (chaining and tapes for distance and optical measurement of angles) and vertical coordinates (use of telescopic levels to transfer vertical location horizontally, and measuring rods for measuring vertical distance) had very little to do with each other, and the measurement campaigns were often separate. Modern geodetic practice makes use of satellite measurements which result in three-dimensional position differences between stations. But, as I pointed out in the message I just sent, there is generally a vertical datum associated with a given horizontal datum. By associated, I mean that, e.g., I suspect that well over 90% of all maps in NAD27 use NGVD29. So there is not a lot to lose by defining a codepoint for a pair; the combinatorial explosion won't arise. E.g., ED50 and NAVD88 is simply not a sensible combination - I'm willing to bet there are no actual maps in that combination. This wouldn't be so confusing if it were not for nautical charts, which use different vertical datums. The question is "will my ship hit the bottom", which drives the reference surface to Mean Lower Low Water. I hold that the presence of such charts makes it necessary to be unambiguous about the height reference system. There are current charts with NAD83 horizontal datums and MLLW vertical, and current maps with NAD83 horizontal and NAVD88 vertical. So NAD83 does not have an unambiguous associated vertical datum. -- Greg Troxel <gdt@ir.bbn.com> _______________________________________________ dhcwg mailing list dhcwg@ietf.org https://www1.ietf.org/mailman/listinfo/dhcwg
- [dhcwg] Fwd: Working Group Last Call: Location Co… Ralph Droms
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Robert Elz
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… John Schnizlein
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Robert Elz
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Andrew Daviel
- RE: [dhcwg] Fwd: Working Group Last Call: Locatio… Marc Linsner
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Henning Schulzrinne
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… creediii
- [dhcwg] Unit of Measurement...? James M. Polk
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… John Schnizlein
- [dhcwg] Re: Unit of Measurement...? James M. Polk
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… James M. Polk
- RE: [dhcwg] Fwd: Working Group Last Call: Locatio… Marc Linsner
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Henning Schulzrinne
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Greg Troxel
- [dhcwg] Re: Unit of Measurement...? Henning Schulzrinne
- [dhcwg] Re: Unit of Measurement...? Greg Troxel
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Greg Troxel
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Greg Troxel
- [dhcwg] Re: Unit of Measurement...? Carl Reed
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Dominic Pinto
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Carl Reed