[dhcwg] Re: Unit of Measurement...?

Greg Troxel <gdt@ir.bbn.com> Thu, 10 July 2003 17:36 UTC

Received: from optimus.ietf.org (ietf.org [132.151.1.19] (may be forged)) by ietf.org (8.9.1a/8.9.1a) with ESMTP id NAA13605; Thu, 10 Jul 2003 13:36:38 -0400 (EDT)
Received: from localhost.localdomain ([127.0.0.1] helo=www1.ietf.org) by optimus.ietf.org with esmtp (Exim 4.20) id 19afKs-00056d-AA; Thu, 10 Jul 2003 13:36:02 -0400
Received: from odin.ietf.org ([132.151.1.176] helo=ietf.org) by optimus.ietf.org with esmtp (Exim 4.20) id 19aJno-0001X0-6m for dhcwg@optimus.ietf.org; Wed, 09 Jul 2003 14:36:28 -0400
Received: from ietf-mx (ietf-mx.ietf.org [132.151.6.1]) by ietf.org (8.9.1a/8.9.1a) with ESMTP id OAA13325 for <dhcwg@ietf.org>; Wed, 9 Jul 2003 14:36:24 -0400 (EDT)
Received: from ietf-mx ([132.151.6.1]) by ietf-mx with esmtp (Exim 4.12) id 19aJnl-0001dF-00 for dhcwg@ietf.org; Wed, 09 Jul 2003 14:36:25 -0400
Received: from fnord.ir.bbn.com ([192.1.100.210]) by ietf-mx with esmtp (Exim 4.12) id 19aJnj-0001dB-00 for dhcwg@ietf.org; Wed, 09 Jul 2003 14:36:23 -0400
Received: by fnord.ir.bbn.com (Postfix, from userid 10853) id AF32720E0; Wed, 9 Jul 2003 14:36:21 -0400 (EDT)
To: Henning Schulzrinne <hgs@cs.columbia.edu>
Cc: dhcwg@ietf.org, geopriv@mail.apps.ietf.org
References: <000501c340ab$46784be0$220d0d0a@mlinsnerzk7abh> <000501c340ab$46784be0$220d0d0a@mlinsnerzk7abh> <4.3.2.7.2.20030708235923.053d2718@localhost> <3F0C533D.1010605@cs.columbia.edu>
From: Greg Troxel <gdt@ir.bbn.com>
Date: Wed, 09 Jul 2003 14:36:21 -0400
In-Reply-To: <3F0C533D.1010605@cs.columbia.edu>
Message-ID: <rmiisqbzgwq.fsf@fnord.ir.bbn.com>
Lines: 35
User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Subject: [dhcwg] Re: Unit of Measurement...?
Sender: dhcwg-admin@ietf.org
Errors-To: dhcwg-admin@ietf.org
X-BeenThere: dhcwg@ietf.org
X-Mailman-Version: 2.0.12
Precedence: bulk
List-Unsubscribe: <https://www1.ietf.org/mailman/listinfo/dhcwg>, <mailto:dhcwg-request@ietf.org?subject=unsubscribe>
List-Id: <dhcwg.ietf.org>
List-Post: <mailto:dhcwg@ietf.org>
List-Help: <mailto:dhcwg-request@ietf.org?subject=help>
List-Subscribe: <https://www1.ietf.org/mailman/listinfo/dhcwg>, <mailto:dhcwg-request@ietf.org?subject=subscribe>

  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