Subject: [dhcwg] Fwd: Working Group Last Call: Location Configuration Information for GEOPRIV

From: Ralph Droms <[email-masked]>
Date: Fri, 20 Jun 2003 06:23:31 -0400

FYI ... announcing a WG last call on draft-ietf-geopriv-dhcp-lci-option-01.txt.

Please review the draft and respond to this WG
last call. If you support acceptance of the document
without change, respond with a simple acknowledgment,
so that support for the document can be assessed.

- Ralph

>To: [email-masked]
>Subject: Working Group Last Call: Location Configuration Information for 
>GEOPRIV
>Date: Wed, 18 Jun 2003 22:51:40 -0700
>From: Allison Mankin <[email-masked]>
>
>This begins WGLC for Location Configuration Information for GEOPRIV,
>draft-ietf-geopriv-dhcp-lci-option-01.txt.  The WGLC will take place
>in both the GEOPRIV and DHC WGs, and in consequence, will be allowed
>three weeks rather than two, concluding Jul 10.

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---

From: Robert Elz <[email-masked]>
Date: Fri, 20 Jun 2003 19:38:39 +0700

The definitions of altitude in that draft need work.

Metres (positive and negative) is simple enough, but relative to
what?   Unless I missed it, I could find no definition of what is 0.

If the assumption is that it is to be "mean sea level" then which sea?
If it is to be above (+ or - below) ground level, which ground level
within the designated area given by the latitude and longitude (sloping
ground).

When measured in floors, the "0" means ground level is fine, but
how are floors counted?   Is there some standard, or does anyone
count them however they like.   Can one assume that if some device
is at altitude (as a floor) N, then something at N+2 will be two
floors up?    If N==12?   How do buildings with offset floors work,
where in one side of the building you can go up (naturally) a floor
at a time, but on the other side all the levels are offset by a
half a floor?   Just to confuse things more, there's a building
not too far away from where I am which has two "level 2" floors,
that is, the elevator stops twice between floor 1 and floor 3
(they're called 2b and 2d - though I have not the vaguest idea
what the 'b' and 'd' mean - 2d is above 2b).   There needs to be
more text describing just what floor numbers mean, and perhaps how
the numbers are meant to be translated to and from the external
designations that people use.

kre

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From: John Schnizlein <[email-masked]>
Date: Fri, 20 Jun 2003 15:44:23 -0400

Thank you for the probing questions. Clarification embedded in context:

>what?   Unless I missed it, I could find no definition of what is 0.

Zero is defined by the datum, at least for those we know.
To cover any cases where it is not, we propose clarifying text as follows:
   If MU = 1, an AltRes value 0 would indicate unknown altitude. 
   The most precise Altitude would have an AltRes value of 30. 
   Unless otherwise defined by datum, a value of 0 is with respect 
   to mean-low-tide.

>within the designated area given by the latitude and longitude (sloping

It is our understanding that mean low tide is well defined, since
political and personal land-ownership boundaries are based on it.

The reason this is a twos-complement value is to cover below zero
altitudes as well as above zero altitudes.

We are not attempting to represent any slope of the ground. The
altitude is that of a point within the horizontal resolution of the
horizontal location values.

>designations that people use.

Yes, the text about what floor values mean was mistakenly dropped
in the editing process. We had lovely arguments about it before 
resolving intermediate floors with the 8-bit fraction part of the
30-bit twos-complement value. Your observation that local customs
sometimes include intermediate floors answers the question about
interval scale: one cannot assume just N floors between values of
the value that differ by N. Although not interval-scale, the value
is monotone (ordinal-scale).

We propose clarifying text for intermediate floors as follows:
  The values represented by this MU will be of local significance.  
  Since buildings and floors can vary due to lack of common control, 
  the values chosen to represent the characteristics of an 
  individual building will be derived and agreed upon by the 
  operator of the building and the intended users of the data.  
  Attempting to standardize this type of function is beyond the 
  scope this document.

  Sub-floors can be represented by non-integer values.  
  Example (1): a mezzanine between floor 1 and floor 2 could be 
  represented as a value=1.1.  Example (2): two mezzanines between 
  floor 4 and floor 5 could be represented as values=4.1 and 4.2 
  respectively.

  Floors located below ground level could be represented by 
  negative values.

  Larger values represent floors that are above (higher in altitude)
  floors with lower values.

John

---

From: Robert Elz <[email-masked]>
Date: Sat, 21 Jun 2003 17:16:49 +0700

Date:        Fri, 20 Jun 2003 15:44:23 -0400
    From:        John Schnizlein <[email-masked]>
    Message-ID:  <[email-masked]>

  | To cover any cases where it is not, we propose clarifying text as follows:
  |    If MU = 1, an AltRes value 0 would indicate unknown altitude.

That's fine, and a good idea.

  |    The most precise Altitude would have an AltRes value of 30. 
  |    Unless otherwise defined by datum, a value of 0 is with respect 
  |    to mean-low-tide.

  | It is our understanding that mean low tide is well defined, since
  | political and personal land-ownership boundaries are based on it.

In most places it is, but consider you're somewhere in the middle of
Panama, which low tide do you consider, the Atlantic, or the Pacific?
There's a considerable distance in height between the two.

I'd be tempted to pick the mean earth radius, and define that as 0,
except then heights would be meaningless just about everywhere when
compared to normal usages (not above ground level, not mean low tide
sea level, not ...).   Good luck finding some way that defines 0
precisely.

  | The reason this is a twos-complement value is to cover below zero
  | altitudes as well as above zero altitudes.

Yes, that was obvious (I didn't think I said anything to question that).

  | We are not attempting to represent any slope of the ground. The
  | altitude is that of a point within the horizontal resolution of the
  | horizontal location values.

No, I didn't think you were - but because the 0 point wasn't specified
anywhere that I could see (if it is there and I missed it, perhaps it
needs better highlighting), one possible definition of 0 may have been
"ground level at the latitude and longitude specified".   That would be
fine when all 34 bits of precision were used, but when things are being
deliberately vague, there would be no particular 0 point in that case.

But given that sea level is the intended 0, the point about slopes
is completely irrelevant, and you can ignore it.

  | Yes, the text about what floor values mean was mistakenly dropped
  | in the editing process.
  | 
  | We propose clarifying text for intermediate floors as follows:

That looks fine, and is about what I expected would be the best choice.

kre