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

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

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

At 08:38 AM 6/20/2003, Robert Elz wrote:
>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.

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.

>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).

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.

>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.

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