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

From: [email-masked]
Date: Thu, 10 Jul 2003 11:28:18 -0400

To all:

As someone who has been working on location referencing for Intelligent 
Transportation Systems for years for the USDOT ITS standards program, I 
have been following with interest the recent discussion in this group on 
datums.  FYI, what follows is a set of extracts from a new SAE standard 
about to go to ballot (ATIS committee), called the Location Referencing 
Message Specification (LRMS).  The LRMS specifies data concepts and 
messages for a variety of coordinate based and non-coordinate based 
location referencing methods.  Note in the extracts that much of what you 
have been discussing is in the standard.  With respect to units, altitude 
is in 'Distance' units, which are a choice of integer or real expressions 
for different English and Metric units.  A complete XML Schema is part of 
the standard, and I would be glad to provide the complete draft to members 
of this group who would be interested in being official reviewers of the 
standard.

Also related to the discussion on WGS-84 and ITRF, an ISO TC204 sub-working 
group is developing an international location referencing standard for ITS 
location referencing between navigation databases.  While this standard is 
not complete, ITRF has been accepted as the standard geodetic datum, being 
more acceptable to our non-US members than WGS-84.

Dr. Cecil W. H. Goodwin
1564 Blue Heron Ln
Jacksonville Beach, FL 32250
Phone/Fax: 904-246-2264
Email: [email-masked]

--------------- excerpts --------------

5.15 Data Frame: GeoLocation:frame

Use: The GeoLocation sub-profile conveys a two- or three-dimensional 
geographic location.  The standard horizontal datum for a spot location is 
WGS-84; other horizontal datums can be specified by 
HorizontalDatum.  Vertical Datum is an option within height.

ASN.1 Representation:

GeoLocation ::= SEQUENCE {
      latitude                Latitude,
      longitude             Longitude,
      horizontalDatum   HorizontalDatum OPTIONAL,
      height                  Height OPTIONAL
          }

------------------------------------

5.26 Data Frame: Height:frame

Use: A vertical height expressed as either an altitude with respect to a 
vertical datum, or a vertical level from -7 to +7, where level 0 is the 
ground surface, or ground level of a structure.

ASN.1 Representation:

Height ::= CHOICE {
    altdatum  SEQUENCE {
       altitude       Distance,
       verticalDatum  VerticalDatum OPTIONAL
                       },
    verticalLevel  VerticalLevel
        }

--------------------------------------

6.17 Data Element: HorizontalDatum:cd
Use: The underlying horizontal geodetic datum for a geographic coordinate.

ASN.1 Representation:

HorizontalDatum ::= ENUMERATED {
  wgs-84                (0),
  wgs-84egm-96     (1),
  nad83                 (2),
  nad27                 (3)
  }

------------------------

6.59 Data Element: VerticalDatum:cd

Use: The underlying vertical geodetic datum for a geographic coordinate.

ASN.1 Representation:

VerticalDatum ::= ENUMERATED {
        wgs-84 (0),
        navd   (1),
        ...}

---------------------------

6.60 Data Element: VerticalLevel:qty

Use: Vertical level specified as an ordinal class expressed positive 
(above) or negative (below) the ground surface or ground level of a 
structure. (vertical level code = 0).

ASN.1 Representation:

VerticalLevel ::= INTEGER (-7..7)

XML Representation:
<xs:simpleType name="VerticalLevel" >
       <xs:restriction base="xs:byte">
          <xs:minInclusive value="-7"/>
          <xs:maxInclusive value="7"/>
       </xs:restriction>
</xs:simpleType>

----------------------

---

From: "James M. Polk" <[email-masked]>
Date: Mon, 14 Jul 2003 10:00:42 -0500

Cecil

Thank you for your comments, and I would be interested in your offer to 
review the document.

 From what subsections you provided, sections 5.26 and 6.60 seem flawed (or 
I don't understand the integer numbering reference -7 to +7). As our 
discussion here has made painfully clear, this LRMS also doesn't answer the 
question of "what is vertical (or altitude) = 0?" other than when 
referencing the ground level of a building.

For example, I'm in Vienna, Austria right now - what is ground level? There 
are a few fairly large water bodies within this city that appear to be a 
substantially below ground level relative to the ground level right next to 
it (all rivers here seem to have concrete vertical walls *up* to the street 
or dirt ground level). Some would argue that mean low sea level doesn't 
mean anything to a country that is land-locked (like Austria is).

By contrast, in San Francisco, ground level varies quite a bit from 
building to building that happen to be right next to each other in sections 
of that city, yet there is an ocean mean (or mean high, or mean low) sea 
level to relate to the ground level close by.

Were these two sections (5.26 and 6.60) intentionally left this vague, or 
is there still some work here to be done?

Please don't take my comments as an attack, just an observed hole or 
inconsistency that I'm open to have filled in/resolved.

At 11:28 AM 7/10/2003 -0400, [email-masked] wrote:
>Also related to the discussion on WGS-84 and ITRF, an ISO TC204 
>sub-working group is developing an international location referencing 
>standard for ITS location referencing between navigation databases.  While 
>this standard is not complete, ITRF has been accepted as the standard 
>geodetic datum, being more acceptable to our non-US members than WGS-84.
>
>Dr. Cecil W. H. Goodwin
>1564 Blue Heron Ln

cheers,
James

                                *******************
                    The answer is "42", what's the question?