Dear TC -

We have fine tuned the requirements etc for the OASIS where GML application
schema. Before asking the OGC GML gurus to complete the schema, we in this TC
need agreement on the following recommendations that are based on stated
requirements in teleconferences as well as in the comments received during the
CAP comment period. Also, please be aware that GML is grounded in an ISO
document known as 19107: Spatial Schema. This document defines an abstract model
for describing spatial characteristics of geographic features. I have included
the introduction to that international standard below.

1. Recommend that the schema be based on GML 3.2.1. GML 3.2.1 is an OGC
standard that has also been approved by ISO TC 211 as an International
Standard. The draft OASIS where schema developed a couple of years ago is based
on GML 3.1.1. Issue: Some of the existing OASIS EM standards need to be double
checked to determine if there are any backwards compatibility issues.

2. The where schema shall support the ability to express coordinate
reference systems (CRS) other than just the current default WGS-84 2d geographic
CRS. CRS: coordinate system that is related to the real world by a datum.

3. The where schema shall support 2d point geometries. Point: 0-dimensional
geometric primitive, representing a position. Point=<(x1, y1)>.

4. The where schema shall support the semantic concept of "floor". This is
to be consistent with current OASIS and IETF standards that are in use by the EM
community as well as the i3 architecture of the NextGen 911 deployment.

5. The where schema shall support 3d point geometries. Point: 0-dimensional
geometric primitive, representing a position. (I added this one as 3d point
geometries are seen as highly important in many EM applications)

6. The where schema shall support 2d multi-point geometries. GM_MultiPoint
is an aggregate class containing only points. Examples of multi-point
geometries are multiple hot spots in a wildfire or multiple radiological sensor
locations.

7. The where schema shall support 2d linestrings. line string: curve
composed of straight-line segments. Therefore, Linestrings are a set of
connected line segments of the form: Linestring =<(x1, y1), (x1, y2), (x2,
y2), (x2, y1), (x1, y1)>. Linestrings can be used to represent roads,
streams, utility lines, etc. NOTE: Should linestring concept be extended to deal
complexes, such as curves and splines? GML supports this capability.

8. The where schema shall support 2d envelopes (area of interest). The
simplest representation for an envelope consists of two DirectPositions, the
first one containing all the minimums for each ordinate, and second one
containing all the maximums. GM_Envelope = <lowerCorner = (x1, y1),
upperCorner = (x2, y2)>

9. The where schema shall support 2d polygons (AKA rings). A polygon is
a planar surface defined by 1 exterior boundary and 0 or more interior
boundaries. Therefore, a polygon consists of a <Polygon> element
with a child <exterior>, <LinearRing> and <coordList>
elements. There must be at least four pairs with the last being identical to the
first. (a boundary has a minimum of three actual points.) No two pairs may be
separated by more than 179 degrees in either latitude or longitude.
<Exterior> specifies this shape as defining the outside of an area, and
<LinearRing> states that the coordinates should be connected with straight
lines. Within <coordList> the coordinates of the points are entered as
pairs of latitude and longitude values, separated by spaces. There must be at
least four pairs with the last being identical to the first. (a polygon has a
minimum of three actual points.) No two pairs may be separated by more than 179
degrees in either latitude or longitude.

10. The where schema definition for polygon shall also support
interior rings (AKA holes, islands, donuts). Interior: set of all direct
positions that are on a geometric object but which are not on its
boundary.

11 The where schema shall support the ability to encode multiple instances
of a phenomenon, such as a chemical plume over time. This needs to be discussed
a bit more as there are several ways this can be done using GML.

The above requirements and definitions are consistent with GeoRSS as well
as the GML application schema being used by the IETF for all their geodetic
(coordinate) payload definitions.

I plan on discussing these requirements at the next TC
teleconference.

Regards

Carl

Introduction to ISO
19107:

This International Standard provides
conceptual schemas for describing and manipulating the spatial characteristics
of geographic features. Standardization in this area will be the cornerstone for
other geographic information standards.

A feature is an abstraction of a
real world phenomenon; it is a geographic feature if it is associated with a
location relative to the Earth. Vector data consists of geometric and
topological primitives used, separately or in combination, to construct objects
that express the spatial characteristics of geographic features. Raster data is
based on the division of the extent covered into small units according to a
tessellation of the space and the assignment to each unit of an attribute value.
This International Standard deals only with vector data.

In the model defined in this
International Standard, spatial characteristics are described by one or more
spatial attributes whose value is given by a geometric object (GM_Object) or a
topological object (TP_Object). Geometry provides the means for the quantitative
description, by means of coordinates and mathematical functions, of the spatial
characteristics of features, including dimension, position, size, shape, and
orientation. The mathematical functions used for describing the geometry of an
object depend on the type of coordinate reference system used to define the
spatial position. Geometry is the only aspect of geographic information that
changes when the information is transformed from one geodetic reference system
or coordinate system to another.

Topology deals with the
characteristics of geometric figures that remain invariant if the space is
deformed elastically and continuously. for example, when geographic data is
transformed from one coordinate system to another. Within the context of
geographic information, topology is commonly used to describe the connectivity
of an n-dimensional graph, a property that is invariant under continuous
transformation of the graph. Computational topology provides information about
the connectivity of geometric primitives that can be derived from the underlying
geometry.

Spatial operators are functions and
procedures that use, query, create, modify, or delete spatial objects. This
International Standard defines the taxonomy of these operators in order to
create a standard for their definition and implementation. The goals are
to:

a) Define spatial operators
unambiguously, so that diverse implementations can be assured to yield
comparable results within known limitations of accuracy and
resolution.

b) Use these definitions to define a
set of standard operations that will form the basis of compliant systems, and,
thus act as a test-bed for implementers and a benchmark set for validation of
compliance.

c) Define an operator algebra that
will allow combinations of the base operators to be used predictably in the
query and manipulation of geographic data.

Standardized conceptual schemas for
spatial characteristics will increase the ability to share geographic
information among applications. These schemas will be used by geographic
information system and software developers and users of geographic information
to provide consistently understandable spatial data structures.

Carl Reed, PhD

CTO and Executive Director Specification
Program

OGC

The OGC: Helping the World to Communicate Geographically

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