RE: OASIS where GML application schema - requirements - please review

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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