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Subject: 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.


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
The OGC: Helping the World to Communicate Geographically
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