ET-IDM Geneva 1st - 3rd September 2004 The Use of GML for the Unified Definition of Boundary Polygons by Dieter Schröder Deutscher Wetterdienst Postfach D Offenbach

ET-IDM Geneva 1st - 3rd September 2004 Summary Advantages: The approach is excellent! GML provide an easy and flexible possibility to define spatial geometries The definition of spatial geometries is very easy The close connection to XML allows an easy transfer of GML defined geometries between different IT-applications (GIS, data bases, self-written programs) Problems: Details still in developement! The GML standard is still in fundamental motion Although it is downward compatible, basic elements has been set depricated. The connection of a GML geometry and the referenced coordinates is still a problem.

ET-IDM Geneva 1st - 3rd September 2004 Introduction: What is GML? GML: Geography Markup Language GML based on XML: philosophy, basic structures, grammar GML can be regarded as a XML dialect for spatial information GML is vector oriented --> objekts are therefore resolution independent GML transports map content, and not displaying features GML objects can be linked to additional data/information GML objects can specify time contraints and reference points --> Definition of moving objects are possible GML defined data can be exchanged with any IT-system providing an XML interface Application of GML is focused on Internet based mapping tools Examples: NASA Web Map Server ( ESA Web Map Server (

ET-IDM Geneva 1st - 3rd September 2004 Introduction: GML structure Elements A GML file consists of a collection of Features GML has Feature elements which descripes phenomina in the real world Features contain subelements (properties) which identify characteristics of a Feature Subelements can also be or contain Features Example: Feature City have properties cityMember (1... N) which contain Features like River, Road,... GML definitions can contain links. An attribute identifier gml:id can be specified. Conventions Feature_names (upper case) property_names (lower case)

ET-IDM Geneva 1st - 3rd September 2004 Object hierachy of geometries: (complies with ISO 19107)

ET-IDM Geneva 1st - 3rd September 2004 Coordinate Data (I) For spatial information, GML geometries relate to a coordinate reference system (CRS) The coordinate reference system (CRS) must be - defined in GML - link a so-called well known system to the GML definition CRS indicates how the coordinates are to be interpreted CRS indicates also in which order the dimensions are quoted (N/E-, x/y-pairs) The coordinate of features are always properties which can be defined as: (1) one coordinate tuple: pos (2) a list of coordinate tuples: posList (3) a defined or referenced point: pointProperty

ET-IDM Geneva 1st - 3rd September 2004 Coordinate Data (II) (1) x1 y1 (2) x1 y1 x2 y2 (3) x1 y1 Examples:

ET-IDM Geneva 1st - 3rd September 2004 Feature: defined by: Example: gml:LineString (1) seqence of pos or pointProperty (2) a posList x1 y1.. xN yN gml:LinearRing --> additional condition xN yN = x1 y1 x1 y1.. x1 y1 Feature:gml:Curvegml:Ring Note: These elements are the bases for all GML geometries!! Line and Area Definition

ET-IDM Geneva 1st - 3rd September 2004 Surface Definition GML surfaces are defined by certain surface elements or polygons Surface elements are: Triangle, Rectangle, PolygonPatches Polygons consist of one or more LinearRings Polygons can also have an exterior or interior property Note: When a apropriate coordinate system is referenced the construction of a polygon will be no problem even if a pole is encompassed or a date line is crossed!

ET-IDM Geneva 1st - 3rd September 2004 Surface Definition: Example x1 y1.. right_poly

ET-IDM Geneva 1st - 3rd September 2004 Surface Definition: Example x0 y0.. x1 y1... xN yN left_poly

ET-IDM Geneva 1st - 3rd September 2004 Compount Geometries (I) 1. GeometricComplex a given number of element properties which can contain a geometric primitive 2. Geometric Composites (recommended) are restricted to a combination of a particular form Example:..... </gml:CompositeSurface

ET-IDM Geneva 1st - 3rd September 2004 Compount Geometries (II) 3. Geometric Aggregates (recommended) can also combine non-related geometric elements Defined aggregates are: MultiGeometry geometryMember MultiPoint pointMember MultiCurve curveMember MultiSurface surfaceMember (recommended)

ET-IDM Geneva 1st - 3rd September 2004 Extent of Features Self definition of a feature: complexType name=... Additional spatial information can be attached with: extentOf / multiExtentOf A maximum boundary can be specified with the boundedBy property (recommended) xl yl xu yu Note: If an appropriate earth coordinate system is referenced, the upper corner can have smaller values than the lower corner (poles encompassed)! An all-inclusive boundary of several features can be obtrained using Envelope feature (recommended)

ET-IDM Geneva 1st - 3rd September 2004 Coordinate Reference Systems (CRS) GML requires a CRS, otherwise all coordinate interpretation are ambigous The reference to coordinates is achieved by a: (1) so-called well-known CRS (2) definition of an own-purpose CRS on the basis of ISO The reference to a well-known CRS can be done with the srsName atrribute in the feature gml:AbstractGeometryType Example: urn:EPSG:geographicCRS:4326 lead to a definition of a well-known CRS specified by the European Petroleum Survey Group urn:EPSG:geographicCRS:4326 uses latitude/longitude and is equal to WGS84 The World Geodetic System 1984 is the basis of GPS data For a definition of an own CRS you need: (1) Mathematical description of the coordinate system (coordinates points) (2) Position of the origin (3) Orientation of the axis (4) Scale (5) Transformation rules

ET-IDM Geneva 1st - 3rd September 2004 Recommendations 1. For coordinate reference a well-known coordinate systems should be used preferably. WMO recommend the adaption of the urn:EPSG:geographicCRS: CRS are to be specified using GML only if they are not well-known 3. WMO has to define a specified schema document containing the definitions for several features 4. Objects for the definiton of boundary polygons should be restricted in the WMO. 5. For the definition of a simple boundary polygon in earth coordinates, only the GML object Polygonshould be used 6. To define a multiple connected area the CompositeSurface feature should be used. 7. Non connected multiple areas should be defined with MultipleSurface and FeatureCollection. 8. For computation and presentation purpose boundedBy and Envelope can be used. 9. For assignment of additional information (spatial weather phenomina) extentOf and multiExtentOf aliases should be used.