An introduction and overview of Geography Markup Language

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An introduction and overview of Geography Markup Language Lecturer: Dr Dan Cornford d.cornford@aston.ac.

1 An introduction and overview of Geography Markup Language Lecturer: Dr Dan Cornford CS3210, Geographic Information Systems, Aston University, Birmingham, UK March 19, 2009 Dan Cornford GML Introduction 1/19

2 Overview GIS and the internet. Introduction to GML The role of the OpenGIS consortium. Implementing GML. Displaying GML over the web - interactive web based mapping. Future developments of GML. Dan Cornford GML Introduction 2/19

3 GIS and the internet There is a trend for increased web-centric computing. This trend has also affected GIS largely driven the formation of the OpenGIS consortium. In its simplest form this means the display of static maps over the internet - e.g. multimap. This is a naive use of the internet, since these static bitmaps are not particularly useful for querying i.e. contain little information. The future promises the creation of truly platform independent data, structured using XML, which can be queried. Thus the data becomes accessible and we will all be working over the internet using virtual data. Dan Cornford GML Introduction 3/19

4 GML - the encoding standard To facilitate the exchange of data (and subsequent translation into information) we must be able to effectively share data. In general this is not possible using proprietary binary software export formats, and is best facilitated using a format that is both machine and human readable XML. For geographic data it would be very useful if we could use a common XML schema to describe our data. GML (Geography Mark-up Language) provides just such a schema, which is public domain (OpenGIS) and XML based. Current version is GML 3, but we will start by looking at GML 2 which includes only simple features and geometry. Dan Cornford GML Introduction 4/19

5 GML Basics GML is a series of schema for describing geographic data in XML. Two key schema are: feature schema; geometry schema. Feature schema used to define geographical features in an object framework e.g. house, road, county... Geometry schema used to define the geometric features of objects e.g. point, line or polygon. Feature Geometery Xlink <<include>> <<import>> GML also uses the Xlink schema. Dan Cornford GML Introduction 5/19

6 Implementing GML GML2 implements only simple geometries: points, lines and polygons. More complex data types can be composed by combining multiple geometries. It is envisaged that users will define their own object types by extending the GML AbstractFeature class. These can then be grouped extending the GML AbstractFeatureCollection base class. Every feature can have associated properties (that the user is free to define). The properties include one or more associated geometries. Dan Cornford GML Introduction 6/19

7 GML2 example (data) <?xml version="1.0" encoding="utf-8"?> <CityModel xmlns=" xmlns:gml=" xmlns:xlink=" xmlns:xsi=" xsi:schemalocation=" city.xsd"> <gml:name>cambridge</gml:name> <gml:boundedby> <gml:box srsname=" <gml:coord><gml:x>0.0</gml:x><gml:y>0.0</gml:y></gml:coord> <gml:coord><gml:x>100.0</gml:x><gml:y>100.0</gml:y></gml:coord> </gml:box> </gml:boundedby> <citymember> <River> <gml:description>the river through Cambridge.</gml:description> <gml:name>cam</gml:name> <gml:centerlineof> <gml:linestring srsname=" url /gml/srs/epsg.xml#4326"> <gml:coord><gml:x>0</gml:x><gml:y>50</gml:y></gml:coord> <gml:coord><gml:x>70</gml:x><gml:y>60</gml:y></gml:coord> <gml:coord><gml:x>100</gml:x><gml:y>50</gml:y></gml:coord> </gml:linestring> </gml:centerlineof> </River> </citymember>... Dan Cornford GML Introduction 7/19

8 GML example continued (data) <citymember> <Road> <gml:name>m11</gml:name> <lineargeometry> <gml:linestring srsname=" url /gml/srs/epsg.xml#4326"> <gml:coord><gml:x>0</gml:x><gml:y>5.0</gml:y></gml:coord> <gml:coord><gml:x>20.6</gml:x><gml:y>10.7</gml:y></gml:coord> <gml:coord><gml:x>80.5</gml:x><gml:y>60.9</gml:y></gml:coord> </gml:linestring> </lineargeometry> <classification>motorway</classification> <number>11</number> </Road> </citymember> <citymember xlink:type="simple" xlink:title="trinity Lane" xlink:href=" gml:remoteschema="city.xsd#xpointer RoadType ])"/> <datecreated> </datecreated> </CityModel> Dan Cornford GML Introduction 8/19

9 GML2 schema example <?xml version="1.0" encoding="utf-8"?>  <schema targetnamespace=" xmlns:ex=" xmlns:xlink=" xmlns:gml=" xmlns=" elementformdefault="qualified" version="2.03"> <annotation> <appinfo>city.xsd v </appinfo> <documentation xml:lang="en"> GML schema for the Cambridge example </documentation> </annotation>  <import namespace=" schemalocation="feature.xsd"/>  <element name="citymodel" type="ex:citymodeltype" substitutiongroup="gml:_featurecollection" /> <element name="citymember" type="ex:citymembertype" substitutiongroup="gml:featuremember"/> <element name="road" type="ex:roadtype" substitutiongroup="ex:_cityfeature"/> <element name="river" type="ex:rivertype" substitutiongroup="ex:_cityfeature"/> <element name="mountain" type="ex:mountaintype" substitutiongroup="gml:_feature"/> Dan Cornford GML Introduction 9/19

10  <element name="_cityfeature" type="gml:abstractfeaturetype" abstract="true" substitutiongroup="gml:_feature"/>  <complextype name="citymodeltype"> <complexcontent> <extension base="gml:abstractfeaturecollectiontype"> <sequence> <element name="datecreated" type="month"/> </sequence> </extension> </complexcontent> </complextype> <complextype name="citymembertype"> <annotation> <documentation> A citymember is restricted to those features (or feature collections)that are declared equivalent to ex:_cityfeature. </documentation> </annotation> <complexcontent> <restriction base="gml:featureassociationtype"> <sequence minoccurs="0"> <element ref="ex:_cityfeature"/> </sequence> <attributegroup ref="gml:associationattributegroup"/> </restriction> </complexcontent> </complextype> Dan Cornford GML Introduction 10/19

11 <complextype name="rivertype"> <complexcontent> <extension base="gml:abstractfeaturetype"> <sequence> <element ref="gml:centerlineof"/> </sequence> </extension> </complexcontent> </complextype> <complextype name="roadtype"> <complexcontent> <extension base="gml:abstractfeaturetype"> <sequence> <element name="lineargeometry" type="gml:linestringpropertytype"/> <element name="classification" type="string"/> <element name="number" type="string"/> </sequence> </extension> </complexcontent> </complextype> </schema> This is a rather trivial and contrived example of a GML2 application schema! Dan Cornford GML Introduction 11/19

12 Structure of GML The abstract base types defined by the GML feature schema can be extended by the user. It is envisaged that some commonly used application schema will be defined for specific purposes. AbstractFeatureCollection Road * classification: string lineargeometry: LineStringPropertyType number: string * River centerlineof: LineStringPropertyType CityModel datecreated: month CityMember { _Feature: _CityFeature } AbstractFeature Dan Cornford GML Introduction 12/19

13 Feature-Property model of GML GML 2 applies a feature-property pattern in its schema. In GML 3.x feature becomes an object since a broader range of types is allowed, but the pattern is maintained. UpperCamelCase is used to define objects (and their types) e.g. Road, Bridge, Campus, Building. lowercamelcase is used to define properties (and their types). e.g. roadclass, bridgeconstruction, centrelineof, pos. In general, properties define the association role, and any objects contained in the property types the associated objects. Dan Cornford GML Introduction 13/19

14 Let s build an application schema! Dan Cornford GML Introduction 14/19

15 GML in use Both UK government and Ordinance Survey (MasterMap) committed to GML. All GIS companies have some GML presence e.g. some sell GML SVG converters. GML is more than a data sharing format in time will allow true distributed GIS. There are some problems for instance the size of GML files is typically 20 times bigger than specialist binary files compression. However, it seems likely that for some time large databases will remain in proprietary formats, being converted to XML (GML) on the fly. Dan Cornford GML Introduction 15/19

16 GML and the DNF The Digital National Framework (DNF) is a major project by the UK OS which replaces their detailed LandLine mapping product called MasterMap. Contains their most accurate data, and has topology (none existed in LandLine). Served in GML, which they have extended to include: query boxes (bounding rectangles); different feature types: topographic, cartographic and boundary; topological properties: polygons defined by oriented (clockwise / anti-clockwise) boundaries. Future may see this available on demand over the web (for a price!). Dan Cornford GML Introduction 16/19

17 GML 3.x GML 2.0 limited to simple geometries (points, poly-lines and polygons). GML 3.x includes: new geometry classes and topology; feature time stamps, histories and events (temporal GIS); units of measure; meta-data; coverages; observations. GML 3.x is not an end point, further development seems very likely. GML is still maturing in usage. Dan Cornford GML Introduction 17/19

18 GML and GIS Q: So will GML really revolutionise GIS? A: Probably not. BUT it does represent a much more open approach to data sharing / interoperability. Together with other innovations such as the catalogue services specification, which allows users to search for data on specific topics or regions GML may help overcome the biggest problem facing GIS finding and accessing data. Removing the data (time + cost) bottleneck may release GIS into a much wider user community. Keep an eye on this quickly developing technology... Dan Cornford GML Introduction 18/19

19 Summary The key points were: GML 2.x is an XML based language for describing simple geographic features. The OGC are the main innovators in this area, motivated by interoperability. GML is based on features, which can have certain properties (possibly multiple) you should be able to read and write simple GML. GML and other innovations may help overcome the data bottleneck. To use GML, application schema need to be created for each information community. Dan Cornford GML Introduction 19/19

Supporting GI standards with a model-driven architecture

Supporting GI standards with a model-driven architecture Roy Grønmo SINTEF Telecom and Informatics Forskningsveien 1, Pb 124 Blindern N-0314 Oslo, Norway T+47 22 06 74 32 roy.gronmo@informatics.sintef.no