Links and Resources

1. Spatial feature representation

OpenGIS Abstract Specifications 

Start with Topic 0 Overview -Introduction and roadmap to the Abstract specification.

OpenGIS Implementation Specification for Geographic information - Simple feature access - Part 1: Common architecture

It describes the common architecture for simple feature geometry. The base Geometry class has subclasses for Point, Curve, Surface and GeometryCollection. Each geometric object is associated with a Spatial Reference System, which describes the coordinate space in which the geometric object is defined. The extended Geometry model has specialized 0, 1 and 2-dimensional collection classes named MultiPoint, MultiLineString and MultiPolygon for modeling geometries corresponding to collections of Points, LineStrings and Polygons, respectively. MultiCurve and MultiSurface are introduced as abstract superclasses that generalize the collection interfaces to handle Curves and Surfaces.

2. UML ...

UML stands for Unified Modelling Language. It is a meta-languages that can be used to model spatial information in order to represent it in digital format.

Explore UML building blocks

3. XML

XML stands for eXtensible Markup Language. It was designed to store and transport data.

Make familiar yourself with XML. There are available XML tutorial  and videos for beginners on youtube.

XML

• tags are not predefined like HTML tags are. It means the author must define both the tags and the document structure.
• stores data in plain text format. This provides a software- and hardware-independent way of storing, transporting, and sharing data.
• data in XML format can be available to all kinds of "reading machines" like people, computers, voice machines, news feeds, etc.
• does not carry any information about how to be displayed

XML documents that conform to the syntax rules are well formed XML documents.

An XML Schema describes the structure and the building blocks of an XML document. Read XML Schema Tutorial.

The XML Schema language is also referred to as XML Schema Definition (XSD). An XML document validated against an XML Schema is well formed and valid.

With XML Schema:

• XML files can carry a description of its own format.
• independent groups of people can agree on a standard for interchanging data.
• data can be verified

Elements in an .xsd schema have to be defined either as simpleTypes or complexTypes. A complex type supports attributes, empty content of the corresponding XML instance, and nested elements that may enforce a specific sequence of sub-elements.

An XML schema has the following properties

• It is extensible: a schema can be reused in other schemas; multiple schemas can be referenced in the same XML document; own data types can be created, based on the standard types
• It is written in XML. A XML schema file has the extension .xsd
• It supports data types
• It supports namespaces: a namespace declaration has the following syntax: xmlns:prefix=“URI”; it helps to solve name conflicting by using a prefix

XML documents can have a reference to an XML Schema.

4. Geography Markup Language - GML

GML

Any communication process involves the presence of a sender, a recipient and a communication channel. To understand the message transmitted by a sender (through an individual or a computer system) the receiver needs to know the encoding mechanism, i.e. the  language used. Thus, in order to communicate, a common language is required. In digital geographic domain, the common language for communication is Geographic Markup Language.

Geography Markup Language (GML) is an open standard encoding geographic features and their (spatial and non-spatial) properties using the XML (eXtensible Markup Language) framework.  GML  is being specified by the OGC and was adopted as ISO Standards (ISO 19136:2007 ) in 20017. It is the basis of communication within geospatial community, by enabling a vendor-neutral exchange of spatial data. Each storage data format (e.g. shapefile, Oracle, PostGIS) can be converted to GML and then published online via web services. Uses of GML: interoperable exchange of geographic data; common format for transferring data between different GIS, Data storage.

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