Standards are a fundamental part of modern society; an organised way for ensuring best practice, common design, safety and many other benefits across every field of industry and science. A series of bodies exist to coordinate and promote the generation of standards. These bodies now play a crucial role in geographic information (GI) science.
ISO, the International Organisation for Standardisation, is very active in the field of standards for GI. The ISO Technical Committee – TC 211 – is in the process of producing standards for many aspects of spatial data, including metadata; spatial referencing by coordinates; imagery and gridded data; and data quality. For more details of ISO, and especially ISO/TC 211, visit the TC 211 official website.
BSI, the British Standards Institution, is the oldest national standards setting body in the world, including BS 7567 (aka NTF) and BS 7666 (Spatial data-sets for geographical referencing). Ordnance Survey is involved with the BSI's technical committee – IST/36 – which is responsible for the UK participation in the area of GI in international committees.
More recently, OGC, the Open GIS Consortium, has been established. OGC is an expanding organisation dedicated to the creation of standards in the field of interoperable geospatial systems. The membership includes most of the GIS and database vendors, several major spatial data users and a few spatial data producers. OGC develops interface specifications for geospatial data and systems, and is increasingly involved in prototyping services for serving and accessing spatial data over the Internet. One of the key areas of activity is encouraging the adoption of standard formats for geographic data exchange based on eXtensible Markup Language (XML) – see the XML and GML page. The OGC public web site contains a wealth of information about these activities.
Metadata is a word that frequently crops up when discussing GIS. It can be described as data about data. It can be very useful if files of digital information include additional information describing the contents of the main part of the file. This concept is inherent in many commonly encountered file types. For example, many graphics file formats like .GIF or .JPG contain a header component that does not specify the image itself but describes the palette of colours present in the image. Similarly, many web pages carry metadata contained in meta-tags at the top of the file.
Metadata is very important in GIS because many different datasets exist, and it is essential to know certain things such as who created it and when, the types of feature it contains, the geographical bounding area and the precision, accuracy and scale. Many standard GIS file types have a header or separate metadata file as part of the data format, or supply the metadata as a written report. A good analogy is to liken metadata to the nutritional information displayed on food packaging.
There has been much effort over recent years in the GI community to create metadata reference archives so that the full range of available datasets can be identified and made accessible. By standardising the way in which metadata is stored it is possible to identify resources that contain common types of information. Therefore, if you are interested in forestry you can access a metadata gateway and find references to forestry information stored across the globe.
At its simplest level a gazetteer is a dictionary of geographical names. Every record contains a description of the location, providing the user with a simple means of identification and reference. An electronic gazetteer works in exactly the same way. It is a file or database listing every feature of a particular type (such as a building, a road or a pond) within a defined area. The user can locate the position of the feature and query any additional information attached to it.
Gazetteers in England and Wales
Recent developments in the UK mean that a definitive national gazetteer of certain key geographical features is closer to becoming a reality. Reliable and consistent gazetteers are vital for different parties to be able to refer to and locate a feature easily and without ambiguity.
The first step towards this goal was the New Roads & Street Works Act (1991), which required utilities and contractors to notify local authorities of road works. It also specified that local authorities should jointly maintain a National Street Works Register.
More significantly, in 1993 the first draft of the British Standard BS7666, Spatial data-sets for geographical referencing, was published.
BS7666 currently contains four parts:
1) Specification for a Street Gazetteer – an up-to-date list of all streets in an administrative area. Every entry is allocated a Unique Street Reference Number or USRN.
2) Specification for a land and property gazetteer (LPG) – an up-to-date list of all land and property units in an administrative area. Each record is called a Basic Land and Property Unit (BLPU) and holds data relating to its provenance. A BLPU also holds a grid reference locating its central point and a Unique Property Reference Number (UPRN).
3) Specification for addresses
4) Specification of a data-set for recording public rights of way.
XML and GML
In general, mark-up languages use tags to associate a rule to interpret the content of a set of information. In HTML this means the visual formatting and association of HTTP hyperlinks with text and images. For example, the tag can be used to instruct a browser application to display a piece of text in a certain style.
In a similar way, eXtensible Markup Language (XML) uses tags to give meaning and context on the content of a set of information. In an XML document, tags could encode the fact that Southampton is a city. But alternatively they could state that, in a particular document, Southampton refers to a node on a network of shipping lines, or in another, a football club. This is where the extensible bit comes in. In XML you can define your own set of tag types as long as the tag set applicable to your document is defined in a separate schema. XML provides for self-describing data. This makes it very useful as a standard format for exchanging information because computer programs can interpret the content of XML packets without any prior exposure. XML makes system development more flexible and is rapidly becoming the standard for information interchange on the Internet.
The emergence of XML has led to the creation of a wide range of mark-up languages specific to particular subject matter such as maths, chemistry and medicine. There is also a Geography Mark-up Language (GML) that allows spatial data to be stored and transferred between systems over a network. It allows points, lines and polygons to be encoded along with their attributes and the spatial reference system on which they are based, for example, the National Grid. There is already a lot of interest in GML from other communities, including the mobile phone industry and the general Internet community. GML is fast becoming the definitive method of describing geographical data and simple location information on the Internet. The specification for GML 2.0 can be viewed on the OGC public web site. Ordnance Survey has adopted GML as the format for OS MasterMap.