3.5 XML

XML  is a means of describing data. It is a method of encoding data in text which renders the data easy to store, transport and interpret. It is not a language but provides a framework that allows users to write their own Mark-up languages for their own specific needs. GML is such a language which is concerned with the description of geographic content. It describes data or objects that have a spatial element by encoding geometry and spatial reference systems. The fact that it is based on XML means that it can be read and edited using any text editor, is easy to transport and transform and, crucially, can be easily integrated with non-spatial data.

This final point is incredibly important. For many years spatial data has been stored, viewed and analysed separately from non-spatial data. In recent years GIS developers have striven to integrate spatial data and analysis. GML enables the integration of geographical information by using a set of rules and guidelines (XML) which can be applied to any data type for any purpose. This means that geographic information can be integrated with a massive range of non-geographic data types thus, greatly enhancing the value and accessibility of spatial information.

The Ordnance Survey’s MasterMap dataset is a digital database of vector information describing geographical features on the ground in incredible detail. Individual building outlines, bollards, trees and road networks all have their own unique identifier called a TOID (Topographic Identifier) as well as information on their geometry and attributes. MasterMap is based on GML. The OS use GML because it's “well defined geometric primitives coupled with a structured mechanism for defining features ensures that when spatial data is exchanged in GML it can be interpreted and understood by everyone.”

Below is a sample of MasterMap for my house viewed in a GIS, and its associated GML.

MasterMap Data Around My House

Crown Copyright/ Database right 2009
An Ordnance Survey/ Edina supplied service

A Sample of the GML for the above MasterMap Data