Contour data

GML

This section describes how OS Terrain 5 is defined in GML version 3.2.1. An understanding of XML and XML schema is required.

Overview

The OpenGIS GML encoding standard is an XML grammar for expressing geographic features. GML serves as a modelling language for geographic systems as well as an open interchange format for geographic transactions on the Internet. As with most XML-based grammars, there are two parts to the grammar: the schema that describes the document, and the instance document that contains the actual data. A GML document is described using a GML schema. This allows users and developers to describe generic geographic datasets that contain points, lines, and polygons.

The GML conforms to GML 3.2.1 simple features (level 0), which is a subset of the full GML specification, intended to make it easier for GIS vendors to provide a minimum level of support for GML.

GML schema

XML schemas are used to validate the format and content of the GML. The GML specification provides a set of schemas that define the GML feature constructs and geometric types. These are designed to be used as a basis for building application-specific schemas, which define the data content.

The OS Terrain 5 application schemas, which are referenced by the data, are available at https://www.ordnancesurvey.co.uk/xml/terrainschema/index.html

The user may need to be connected to the Internet to access these online schemas while working with OS Terrain 5 in GML unless their software supports local copies of the schema. Depending upon the software that is being used to read the data, the user has the following options:

• The software does not use the schema, and therefore does not need to be connected to the Internet.

• The software needs the schema but can reference it from a local copy if it is downloaded in advance.

• The software needs the schema and can only reference it from the online version, and therefore needs to be connected to the Internet.

Data structure

Layer name	Layer description
<tile name>.gml	GML data file.
Metadata_<tile name>.xml	A metadata file for contour data providing information on the flying date and so on.

Detailed GML model

Diagram of OS Terrain 5 GFigure 2: GML data modelML data model.

Spot heights

The spot heights have been created by an automated process to select the highest mass point that is at least 1 metre above and within every closed contour. If the point is the same height as the contour, it will be removed. Some spot heights could be lower than the surrounding contour due to genuine depressions. The z value is rounded to one decimal place.