18 September 2001

Meeting the master challenge

Eddie Curtis and Ian Painter, two members of the team who created OS MasterMap, outline the technical challenges they faced.

Ordnance Survey first introduced digital mapping in 1973 when it started digitising its large-scale mapping archive in order to streamline the map production process. It took the next step when it created the Land-Line product, supplying the digital data to customers in electronic form. This allowed customers to display Ordnance Survey maps in their own systems and electronically overlay their own data.

OS MasterMap is the next evolution of Ordnance Survey large-scale data. While Land-Line is largely a cartographic product, OS MasterMap is designed to enable sophisticated analysis in GIS applications and to allow closer integration with third party data. At the heart of this evolution is the creation of object-based data - making features represent objects in the real world rather than the graphical elements of a map.

The most noticeable addition to achieve this goal is the provision of area features, or polygons, alongside the lines and points familiar to Land-Line users.

To complement this rich feature set, numerous additional attributes have been added to describe the nature of the feature as well as just its location. Furthermore, OS MasterMap supports closer integration with user data by providing each feature with a unique identifier, allowing users to associate their own information with the OS MasterMap features.

To enable the DNF and the OS MasterMap product, Ordnance Survey had two main technical challenges to meet. Firstly, enhancing the content of the Land-Line map into real-world objects. Secondly, moving to a whole new paradigm of data management, storing its data in a single seamless database.

Enhancing the content
The main obstacle to using Land-Line for geographical analysis is its inherent cartographic history. To a person looking at a graphical display of Land-Line it is clear where the building, roads, fences and walls are. However, to a computer with no understanding of the physical world there are just lines and points in different locations.

Due to the enormous amount of detail in Ordnance Survey large-scale mapping it was not practical to use human operators to identify, describe, and recode every real-world feature shown on the map. Ordnance Survey therefore had to automate the vast majority of this work. To achieve this, its software developers built systems to interpret the Land-Line maps and convert the cartographic information into real world objects.

Built on Laser-Scan's object-oriented Gothic architecture, the conversion process used a highly complex rule base, alongside a detailed object-orientated data model. This approach enabled the automatic classification of area features as well as inferring numerous closing links to further subdivide areas (polygons) into more meaningful units. However, before achieving this goal, over 60 million geometric inconsistencies within the Land-Line data had to be resolved without affecting the positional accuracy. The result, after a year of processing and less than 1% of manual editing, was the total re-engineering of the large-scale data and first stepping stone to the creation of OS MasterMap.

Managing the new data
In the past Ordnance Survey has held its maps as a set of separate map files. With OS MasterMap the entire mastermap of Great Britain is held as a single seamless database. Instead of finding a map location using a National Grid map sheet reference, Ordnance Survey data is now retrieved by defining exactly the area and data of interest. Features are now selected="selected" individually from the archive to meet the needs of each customer.

Managing the DNF database presents some unique technical challenges, including a relatively complex data model. The DNF uses topological polygons to represent areas. This means defining areas by referring to the lines that surround them instead of holding the coordinates within the area features themselves. So, besides dealing with a very large number of features, Ordnance Survey has to manage the fact that they are all connected to each other and those relationships must be properly maintained. OS MasterMap is an unbroken web of 400 million features stretching from Land's End to John O'Groats. There are no natural divisions in this network (each feature is next to another, which is next to another, and so on, from one end of the country to the other) so the data management systems at Ordnance Survey must present this giant dataset as a single entity to the user.

Because OS MasterMap is so complex, Ordnance Survey's data management system has to provide a variety of ways to select from the database to make using the DNF easy for customers. The DNF features can be selected="selected" by combinations of area and theme.

Change-only information can be supplied by selecting all features changed since a given date. Features can also be retrieved by their unique identifier (known as a Topographic Identifier or TOID). Ordnance Survey has developed bespoke indices within the database to ensure that for all these patterns of access the features can be retrieved efficiently.

Another challenge for the data store is the wide variety of feature sizes. The area features range from something as small as a garden shed to something as big as Loch Ness. The systems have to reliably deal with either extreme from one moment to the next.

The system is engineered to provide a variety of output formats. OS MasterMap is available in two geography mark-up language (GML) formats: independent polygons, and topological polygons. This, combined with the fact that each customer is given a selection of features specific to them, means that every file of OS MasterMap data has to be individually created from the database for each customer.

The systems producing the files must therefore be high performance to ensure that customers are supplied in a timely manner. They must also be highly scaleable to ensure that growing customer demand does not result in a reduction in service.

To create OS MasterMap, Ordnance Survey has brought together a wide range of technical developments, including customer-specific feature selection and high performance, each one challenging in its own right. It has done so on an industrial scale creating a product which is not only a sophisticated data product, but also one of the largest databases of its kind in the world.

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