OS MasterMap and AutoCAD explained.

17 February , 2008

This weeks post is focused on the premier digital mapping dataset of Great Britain – OS MasterMap®. While many mapping software users have understood, and have been using OS MasterMap for a number of years now, the CAD community has seen a slower take up of the data, mainly due to the difficulty in reading the MasterMap information into the DWG.

This post is designed to provide an introduction and summary to some of the ways the data can be managed within an AutoCAD environment.

What is OS MasterMap?

OS MasterMap is the latest generation of Ordnance Survey’s most detailed geographic data of Great Britain. It provides consistent and up to date information down to address, street and building level and is available in four data layers: Topography, Address, Imagery and Integrated Transport Network (ITN). It replaces the old OS Land-Line product, which will be withdrawn from the market in September 2008.

The most commonly used layer is the Topography Layer, which provides detailed mapping of around half a billion landscape features; such as buildings, land area classifications, roads, tracks, paths, railways, water features and administrative boundaries. From the infrastructure design perspective, this is excellent for showing the detailed geographic context around a specific site.

Historically Land-Line has been supplied in a CAD friendly format as tiled DXF or NTF files, in a simple data structure of points, lines and text. Due to its simplicity, this data can be quickly and easily loaded into AutoCAD. In contrast, OS MasterMap is delivered to the users in GML files (Geography Mark up Language), a format based on XML, designed by the Open Geospatial Consortium to store spatial information. GML cannot be read by vanilla AutoCAD and due to the increased data intelligence, OS MasterMap files are much larger than their Land-Line equivalent.

As such, the management of the data within an organisation requires careful thought, especially when considering how data can be accessed and utilised within an AutoCAD environment.

Why use OS MasterMap?

OS MasterMap data is a far richer dataset than Land-Line. No longer limited by tiled boundaries, OS MasterMap is a seamless database of information offering the customer the flexibility to receive only the extents of the data they require.

OS MasterMap data is polygonised, forming intelligent objects that represent real world features such as a building, a park or road junction. Each object has rich attribution, including an id known as a TOID (Topographic Object Identifier) a unique, numeric value given to every feature in the dataset. As these TOIDs stay constant for the lifetime of the feature in the dataset, it is possible to use them as a framework for linking other ‘user specific’ data to the geographical features.

The problem with the DWG file

When MasterMap data is translated into a standard AutoCAD DWG using an external translator it will convert the MasterMap entities into CAD features. However the standard DWG cannot understand attributes on the entities therefore losing the intelligence assigned to the data by the Ordnance Survey. This intelligence is a major advantage of the MasterMap data, as it allows you to produce powerful themes, link the data to external datasets and manage the life cycle of the data.

A further disadvantage of loading MasterMap data into the DWG is storage and speed problems. The DWG was not designed to store large amounts of geospatial data. The more data loaded into the DWG, the bigger the impact on performance. Even when dealing with a small dataset, the DWG file sizes can become very large before any other design data is added.

How can AutoCAD Map manage OS MasterMap?

As AutoCAD Map 3D is based on open data standards the user has the flexibility to work with virtually any spatial data available. So whether the MasterMap is supplied directly by the OS in GML/GZ format, or the user receives the data in other geospatial formats they can read the data directly into their DWG. Allowing them to select the most appropriate method for managing MasterMap data dependant on what they want it for. The options include:

1 – Load raw MasterMap GML/GZ data into the DWG

2 – Connect to a geospatial file holding MasterMap data  

3 – Access MasterMap data stored within a spatial database

Option 1 – Loading raw GML/GZ files into the DWG

AutoCAD Map allows the user to import the OS MasterMap GML data directly into the DWG without the need for an external loader. Within this import process, the user has full control over which layers and attributes to import, and can create a spatial query when loading the data to control the geographical extents of the data required.

The DWG used within AutoCAD Map has an extended ability to store attributes on all CAD entities. This allows the intelligence inherent within MasterMap to exist within the AutoCAD environment. Using that intelligence, users can theme and style the data according to their own requirements. The issues of DWG speed and size still remain however.

Option 2 – Connect to a geospatial file

An alternative to importing the MasterMap data into the DWG is to ‘Connect’ to the data. AutoCAD Map uses an engine called Feature Data Objects (FDO) for accessing features and geospatial data regardless of the underlying data store.

There is a great write up on FDO at Neal Niemiec’s blog

When you access data through the FDO tool, you use an ‘FDO provider’ to connect directly to a data source. This source could be foreign spatial file formats such as MapInfo TAB files, ESRI SHP files or even OS MasterMap GML data (more about this option here).

When the FDO tool connects to the data it is NOT translated into the DWG as AutoCAD objects, but read natively from the original source and then displayed in the Model space as you would display normal CAD entities.

Once the data is connected within AutoCAD Map it displays all the features within the file and the FDO source becomes a layer in the Display Manager, through which the user can then perform styling and querying.

With this method, MasterMap data held within foreign file formats can be accessed very quickly and in large quantities without the risk of any performance issues. As the data is connected and not imported, it takes up the minimum of space within the file, resulting in a very small DWG.

Option 3 – Access the data within a spatial database

An increasingly popular way of storing MasterMap data and many other geospatial datasets is within a spatial database. Database storage makes spatial data easier to manage, and presents a seamless dataset to the user. With a spatial database you are no longer required to search for the right file in order to obtain the information required – it is all available by simply connecting to the database.

There are a number of different spatial databases on the market- examples include Oracle and MySQL. Many organizations in the UK have implemented spatial database solutions and are storing huge amounts of MasterMap data within them. AutoCAD Map is able to connect directly to databases such as Oracle and MySQL, and will also connect to ArcSDE, which is a widely used spatial database tool from ESRI.  

Database connections use the FDO engine as in option 2, meaning all the same advantages are gained here also.

Over the future posts I will try to highlight some tips on using OS MasterMap within the DWG so check back soon.