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Figure 2: vector data is stored as coordinates
The raster model encodes the features as pixels within a geographically-referenced image. The
representation of the feature is made by the colour of the pixel (see figure 3).
Figure 3: raster data is stored in a grid made up of pixels
These models result in datasets that are very different in terms of how the data can be used. The main
difference is that with vector models, multiple attributes can be stored alongside the coordinates that supply the
geometry. In raster models, usually only one piece of information can be stored, apart from the colour, against
the pixel. Typical examples include a height measurement or a value representing a type of land cover.
OS MasterMap Topography, Address and ITN Layers have much in common with each other in terms of
their structure and how they can be used, and much of the information within this general guide can be
applied to all vector layers. The Imagery Layer has been geographically referenced to align with the other
layers to the greatest extent possible, as shown in figure 1.
The Digital National Framework (DNF)
The OS MasterMap products are different from other Ordnance Survey products. OS MasterMap has been
designed to facilitate the adoption of a larger geographic concept called Digital National Framework (DNF).
DNF is a model for an industry-standard way of integrating and sharing business and GI from multiple sources.
GI increasingly needs to underpin mainstream information services, yet it has proved difficult for it to move
from a niche or specialised data type. Ordnance Survey has recognised that GI needs to move from simple
and relatively unintelligent maps and pictures to computer records that mainstream information technologies
can recognise and handle logically, reliably and in increasingly automated processes.
OS MasterMap Topography Layer user guide chapter 1 v1.12 – 03/2014 © Crown copyright Page 9 of 150