User Guide - Digimap

Example
Example class model
Topological structuring layers
Within the Topography Layer there are three topological structuring layers that determine how area features
interact. A structuring layer contains features that do not cut across each other and are permitted to connect
to each other spatially, sharing common geometry at their edges. For example, most topographic features
participate in a single topological structuring layer, but features that are underground or above cartographic
level, or that represent pylons, cliffs and slopes are not considered to interact with other features in the
Topography structuring layer. During capture and maintenance of the data processes ensure that vertices of
the geometry of features are coincident where they should be, so that the features topologically structure
with each other. The full structuring layer definitions are given in the following table.
Structuring layer
Topography
Landform
Pylons
Boundaries
Rule
Feature Type = ‘TopographicLine’ or ‘TopographicArea’
descriptiveGroup not equal to ‘Landform’
physicalLevel = ‘50’
physicalPresence = ‘Closing’, ‘Edge/Limit’, ‘Obstructing’ or ‘Overhead’
Feature Type = ‘TopographicLine’ or ‘TopographicArea’
descriptiveGroup = ‘Landform’
physicalPresence = ‘Closing’, ‘Edge/Limit’, ‘Obstructing’ or ‘Overhead’
Feature Type = ‘TopographicLine’ or ‘TopographicArea’
physicalLevel = ‘51’
physicalPresence = ‘Closing’, ‘Edge/Limit’, ‘Obstructing’ or ‘Overhead’
Feature Type = ‘BoundaryLine’ or ‘CartographicSymbol’
descriptiveGroup = ‘Political Or Administrative’
Seamless data supply
A principle of OS MasterMap is that data is seamless, that is there are no fixed units of data supply. The
nominal boundary of each packet of OS MasterMap data is defined by the user’s data selection polygon, and
by the data chunking method applied to break the supply into manageable units (if used).
With respect to the nominal boundary, data is supplied unclipped. This means that all features whose
geometry overlaps the nominal boundary are supplied in their entirety.
Inconsistent features
There are occasions when data update will temporarily leave a feature in an inconsistent state. This occurs
when neighbouring data are updated and the edits are applied to the seamless database at different times.
According to the type of feature the following results may be realised:
OS MasterMap Topography Layer technical specification chapter 7 v1.3 – 07/2007 © Crown copyright Page 89 of 140