# Topology

Within OS MasterMap Topography Layer, there are four topological structuring layers that determine how features interact with each other. 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.

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:

<table><thead><tr><th width="246">Structuring layer</th><th>Rule</th></tr></thead><tbody><tr><td>Topography</td><td><p>Feature Type = ‘TopographicLine’ or ‘TopographicArea’ descriptiveGroup not equal to ‘Landform’</p><p>physicalLevel = ‘50’</p><p>physicalPresence = ‘Closing’, ‘Edge/Limit’, ‘Obstructing’, ‘Overhead’ or ‘Moveable’</p></td></tr><tr><td>Landform</td><td><p>Feature Type = ‘TopographicLine’ or ‘TopographicArea’ descriptiveGroup = ‘Landform’</p><p>physicalPresence = ‘Closing’, ‘Edge/Limit’, ‘Obstructing’ or ‘Overhead’</p></td></tr><tr><td>Pylons</td><td><p>Feature Type = ‘TopographicLine’ or ‘TopographicArea’ physicalLevel = ‘51’</p><p>physicalPresence = ‘Closing’, ‘Edge/Limit’, ‘Obstructing’ or ‘Overhead’</p></td></tr><tr><td>Boundaries</td><td>Feature Type = ‘BoundaryLine’ or ‘CartographicSymbol’ descriptiveGroup = ‘Political Or Administrative’</td></tr></tbody></table>


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