This section describes the method of data capture and maintenance and is included for technical information - for details of the actual data provided please see later sections.
The boundary data is maintained as "Lines" and "Pins".
The boundary geometry for each Theme is captured as a series of connecting "Lines" using OS MasterMap as a primary reference or source.
Theme boundary features that are mered exactly to features on OS MasterMap are cloned (copied) and will share OS MasterMap geometry, and if not mered, are captured using a variety of other source material.
Every section of line will be assigned a "capture code" (please note that this code is not included in the polygon data provided with this document.)
A - Snapped exactly to MasterMap
B - Snapped exactly to LandLine
C - Digitised from written description - usually following an unmapped feature such as edge of ploughline
D - Digitised from a georeferenced scanned image of designation map
E - Digitised by interpretation of source material - usually a straight line, "undefined", alignment. For example, a line across a saltmarsh channel or a track or road.
F - Snapped to existing digital source data such as SSSI dataset - often when no other alignment can be found. For example, part of a boundary across open moorland.
G - Taken directly from GPS derived data
H - Taken from geo-referenced digital air photographs
J - Snapped to OS Land-Form Profile contour data
Where two or more Themes share a boundary features, the geometry will be captured once.
Lines have no Theme attributes - these are held within the "Pin" attributes.
The end nodes or vertices of every Line will always join exactly with the end node of another.
The Lines that together describe a self contained area of land - which describe part of a Theme or an excluded area - are referred to as a "Collection of Lines".
A Collection of Lines that describes one or more Themes will have one or more "Pins" placed within the containing Lines.
A Pin is a point object.
The Pin will have an attribute describing the Theme and a unique integer number, known as the "Theme ID" or "ThemeID" which in turn will refer to further information about the particular "Thing" described by the Collection of Lines.
For example, the ThemeID for "The Quantocks" SSSI is 1001391.
The Lines and Pins data is stored in a spatial database (currently Oracle Spatial) from which draft and final data is produced using a variety of software tools.
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