This is the second part to my discussion on postcodes.
Postcode boundaries are based on the street address delivery points of the national post offices which represent the postal delivery areas of the postal services. The postal systems are designed to assist the national postal services to deliver post and are designed to support the identification of a postal address where a delivery can be made. As such no official demarcation of a postal area is made, however boundaries can be created using the associated addresses to define contiguous areas that represent the postcode area. Postcode Boundaries only include area related postcodes, not company-related postcodes or Post Office Boxes. Using data from the official Post Office sources guarantees best possible completeness and accuracy of the Digital Boundaries.
Therefore the most important source data for building postcode boundaries is the address list with postcode and coordinate for the location of that address. The address and postcode is defined by the government agency and georeferenced either by the government agency or commercial organisations. This georeference is the crucial element for the boundaries, as boundaries should contain all relevant and appropriate postcodes. Coordinates are collected via imagery, ground survey and base map reference data and continuously collected by source agencies. Source agencies can be national mapping bodies such as the UK's Ordnance Survey whose responsibility is to accurately map the nation, or commercial agencies such as TomTom and Here who collect the data for accurate vehicle navigation. Note some postcodes are non geographic, post boxes, a number of large commercial entities. Therefore there maybe postcodes in the national source for which no boundary can be created.
Simplistic postcode boundaries can be created using Thiessens/Voronoi boundary creation methods using the postcode centroid. This provides a minimum bounding area for the postcode but will not be an accurate representation of the area a postcode covers. Boundaries created in this way can provide basic visualization capabilities.
When it comes to digitizing postal boundaries, in-depth specialist knowhow is required. There are often no existing official boundary data sources available, although administrative data boundary data can provide a source base reference, meaning that data engineers must manually review and capture boundaries based on a wide variety of source data such as satellite and aerial imagery. Detailed boundaries can then be digitized along roads, lakes, buildup areas and when warranted, match administrative boundaries.
Remember it's about delivering post not creating neat polygons. These characteristics mean a postcode boundary is a representation of the area, you can get better representations of those areas but there is no official demarcation line between different postcodes.
Lastly, postcodes change regularly, changes will include new postcodes, postcodes being retired, splitting postcodes, additional addresses associated with a postcode. These changes reflect the real world where there are new developments, areas being demolished, population increase and decrease plus more efficient postal delivery.