11.1.1
Elementary Street Units (ESUs) are subsets of Streets forming a continuous length. Primarily, they are recorded to relate the Street reference to a physical extent.
11.1.2
ESUs also enable the construction of Street network topology. They are primarily junction to junction (or end), but sometimes they end at other points.
11.1.3
Other topological features are the direction of the lines and the nodal connections.
11.1.4
These other topological features must be included in the LSG to construct a rigorous topology suitable for using the network for purposes such as routing.
11.1.5
Therefore, unlike the ASD Records, topology One-way Exemption Records are attributes of the ESU and not of the Street.
11.1.6
In some LSGs, Streets are split into very short units, to tie in with other information systems. This leads to problems of inconsistency when LSGs are amalgamated at the national level.
11.1.7
In BS7666-1:2006 an ESU is defined as a continuous length of Street between adjacent junctions, from junction to Street end or between other defined criteria, see Section 11.3.
11.1.8
Level 2 and Level 3 Street gazetteers include ESU Records, as defined in Section 6. ESU Records are the core element of the LSG. To conform with basic data capture principles these core elements must be collected once.
11.1.9
As many ESU coordinates as is necessary must be used to define the true shape of a Street centreline.
11.1.10
ESU coordinates are transferred within the data transfer file as a type 14 Record. Each ESU coordinate Record is numbered, with the first as number 2, the second as number 3 and so on, to enable the points to be plotted as a polyline and form the centreline of a Street section.
11.1.11
Attribution for ESU records is transferred within the DCA Street Update transfer file as a type 13 Record. For further details of the transfer format, see the DTF current version in Related Documents.
11.1.12
Street coordinates are transferred within the DCA Street Update transfer file as a type 14 coordinates Record, as defined in the DTF current version in Related Documents.