In 2018, the Central Statistics Office (CSO) carried out a review of the Census 2016 Settlement boundaries. The review highlighted some issues with the methodology used to produce Settlements. In particular, the risk of statistical disclosure between Settlements and Statistical small areas (SSA).
As part of the review, the CSO began formulating a methodology to objectively identify and delineate Settlements moving forward and align them to SSAs. It was decided to broaden the exercise to include stakeholders and redefine Settlements. A working group was formed between the CSO, the Department of Housing, Local Government and Heritage (DHLGH), and Ordnance Survey Ireland (OSi, now Tailte Éireann), and a formal methodology was developed and agreed.
Built Up Areas (BUAs) are a new statistical Geography released as part of Census 2022, created by combining small areas to identify the footprint of urban centres across Ireland. They are generated using an objective algorithm run across the State that is used to group buildings together into BUAs.
The key difference between Settlements and BUAs are:
1) The criteria used to define them
2) How they are drawn
In terms of how they are drawn, Settlements were drawn manually by CSO staff following the census, whereas BUAs are drawn automatically by Tailte Éireann. The criteria are quite similar. Settlements used 50 occupied dwellings as a starting point, BUAs on the other hand use 100 buildings, not just dwellings.
The methodology was agreed between CSO, OSi (now Tailte Éireann) and other Government Departments including DHLGH.
There are lots of ways to define 'urban areas', two of the most common are:
1) Land cover definition (e.g. urban areas are demarcated by a certain % of artificial land cover)
2) Population definition (e.g. urban areas are demarcated by a certain population density and/or total population)
Different definitions suit different users and purposes, BUAs represent a land cover definition, while Settlements ultimately represent a population definition, they are simply different measures of the same subject.
Legal Towns were abolished in 2014 following the passing of the Local Government Reform Act. In response to user needs, the CSO revised Settlements to ensure they aligned with Legal Towns in so far as possible. The CSO will make total population figures for 2022 available using the 2016 Settlement boundaries to facilitate comparisons for users, but the BUAs will be the primary basis on which the CSO reports town populations for Census 2022 and the urban geography the CSO will use to produce detailed attribute data.
The two definitions are quite similar, in general BUAs have less green space than Settlements around their fringes.
As they are defined differently, the population figures won’t exactly match between Settlements and BUAs. Users can use the figure/definition that is most appropriate for their use case, but BUAs will be the primary way in which the CSO will report on urban data for Census 2022.
In Census 2016, there were 874 settlements. These are replaced in Census 2022 with 867 BUAs/Towns. There were a small number of settlements that were not possible to recreate given there is a requirement for Statistical Small Areas (SSAs) to respect the CSO Electoral Divisions (CSO EDs). Certain CSO EDs are built entirely off an SSA and, therefore, it was not possible to split or redraw these.
Generally, this case arose because the BUA/Town is bisected by a CSO ED and the resulting parts are too small to be considered Statistical Small Areas (SSAs). The SSAs are the building block for the BUAs/Towns and they must respect the hierarchy of CSO EDs. An aim of the BUAs/Towns is to ensure statistical confidentiality and this is achieved by geographical nesting boundaries. In Census 2016, this was not the case for the relationship between Settlements and Electoral Division. Typically, an SSA was not included in the BUA because (1) did not meet the BUA criteria, (2) it was bisected by a CSO ED and/or (3) the SSA has changed when reviewed and is no longer in the BUA.
BUAs are now an integral part of the CSO's statistical small areas product (SSA), as it is combinations of statistical small areas that define built up areas. After each census, statistical small areas are reviewed to ensure they fall within agreed limits. Reviewing the boundaries of BUAs (as they relate to the shape of statistical small areas) after each census cycle will be required moving forward.
All statistical boundaries are reviewed after each census to ensure they are within agreed tolerances in terms of the number of dwellings and/or the total population living there. Each cycle, changes are made and these are flagged to users.
No. The creation of CSO Electoral Divisions (EDs) is an independent exercise designed to ensure the set of EDs mirrors the legally defined set as closely as possible while ensuring they meet minimum population totals and are suitable for the dissemination of statistics. Small Areas (and by extension, Built Up Areas) nest to CSO EDs, not the other way around.
The definition of BUAs was agreed in consultation with partners including the DHLGH. The methodology is primarily designed for monitoring urban development into the future and to protect against statistical disclosure. To understand how census boundary reviews may impact on policies and decisions, users should consult with DHLGH.
As previously outlined, this methodology has been agreed between the CSO, the DHLGH and Tailte Éireann. It leverages modern technology to objectively define boundaries based on a land cover definition. The CSO is confident that this methodology provides an effective solution to managing boundary reviews for the future. Importantly, it also eliminates the risk of statistical disclosure.