‘Water’ from OS Select+Build can be accessed under the 'Downloads' section if you have an OS Data Hub account and are a Public Sector Geospatial Agreement (PSGA) member by: OS Select+Build > Create a new recipe > give the data package a name and from ‘Themes’ select Water → Water Features → Water > Create recipe. On the next page Click ‘Add data package’ under the data package just created and give the package a name, select ‘British National Grid’ as the Coordinate Reference System, GeoPackage as the file format, select the first of the current month as the initial supply date and select how often (if at all) you’d like this package updated. Click ‘Create data package’ and download file once ready. NOTE - This produces a fairly large download file (about 1GB).
‘Land’ can be downloaded from OS Select+Build using the same instructions as above, except when creating a new recipe the ‘Themes’ to select are Land → Land Features → Land. NOTE - This produces a very large download file (about 20GB).
Define and create GB shoreline outline
The ONS GB Shoreline is created by taking full resolution extent of the realm boundaries (also known as low water mark) and ‘cookie-cutting’ out selective areas of sea, tidal water and tidal land taken from Ordnance Survey data.
The areas selected for ‘cookie-cutting’ broadly fall into four categories:
Assigned as Sea in ‘Water’ from OS Select+Build (e.g. the area between low and high water marks)
Assigned as i) tidal water in ‘Water’ from OS Select+Build, ii) intersect with the Built Up Areas 2022 geography and iii) can be considered to be near the sea (e.g. the tidal water around Southampton is considered to be near the sea, the tidal River Ribble going through Preston is not)
Assigned as i) tidal water in ‘Water’ from OS Select+Build, ii) does not intersect with the Built Up Areas 2022 geography but iii) can be considered a substantial water body (e.g. Christchurch Harbour is considered to be substantial, but a tidal river like the Beaulieu River in Hampshire is not)
Assigned as tidal land in ‘Land’ from OS Select+Build and touch selected tidal water features
The areas of sea, tidal water and tidal land that are used for the ‘cookie-cutting’ are selected using an automated process run in R. The commented code explains the process step-by-step. It also includes commentary as to why certain steps were necessary and how certain areas were included/excluded.
Buffer GB shoreline
Due to the complexities of the GB shoreline geometry, running a simple buffer (e.g. geos_buffer or st_buffer in R) on the whole dataset can take a long time to complete.
Instead of buffering the entire dataset, it is chunked by 10km OS grid tiles. So rather than buffering one very complex polyline it buffers multiple simpler polylines.
A polyline version of the shoreline is used for the majority of the buffering. However, the polygon version is used for objects smaller than the minimum size of the buffer. This is because geos_buffer and st_buffer both struggle to buffer polyline features that are smaller than the minimum area of a possible buffer.
Identify Built Up Areas (2022) that intersect GB shoreline
As Built Up Areas are constructed from 25 metre grid squares, edges that would be considered touching the shoreline do not always intersect with it.
As such, a buffer is used to make the shoreline wider, enabling more consistent and realistic intersections with Built Up Areas. Width of buffer can be edited based on user requirement.
This allows the following outputs to be created for all Built Up Areas that touch the GB shoreline:
'BUA_Perimeter_KM' is the total perimeter in kilometres of the Built Up Area
'BUA_Shoreline_KM' is length of each Built Up Area's perimeter that intersects the GB shoreline
'BUA_Shoreline_PCT' is the percentage of each Built Up Area's perimeter that intersects the GB shoreline
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