Chapter 5: Site Feature#

Welcome back! In our previous chapters, we've zoomed in on specific points and objects: the Unique Property Reference Number (UPRN) for an addressable location, and the Topographic Identifier (TOID) and Building Feature for the detailed outline and attributes of an individual building. We also saw how the Linked Identifiers Dataset helps us connect these different IDs.

But often, buildings aren't isolated islands. They are part of a larger area – a campus, a hospital complex, an industrial estate, or even a housing development. When you're working on a BIM project for a building within such an area, you don't just need context about that single building; you need context about the entire site it sits on.

This is where the concept of a Site Feature becomes essential.

What is a Site Feature?#

Imagine you're planning a new building on a university campus. You care about the specific building plot (maybe a UPRN or a building TOID), but you also need to know about the entire university campus. What are its boundaries? What's the main purpose of the land? How many other buildings are part of it?

A Site Feature is a geographical area, typically represented as a polygon (a shape with many sides forming a closed loop), that defines a functionally unified area of land. Think of it as drawing a boundary around a place that operates as a single unit, like:

A university campus
A hospital complex
An industrial park
A large retail park
A railway station area
Even a specific residential estate

This polygon defines the extent of the site. Just like a Building Feature has attributes describing the building, a Site Feature has attributes describing the site as a whole. These can include things like:

The primary use of the site (e.g., Education, Health, Industrial)
Classifications that align with planning or land use schemes
The number of buildings or addresses within that site's boundary
Sometimes, a link to a 'main' building within the site

The data for Site Features comes from Ordnance Survey's National Geographic Database (NGD), as summarised in documents like the OS NGD – Site Summary and described within the broader OS NGD Land Use Features collection.

Why is the Site Feature Important for BIM?#

For anyone involved in designing, constructing, or managing buildings that are part of a larger development or facility, the Site Feature provides crucial context:

Site Context and Boundaries: It gives you the official boundary of the area you're working within. This is vital for site planning, understanding property limits, and validating that your project location is correct within the larger development.
Land Use and Planning: The attributes of a Site Feature often include official land use classifications (like NLUD or OS Land Use tiers, mentioned in OS NGD – Site Summary and OS NGD – Land Use Features Summary). This helps you quickly understand the permitted or actual use of the land, which is critical for planning permission and compliance.
Aggregating Building Data: If you have BIM models or data for multiple buildings within a single site (like a hospital), the Site Feature can act as a container. You can easily select all buildings within that site polygon or linked to that siteid to perform site-wide analysis.
Understanding the Environment: Knowing the nature of the surrounding sites (e.g., adjacent to a residential area, next to a railway line) provides essential context for your project's design and impact assessments.

It helps you zoom out from the individual building and understand its place within a larger functional area.

Key Information in a Site Feature#

A Site Feature combines its geographic shape with descriptive attributes:

Geometry: The defining aspect is usually a polygon representing the area's boundary.
Attributes: These describe the site. Based on sources like the OS NGD – Site Summary and OS NGD LandUseFeatures bSDD Data Dictionary, key attributes often include:
- siteid or osid: A unique identifier for the specific site area. This is the primary ID for linking!
- geometry: The polygon shape of the site boundary.
- description: What kind of site is it? (e.g., 'University', 'Hospital', 'Industrial Estate').
- oslandusetiera/oslandusetierb or nlud_code/nlud_groupdescription: Land use classification systems used nationally or by OS.
- mainbuildingid: The osid of a Building Feature considered the primary building for this site (useful for finding the site if you know the main building).
- matcheduprn: A primary UPRN associated with the site (another way to link).
- addresscount_total / addresscount_residential / addresscount_commercial: Counts of different types of addresses found within the site boundary.
- status: Like buildings, sites can have a status (e.g., 'Active', 'Under Development').

These attributes provide a rich summary of the site, enabling planning analysis and data aggregation.

How Do You "Use" a Site Feature?#

Working with Site Features typically involves spatial operations because the primary data is the polygon boundary.

Find the Site: You might find a site in several ways:
- By Location: If you know the location (coordinates) of a building or point within a site, you can perform a spatial query to find the Site Feature whose polygon contains that point.
- By Linking: If you have the UPRN of a primary address on the site, you might use the matcheduprn attribute to find the site. If you know the osid of a Building Feature that is the main building, you might use the mainbuildingid attribute.
- By Name or Description: Sometimes you can search for a site by its name (e.g., "Royal Infirmary") or description ("Hospital").
Access Geometry and Attributes: Once you've identified the Site Feature(s) you're interested in, you can access their polygon geometry (to see the boundary on a map) and read their attributes (like description, oslandusetiera, addresscount_total, etc.).

Example Input: Coordinates (51.5074, -0.1278) (for a location near the centre of London) or a UPRN like 10002331234 which is known to be within a site.
Process: Load the Site Feature data. Perform a spatial query to find sites whose polygon contains the input coordinates, OR use the UPRN to find a linked building, then find a site containing that building's location or linked by mainbuildingid/matcheduprn.
Example Output: The system returns the polygon boundary for 'Westminster University', and its attributes: description: University, oslandusetiera: Education, addresscount_total: 50.

The Data Structure#

Site Feature data is inherently spatial, combining geometry and attributes. When you load this data into a GIS or a spatial data tool, you get a collection of polygons, each with a set of descriptive columns.

Conceptually, the structure looks like this:

Property Name	Description	Data Type	Example Value
`siteid` / `osid`	Unique identifier for the site	String	abc123-def456-ghi789...
`geometry`	The polygon shape of the site boundary	Geometry	POLYGON ((...))
`description`	Type of functional area	String	University
`oslandusetiera`	Broad OS Land Use Category	String	Education
`mainbuildingid`	OSID of primary building (if applicable)	String	def456-ghi789-jkl012...
`addresscount_total`	Total count of addresses within site boundary	Integer	50
... (other attributes)	...	...	...

Each row represents one Site Feature polygon, providing its unique ID, shape, and characteristics.

Behind the Scenes: Finding a Site#

Here's a simplified workflow showing how you might find a Site Feature based on a location you're interested in (perhaps derived from a UPRN):

sequenceDiagram
    participant YourSystem as Your BIM/GIS System
    participant SiteFeatureData as OS Site Feature Data (e.g., GeoPackage)

    YourSystem->>YourSystem: Start with a location (point coordinate) for a building.
    YourSystem->>SiteFeatureData: Query/Select polygon feature that CONTAINS this point location.
    SiteFeatureData-->>YourSystem: Find the matching Site Feature record.
    SiteFeatureData->>YourSystem: Return the Geometry (polygon) and all Attributes for that feature.
    YourSystem->>YourSystem: I now have the boundary and summary details for the site containing the building.

This shows the spatial query ("contains") as a key way to connect a specific point or building location to the larger site area defined by a polygon in the Site Feature dataset.

Let's look at a very simple conceptual Python example using geopandas to load the data and find a site containing a specific point. Assume you have a GeoPackage file named site_features.gpkg.

First, loading the data:

# This is example conceptual Python code
import geopandas as gpd
from shapely.geometry import Point # Need this to define a point

# Load the site feature data from a GeoPackage file
print("Loading site feature data...")
sites_gdf = gpd.read_file('site_features.gpkg')
print("Data loaded.")

# Display the first few rows to see columns like 'siteid', 'description', 'geometry'
print("\nFirst 5 rows of the data (showing key columns):")
print(sites_gdf[['siteid', 'description', 'oslandusetiera', 'geometry']].head())

This code loads the data and shows the structure, including the siteid, some attributes, and the geometry column.

Now, let's say you have the coordinates for a specific building or point and want to find the site it's in:

# Suppose you have the coordinates for a location (e.g., from a UPRN lookup)
# Using example coordinates roughly for a building in central London
building_lat, building_lon = 51.5074, -0.1278 # Example coordinates

# Create a Shapely Point object from these coordinates
# You might need to ensure this point uses the same coordinate system as the sites_gdf!
# For OS GB data, this would likely be Easting/Northing in EPSG:27700
# But for simplicity here, we use lat/lon as a concept
# Assume sites_gdf geometry is also in lat/lon for this example query
point_of_interest = Point(building_lon, building_lat)

# Find sites that contain this point
# .within() or .contains() are common spatial operations
# sites_gdf is assumed to be in the same CRS as point_of_interest for this to work directly
containing_site = sites_gdf[sites_gdf.geometry.contains(point_of_interest)]

# Print the result
print(f"\nSite(s) containing the point {building_lat}, {building_lon}:")
print(containing_site)

This code snippet demonstrates the core spatial query: finding which polygon(s) from the sites_gdf GeoDataFrame contain the point_of_interest. The output would show the row(s) for the Site Feature(s) that boundary the given location, giving you access to all their attributes and the exact site polygon.

Site Feature vs. Building Feature#

It's important to remember the distinction:

Building Feature: Focuses on the individual building's footprint and specific building attributes.
Site Feature: Focuses on a larger area grouping multiple related entities (buildings, land parcels, activities) and describes the area as a whole (its boundary, overall use, counts).

A single Site Feature polygon will often contain many Building Features within its boundary.

Conclusion#

The Site Feature dataset provides the crucial geographic context for understanding functionally unified areas of land like campuses, hospitals, or industrial parks. By providing the polygon boundary and key attributes for these areas, it allows you to anchor your BIM projects within their wider development site, understand site-level planning and land use, and aggregate information for analysis across multiple buildings.

Understanding Site Features, alongside individual Building Features and the identifiers like UPRNs and TOIDs that relate them, gives you a powerful framework for integrating real-world spatial data into your BIM workflows.

Now that we've looked at individual buildings and the sites that contain them, let's go back inside the building and explore how we can represent and link parts of a building.

In the next chapter, we will dive into the Chapter 6: Building Part dataset, exploring how detailed components of a building are represented.

Next Chapter: Building Part

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