Geospatial Data#

Geospatial data describes location, shape or spatial coverage. The Map view supports discovery and bounded inspection; it is not a full Geographic Information System.

Place Is More Than A Name#

“London” can mean:

• a named concept;
• a legal or statistical geography;
• a bounding rectangle;
• a point used to locate the area on a small map;
• an exact boundary polygon;
• a field value occurring in prose.

These representations are not interchangeable. The Explorer records what kind of evidence it has.

Coordinates#

A coordinate locates a position within a Coordinate Reference System, or CRS.

Web data commonly uses WGS 84 longitude and latitude:

{
  "type": "Point",
  "coordinates": [-0.1276, 51.5072]
}

In GeoJSON the order is longitude, then latitude. This is easy to reverse.

The identifier EPSG:4326 commonly names WGS 84. UK analytical data may use British National Grid, EPSG:27700, measured in metres. Plotting one as the other produces incorrect locations.

Geometry#

Common geometry types include:

• Point;
• MultiPoint;
• LineString;
• MultiLineString;
• Polygon;
• MultiPolygon;
• GeometryCollection.

A polygon has rings of positions. A boundary can contain many thousands of coordinates, so a preview must limit complexity.

Feature And Feature Collection#

GeoJSON wraps geometry with properties:

{
  "type": "Feature",
  "properties": {
    "name": "Example Area"
  },
  "geometry": {
    "type": "Point",
    "coordinates": [-0.1276, 51.5072]
  }
}

A FeatureCollection contains several features. The preview loader accepts a bounded number and reports when more existed.

Bounding Box#

A bounding box records minimum and maximum coordinates:

[west, south, east, north]

It is a cheap way to show extent, but it is not the real boundary. An irregular coastal area can occupy only a small part of its box.

Geography Codes And Vintages#

Stable codes are better than labels for joining data. UK statistical geographies commonly use GSS codes.

A code still needs context:

• geography level;
• source organisation;
• release or vintage;
• exact or best-fit derivation;
• boundary variant;
• CRS.

Boundaries and administrative structures change over time. A code without a vintage can be ambiguous for historical comparison.

Exact, Best-Fit And Representative#

• Exact means the source geometry follows the declared geography.
• Best-fit means smaller units were assigned to a larger area under a documented rule.
• Representative point is a locator chosen for display.

A representative point must never be drawn or described as the authoritative boundary.

Evidence Levels In The Explorer#

The Map classifies records from strongest to weakest evidence:

1. explicit coordinates, geometry or bounding box;
2. declared coverage or jurisdiction fields;
3. recognisable geospatial service or file;
4. bounded place or spatial-language match in text.

A record can have several signals. The interface preserves the matched field, value and rule so a user can inspect why it was classified.

Text classification helps discovery. It does not prove that the dataset's geography is complete or current.

Geospatial Files And Services#

GeoJSON#

JSON encoding of features and geometries. It is the primary direct preview format supported by the Explorer.

KML, GML, Shapefile And GeoPackage#

These are common exchange or storage formats. The current prototype can recognise and link them but does not parse their bodies in the browser.

OGC Web Services#

Open Geospatial Consortium standards include:

• WMS for rendered map images;
• WFS for vector features;
• WMTS for tiled map images;
• WCS for gridded coverages;
• OGC API Features for web-native feature access.

Recognition is not the same as successful preview. Service versions, query parameters, CRS support, authentication and Cross-Origin Resource Sharing (CORS) all matter.

ArcGIS REST#

ArcGIS FeatureServer and MapServer URLs expose service and layer metadata. The Explorer can:

1. inspect a service;
2. choose a declared feature layer;
3. request a capped GeoJSON result;
4. render supported geometry;
5. retain the original source when anything fails.

It does not probe private layers or insert credentials.

The current basemap dependency#

The current locator requests four raster tiles from tile.openstreetmap.org at zoom level 5 and draws the hand-authored UK outline over them. It displays OpenStreetMap attribution, but it is not presently a self-contained or offline map. Opening Map discloses the tile requests to that service, and a network or policy failure can leave only the local outline, markers and grid.

The OpenStreetMap Foundation tile-usage policy requires normal clients to honour cache headers and explicitly prohibits bulk download, prefetch and offline archives from its public tile service. A boundary-pack builder must therefore never copy those public raster tiles into an OKF cache.

The boundary proposal treats a basemap and evidence geometry as different layers. A privacy-preserving default could use only a locally published overview boundary and grid. Live OpenStreetMap or another configured basemap could be an explicit enhancement with its own attribution, privacy, availability and usage-policy statement. A publisher could instead ship or self-host a basemap only where its source licence explicitly permits that redistribution and caching.

Proposed boundary packs (not yet implemented)#

Status: planned, not implemented. The current UK locator is a very small hand-authored outline drawn from a handful of hard-coded coordinates. That explains why its coastline can look angular or visibly wrong at the scale shown. It is a temporary presentation aid, not a supplied boundary and not evidence about any record.

The proposed replacement is a boundary pack: an optional, cached sidecar whose geometry is connected to the bundle's semantic entities by stable identifiers. YAML for Linked Data (YAML-LD) would continue to say which geography a record is about and what relationship it has to that geography. The sidecar would say how that geography may be drawn at particular scales. Keeping these concerns separate avoids putting millions of coordinates into an ontology.

The proposal has two complementary tiers:

1. a small Explorer-owned overview pack can replace the temporary UK outline even when a bundle supplies no geometry; and
2. a bundle-specific sidecar can provide dated boundaries for the particular countries, authorities, statistical areas or other spatial entities used by that corpus.

The proposed contract name and fields remain candidates until Explorer issue #84 is implemented and released. The completed contract review records the acceptance detail. An illustrative descriptor reference is:

{
  "geometry_packs": [
    {
      "id": "ons-countries-dec-2025-uk-buc",
      "manifest": "geometry/ons-countries-dec-2025-uk-buc/manifest.json",
      "manifest_schema": "okf-geometry-pack.v1",
      "media_type": "application/json",
      "bytes": 12345,
      "sha256": "<SHA-256 of the exact manifest bytes>",
      "required": false
    }
  ]
}

The root descriptor, rather than the manifest itself, binds the manifest's schema version, media type, exact byte count and detached digest. The Reader would reject an over-budget, short or long response before parsing JSON, then verify the digest. The manifest must not claim its own digest as the binding proof. The names above are illustrative and are not an accepted schema.

The manifest would bind:

• the dated source URL, retrieval time and source-file digest;
• publisher, licence, required attribution and territorial coverage;
• geography type, identifier scheme, code, boundary vintage and variant;
• source and display Coordinate Reference Systems;
• every local file's path, media type, byte size and SHA-256 digest;
• the transformation tool, version and reproducible boundary-generalisation settings;
• zoom or scale ranges for each level of detail (LOD); and
• known omissions and limits, including whether coverage is Great Britain or the whole United Kingdom.

Names alone are unsafe joins. A normal join key would combine identifier scheme, stable identifier, geography type and vintage. For an Office for National Statistics (ONS) statistical geography this might include a Government Statistical Service (GSS) code. A builder must reject duplicate or unresolved keys rather than attaching a plausible-looking polygon by name.

The semantic geography and its geometry representations are also different identified resources. For example, “Coventry local authority district, 2025” is the semantic geography; its BGC polygon in WGS 84 and its vector-tile representation in Web Mercator are representations of that geography. They need their own identifiers, formats, CRS, LOD, derivation and rights. Replacing one representation must not silently create a new geography or change assertions about the original one.

Where ONS publishes a GSS code, the semantic entity can retain the canonical http://statistics.data.gov.uk/id/statistical-geography/{GSS_CODE} Internationalized Resource Identifier (IRI). It must not be changed to HTTPS merely for stylistic consistency. Where no GSS identity exists, the pack can declare an OS identifier (OSID) or source-local scheme, but must not invent one. An explicit join table connects each semantic identity to one logical source feature. Its uniqueness rule applies before tiling: the same feature may legitimately occur in several vector tiles.

Each representation identity also binds its source product or item, geography type, boundary vintage, source publication or version, upstream modification, retrieval and build times, boundary variant, derivation and digest. A dated ArcGIS item can still change, so the builder retains the exact source metadata and bytes.

Use the right detail for the scale#

One shape is rarely suitable at every scale. At a UK-wide zoom, a carefully generalised boundary usually looks better and transfers faster than an exact polygon containing thousands of points. At a local zoom, the Explorer can load a more detailed representation.

The ONS digital-boundary guidance uses four boundary-detail abbreviations: full-resolution clipped boundaries (BFC), full-resolution extent-of-the-realm boundaries (BFE), generalised 20-metre clipped boundaries (BGC) and ultra-generalised 500-metre clipped boundaries (BUC). Some newer ONS products also publish super-generalised 200-metre clipped boundaries (BSC). BSC and BUC are distinct representations and a builder must not treat one abbreviation as an alias of the other. Availability varies by geography and release. BFE is a different coastal extent, not simply a more detailed rung in the display ladder. A sensible initial policy is:

• use BUC for a national overview such as the locator shown above;
• use BGC for regional and local-authority browsing; and
• load BFC only where closer inspection justifies its cost.

For the screenshot's national locator, a suitable first candidate is the dated Countries (December 2025) UK BUC item, converted into a small, whole-file-verifiable TopoJSON overview. Its source metadata, codes, polygon parts and islands would still be checked rather than assuming that the word “UK” in a title proves complete coverage. The coverage statement must say explicitly whether Crown Dependencies and Overseas Territories are excluded. Validation must check expected codes, polygon parts, rings and small islands after generalisation.

These are display choices, not interchangeable evidence. Exact or source-supplied geometry must remain separately identified and must be used for analysis when the task requires it. A display-generalised polygon must not be presented as a cadastral, legal or measurement boundary.

For browser delivery, a small TopoJSON overview is a practical first step because neighbouring polygons can share arcs and the complete file can be verified before it is drawn. PMTiles or Mapbox Vector Tiles (MVT) are better candidates for a large, multi-scale pack. GeoPackage (GPKG) is useful as an auditable builder input. Large raw GeoJSON files are simple but can be slow to download, parse and retain in browser memory.

The output format determines part of the CRS contract. RFC 7946 GeoJSON uses CRS84 longitude and latitude and must not carry an arbitrary alternate CRS; this proposed TopoJSON overview would use the same coordinate convention. Web vector tiles normally use a named Web Mercator tile matrix. Each LOD must state its output CRS or tile matrix, zoom range, geometric error or simplification tolerance and units, quantisation and topology method, feature and coordinate counts, bounds and topology checks. TopoJSON shares arcs only when the builder constructs a coherent topology. The manifest must also retain the source CRS and exact transformation, so display convenience does not erase provenance.

A range-loaded PMTiles archive needs an extra integrity decision. A byte-range request fetches only part of the archive, and its ordinary header and directories do not provide cryptographic hashes for every requested range. The Explorer can either download and verify the complete archive before rendering, or the builder can publish independently digest-bound chunks or a reviewed Merkle-style manifest—a tree of digests—that lets each fetched range be verified. A whole-file SHA-256 digest cannot be verified before partial range rendering and must not be described as trust-before-render.

Source and delivery options#

The OS Places API is useful for address, postcode and Unique Property Reference Number (UPRN) lookup and geocoding. It is not a boundary-download API. Its polygon operation finds addresses within a polygon that the caller supplies; it does not return that polygon. Postcode areas are a separate use case. Code-Point with Polygons and the OS NGD postcode-unit-area feature cover Great Britain, not the complete United Kingdom, and may require licensed access. The current OS NGD Northern Ireland feature-type inventory exposes a postcode-unit point but no postcode-unit-area path, while its overview also discusses polygon use. A builder must therefore inspect and pin the actual Select+Build schema and package rather than assuming either description is complete.

A pinned Office for National Statistics Postcode Directory or National Statistics Postcode Lookup can link UK postcodes to named statistical geographies, but it is a lookup, not postcode-boundary geometry. Northern Ireland polygon coverage needs its own authoritative source and rights review. None of these products should be mistaken for administrative or national borders.

Unless an exact ONS layer is demonstrably UK-wide, Northern Ireland needs its own source, rights, CRS and provenance. A Great Britain product must never be presented as United Kingdom coverage.

Safe caching and failure#

The recommended default is build-time acquisition followed by same-origin, content-addressed delivery. A builder downloads or receives an official dated source, checks its type and size, records rights and attribution, transforms it deterministically, validates every digest and publishes only the derived pack. API keys and access tokens never enter the bundle.

A dated-looking URL is not enough. Pack identity keeps the boundary's validity or as-at date, source publication or version, upstream item or service modification time, retrieval time and pack-build time as separate facts. The builder retains and hashes the exact source bytes and metadata; a mutable “latest” endpoint is only a discovery signal.

“Same origin” must be interpreted from the Reader's point of view. A pack beside a bundle on the same host may still be cross-origin when that bundle is opened in an Explorer hosted elsewhere. The descriptor must define whether a URL is relative to the bundle or manifest, and the Reader must resolve it once without silently proxying it. External producers must enable the required CORS and range responses or provide an explicitly governed same-origin copy. Responses must be readable rather than opaque, and redirects are revalidated against the URL policy.

The Explorer should select the smallest suitable LOD and cache immutable bytes by digest. Small files can be verified completely before use. Range-addressed large files need the full-download or chunk-verification design described above. Cross-origin packs additionally need explicit HTTP Range and CORS support, immutable cache semantics and an object-consistency rule ensuring that every range comes from the same unchanged version. Same-origin delivery is simpler and is the preferred default.

The current publication deliberately retires its old service worker, so this proposal also requires new, bounded cache lifecycle and eviction behaviour if offline use is promised. A missing, corrupt, unlicensed or identifier-incompatible pack must fail closed: the map can retain markers and explain that boundary geometry is unavailable, but it must not silently draw a different or guessed boundary.

Browser storage can be evicted, so “cached” does not automatically mean “available offline”. An offline claim must say whether the complete verified pack is retained or only previously fetched, individually verified chunks are retained. Durable offline assurance needs an explicit downloadable snapshot, not an assumption about the browser cache.

A secure builder and Reader also need contract-level limits for each manifest, pack, layer and LOD. Limits cover files and archive entries, properties and strings, features, rings, coordinates, tiles and ranges, nesting, download, decompressed and retained bytes, processing time and memory. They must reject path traversal, links and device files, decompression bombs, invalid CRS declarations, non-finite coordinates, broken polygon topology and unexpectedly complex geometry. These limits belong in the pack profile and receipt, so a successful small fixture is not mistaken for an unbounded parser claim.

Conversion should run with pinned tools, low privilege and no unnecessary network access. Archive extraction rejects paths that escape its workspace; GeoPackage and SQLite extension loading remains disabled. Integrity proves agreement with the reviewed descriptor, not that an unreviewed publisher or source assertion is true.

Attribution is part of the rendered result, not merely hidden manifest metadata. ONS boundary reuse normally requires the source and Open Government Licence statement together with the applicable OS Crown copyright and database right year. Every pack must retain the exact source wording. The Explorer must display the attribution and licence URI for every visible geometry or basemap layer, including downloadable or offline artefacts. Rights are recorded per source, layer and nation rather than only once for the pack; postcode sources may add Royal Mail or Northern Ireland Land and Property Services conditions.

Geometry remains supplementary. Every visible feature must also have an accessible text or table route exposing its identity, source, vintage, variant, relationships and rights. The evidence must not depend on colour, shape or pointer use. See accessibility.

Bounded Preview#

Remote preview happens only after an explicit user action and is limited by:

• allowed URL schemes and sanitised query parameters;
• JSON response type;
• response bytes;
• feature count;
• coordinate count;
• supported geometry;
• timeout and retry policy.

If a preview cannot load, local metadata remains useful and the source link remains available. Failure should not erase the record.

Sensitive URLs#

Bundle and resource URLs must not contain passwords, tokens or API keys. Sensitive query parameter names are removed from geospatial URLs before they are displayed or requested.

A static bundle is public. Any credential placed in it should be assumed disclosed.

Shared Context#

The Map starts from the same search and facet reduction as Reader or Graph. Selecting a place or evidence class produces a URL-addressable reduction that the other views can use.

This makes geography part of retrieval rather than an isolated picture.

Common Reasoning Errors#

Avoid these conclusions:

• a place-name match proves exact geographic coverage;
• a point proves a boundary;
• a current boundary is valid for every historical record;
• a service URL proves anonymous access;
• a successful preview proves the data licence permits reuse;
• missing spatial metadata proves a record is non-spatial.

Continue#

Use the illustrated Geospatial Map manual for a worked interface journey and Geospatial Map exploration for the implementation contract.

Next#

UK legislation data introduces a second specialist domain where identity, versions and authority are critical.