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Coordinate Reference Systems for Nature Data

Overview​

A coordinate reference system (CRS) is the framework that turns numbers into a place on the Earth. Get it wrong and everything downstream — areas, overlaps, distances — is quietly, precisely incorrect. For UK nature and BNG data, two systems dominate:

  • EPSG:27700 — British National Grid (OSGB36). A projected system in metres. The right choice for measuring areas and distances in Britain.
  • EPSG:4326 — WGS84 latitude/longitude. A geographic system in degrees. Great for web maps and GPS; wrong for area calculations, because a degree is not a fixed length.

Why it matters for nature strategy​

BNG is fundamentally about area — hectares of habitat, multiplied into units. Area is exactly the quantity a CRS mistake corrupts:

  • Calculating area in degrees (4326) gives nonsense. A "square degree" isn't a consistent area, so hectares come out wrong — and the metric turns wrong hectares into wrong unit totals with total confidence.
  • Mixing CRSs silently misaligns layers. Overlay a 4326 layer on a 27700 one without reprojecting and intersections shift or vanish — features that should overlap don't, and vice versa.
  • The failure is invisible. There's no error message. The map still draws; the numbers are just wrong.

How it works — good practice​

  1. Pick one working CRS and enforce it. For UK area work that is EPSG:27700. The WildStack pipeline mandates 27700 everywhere for exactly this reason.
  2. Reprojection is a first-class step, not an afterthought. Datasets arrive in different CRSs — WFD data, for instance, is commonly distributed in 4326 and must be reprojected to 27700 on ingest.
  3. Confirm, don't assume. Portal metadata is sometimes wrong about a dataset's CRS — verify against the actual file before trusting it. (A real example in this stack: a source labelled 4326 in portal metadata was actually 27700.)
  4. Do all measurement in the projected CRS, and reproject to 4326 only for display on web maps.

Nation differences​

British National Grid (EPSG:27700) covers Great Britain. Northern Ireland uses Irish Grid / ITM systems, and the Republic of Ireland likewise — a reminder that "the national grid" is nation-specific, and cross-border datasets need explicit CRS care.

WildStack's take​

WildStack's take

CRS errors are the silent data-corruption of ecological analysis: no crash, no warning, just quietly wrong hectares that flow straight into a BNG unit total. Because the map still looks fine, these mistakes survive all the way to a decision. Two habits prevent almost all of them — enforce a single projected CRS (27700) for every measurement, and verify each dataset's real CRS rather than trusting the portal that published it, which is wrong often enough to matter. It's unglamorous plumbing, but a BNG number computed in the wrong CRS is worse than no number, because it carries false confidence.

Official sources​

Last reviewed​

5 July 2026. A stable technical topic; revisit only if the standard grids or common dataset CRSs change.

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