4.2 Geodetic Datums

Key takeaways

• A datum pins an ellipsoid to the Earth — it's what makes a coordinate meaningful.
• Different datums produce different coordinates for the same physical point.
• Converting between datums is a geometric and sometimes temporal problem (plate motion matters).

Introduction

"Just use lat/lon" is one of the most common — and most dangerous — things a GIS beginner can say. Latitude and longitude are angular coordinates, but on which ellipsoid pinned to Earth in what orientation? That's the datum. This lesson builds the intuition to handle datums correctly.

What a datum is

A geodetic datum specifies:

1. A reference ellipsoid (shape).
2. Its orientation in space (rotation).
3. Its origin (translation).
4. A reference epoch when modern datums — to account for continental drift.

A datum turns the abstract ellipsoid into a physical reference. Without it, saying "the point is at 37.7749° N, 122.4194° W" is like saying "the room is at coordinates 5, 7" without specifying the origin.

Local vs geocentric datums

Two families exist:

Local (horizontal) datums

Defined by a ground survey point and a best-fit ellipsoid over a region. The ellipsoid is not centred on Earth's centre of mass — it's offset to minimise residuals regionally.

Examples:

• NAD27 (North American Datum 1927) — origin at Meades Ranch, Kansas; Clarke 1866 ellipsoid.
• OSGB36 (Ordnance Survey Great Britain 1936) — Airy 1830 ellipsoid, fitted to the UK.
• Tokyo Datum — Bessel 1841 ellipsoid, fitted to Japan.
• Pulkovo 1942 — Krasovsky 1940 ellipsoid, fitted to the former USSR.

Local datums produce smaller errors within the region they were designed for. Outside that region, errors balloon.

Geocentric datums

Centred on Earth's centre of mass. Modern GNSS-friendly.

• WGS 84 — maintained by the US DoD; GPS default.
• NAD83 — North America; tied to the North American plate so it stays still relative to Canada, US, Mexico.
• ETRS89 — Europe; tied to the Eurasian plate.
• GDA2020 — Australia.
• ITRF2020 — International Terrestrial Reference Frame; global, continuously updated.

Geocentric datums give consistent results worldwide. Local datums still matter because historical data is in them.

Why datum choice matters

The same physical location can have different coordinates in different datums:

That difference is about 70 m. A parcel boundary off by 70 m is a legal nightmare.

Datum transformations

Converting between datums is not just rescaling — it's a 7-parameter Helmert transformation (three translations, three rotations, one scale) or a more complex grid shift (NADCON / NTv2 for North America, OSTN15 for the UK). Modern tools apply these automatically if you specify both datums correctly.

In PROJ:

1# Convert WGS 84 to NAD27 (requires a grid shift file)
2cs2cs +proj=longlat +datum=WGS84 +to +proj=longlat +datum=NAD27 <<< "-95.0 40.0"

Or with pyproj:

1from pyproj import Transformer
2t = Transformer.from_crs('EPSG:4326', 'EPSG:27700', always_xy=True)
3# WGS 84 lon/lat to British National Grid easting/northing
4easting, northing = t.transform(-0.1278, 51.5074)

Dynamic datums and plate tectonics

Modern datums like ITRF are dynamic: coordinates change over time as plates move. An ITRF coordinate includes an epoch — the date it was valid.

Australia moves ~7 cm per year northeast. If you take a centimetre-accurate survey in 2010 and compare to a 2020 measurement of the same corner, the coordinates will differ by ~70 cm — all due to plate motion. Datums like GDA2020 account for this by updating with epoch.

For most GIS at metre-level precision, plate motion is invisible. For engineering survey, it's critical.

EPSG codes

Every published datum, ellipsoid, and CRS has an EPSG code — a number maintained by the International Association of Oil & Gas Producers and now the EPSG registry. Examples:

• 4326 — WGS 84 geographic (lat/lon).
• 3857 — Web Mercator on WGS 84 ellipsoid.
• 4269 — NAD83 geographic.
• 27700 — British National Grid.
• 2154 — Lambert-93 for France.
• 25832 — ETRS89 UTM Zone 32N.

epsg.io is the canonical lookup. Module 4.6 covers EPSG codes in detail.

Transformations in practice

Rule 1 — store the datum explicitly

Never store bare lat/lon in a system without an EPSG code or equivalent tag. Your future self will thank you.

Rule 2 — convert, don't relabel

Changing a file's CRS tag without actually reprojecting the coordinates is a common disaster. A shapefile that says it's in EPSG:4326 but has easting/northing values is not in either system — it's broken.

Rule 3 — use proper grid shifts

For country-level datum transformations, a single Helmert transformation isn't accurate enough. Use official grid shift files — NTv2 for Canada, OSTN15 for the UK, GEOCOL24 for Colombia, etc. Modern PROJ ships with most of these.

Rule 4 — document the epoch

For sub-metre work, the epoch of your coordinates matters. A GNSS reading today is ITRF2020 at epoch 2025.3 or similar. If you integrate with a 2010 reference, use proper epoch propagation.

Self-check exercises

Summary

• A datum pins an ellipsoid to the Earth; without it, a coordinate is meaningless.
• Local datums minimise regional error; geocentric datums (WGS 84, ITRF) are global and GNSS-friendly.
• Datum transformations require proper grid shifts for high accuracy.
• Modern geodesy is dynamic — epoch matters for sub-metre work.

Further reading

• IOGP — Geomatics Guidance Note 7-2: Coordinate Conversions and Transformations including Formulas.
• Altamimi, Z. — ITRF2020: An augmented reference frame.
• NGS — Datum transformations in North America.
• EPSG Registry — https://epsg.org for authoritative CRS definitions.