Minna Datum vs WGS 84: Why Nigerian Coordinates Don’t Line Up

Nigeria’s national survey control is still based on the Minna datum, a local datum, while GPS, web maps and most global datasets use WGS 84. For the same point on the ground, the two can differ by roughly 65 to 110 metres depending on where you are, so coordinates that don’t state their datum can’t be trusted.

What a datum is
A coordinate such as 6.5244° N, 3.3792° E only means something relative to a datum: a definition of the Earth’s shape and where its origin sits. WGS 84 is geocentric and is what GPS uses. Minna is a local datum built on the Clarke 1880 ellipsoid, with its origin at station L40, the northern end of the Minna baseline of the primary triangulation network. Because the two are positioned differently relative to the Earth, one physical point gets two sets of coordinates.

How big is the difference?
Using the transformations registered in the EPSG database, the horizontal shift from Minna to WGS 84 is approximately:

Lagos: 100 m
Port Harcourt: 96 to 108 m
Calabar: 96 to 106 m
Abuja: 85 m
Sokoto: 85 m
Jos: 71 to 80 m
Kano: 72 to 78 m
Maiduguri: 64 to 70 m

These figures are indicative, not survey-grade. The registry lists more than a dozen different Minna to WGS 84 transformations, each with its own area of use and a stated accuracy of about 5 to 44 m. A shift derived for one area, such as an offshore oil block, is wrong elsewhere, and even a well-chosen one leaves metres of uncertainty.

Why the transformation is uncertain
The Minna network has documented weaknesses: scale and orientation defects, a poorly defined origin, and no geoid model when it was computed. The Office of the Surveyor-General of the Federation (OSGoF) has said that scale error and the undefined origin hinder accurate transformation parameters. Many of the original pillars have also been destroyed.

Where it bites

Survey plans and records: coordinates in Nigeria are reported in several systems, including Minna, WGS 84, ITRF and Nigeria Transverse Mercator. A plan that doesn’t say which one it uses is ambiguous.
Projections: Minna has its own UTM zone 31N and 32N systems and Nigeria West, Mid and East Belt systems. Picking the wrong zone or belt can produce errors far larger than the datum shift.
GNSS and CORS work: RTK corrections tie you to the reference frame of the base or network, a global frame at a stated epoch. WGS 84 and ITRF agree to centimetres, but plate motion means the epoch matters at that level. Record the frame and epoch as well as the datum.
Merging data: most global datasets (imagery, elevation, OpenStreetMap) are on WGS 84. Combine them with Minna-based data without transforming and nothing lines up.

What is being done
OSGoF started NIGNET, Nigeria’s continuous GNSS reference network, in 2008, and a Nigerian Geocentric Datum project aims to replace Minna. In a December 2023 statement, OSGoF said little had been achieved on realising it and blamed poor funding. Until it’s adopted, you will keep meeting Minna data.

Practical rules

Always ask “which datum, projection and epoch?” before accepting coordinates.
A survey plan should state its datum and projection. If it doesn’t, treat its coordinates as unverified.
Confirm the datum of every dataset before merging. Never assume.
Never fix a mismatch with a hand-applied constant offset. Use a named transformation, and record which one and its stated accuracy.
For high-stakes work, have the surveyor confirm the datum, epoch and transformation in writing.

In recent QGIS versions, Project → Properties → Transformations shows which operation is applied between two coordinate systems, with its area of use and accuracy.

A.B. Wayforward