The problem in one sentence
You have a deed plan with two numbers on it — an X and a Y — and a phone that reports latitude and longitude. They describe the same corner of the same plot, and they do not agree.
This is not an error on either side. They are measured in different units, projected differently, and — the part that catches most people out — referenced to a different model of the earth. Get any one of those three wrong and your point lands somewhere else.
What Cassini actually is
Kenyan cadastral survey uses Cassini-Soldner, a transverse cylindrical projection with its central meridian at 37° East. It keeps distances true along that meridian, which suited the local survey work it was adopted for, and it is still the grid printed on a great many survey plans and deed plans today.
Coordinates are given as an X (east–west) and a Y (north–south) in metres or feet from that origin. Because Nairobi lies west of 37° E and south of the equator, both values normally come out negative — which is correct, not a transcription error.
Underneath it sits the Clarke 1880 ellipsoid, as used by the Arc 1960 datum. That is where the trouble starts.
The datum problem
A datum is the reference model of the earth's shape that coordinates are measured against. Arc 1960 was fitted to survey observations across East Africa, so it hugs the ground closely here. WGS84, which GPS and every web map use, is fitted globally.
Two good models, two different answers for the same physical point. Around Nairobi they differ by roughly 317 metres:
| Point (WGS84) | -1.284600, 36.821900 |
| WGS84 UTM 37S | E 257,633.79 N 9,857,909.55 |
| Arc 1960 UTM 37S | E 257,542.77 N 9,858,213.42 |
| Difference | 91 m west, 304 m north → 317 m apart |
That offset is consistent across the county — right across Nairobi it varies by under 20 centimetres — so it is not noise you can average away. It is a systematic shift, and a converter that ignores it will be wrong by about 317 metres every single time, in the same direction.
Why this matters: 317 metres is several plots wide. A boundary that looks displaced against the modern map is almost always this datum difference, not a change on the ground and not a mistake by the original surveyor.
Worked example — Cassini in metres
A plan quotes a beacon in Cassini metres. Converting it properly means two steps: undo the Cassini projection on the Clarke 1880 ellipsoid, then shift from Arc 1960 to WGS84.
| Cassini X | -19,907.891 m |
| Cassini Y | -141,740.792 m |
| → Arc 1960 UTM 37S | E 257,542.77 N 9,858,213.42 |
| → WGS84 | -1.284600, 36.821900 |
That lands in central Nairobi, a little west of the 37° meridian and just south of the equator — exactly where the negative X and Y said it would.
Worked example — Cassini in feet
Older Kenyan records are commonly quoted in feet, and nothing on the sheet always shouts about it. One international foot is 0.3048 m, so the figures are converted before anything else happens.
| Cassini X | -65,000 ft |
| Cassini Y | -465,000 ft |
| × 0.3048 | -19,812.000 m -141,732.000 m |
| → WGS84 | -1.284521, 36.822762 |
Now the same figures read as though they were metres, which is the single most common mistake with older plans:
| Treated as metres | -65,000 m -465,000 m |
| → WGS84 | -4.207783, 36.415309 |
| Error | 328 km away — well inside Tanzania |
Check the units before you check anything else. A datum mistake puts you 317 metres out and can look plausible on a map. A units mistake puts you in another county and is obvious the moment you plot it — which is why plotting the result is always worth the ten seconds.
How accurate is the conversion?
The projection maths itself contributes essentially nothing. Taking a coordinate from Cassini to WGS84 and straight back returns the original value to within about 1.4 millimetres, so the conversion is not what limits you.
What limits you is the datum transformation. This uses the standard
three-parameter Helmert shift for Arc 1960 to WGS84
(-175, -23, -303 metres), which is a national average.
Local distortions in the original survey network mean the real offset
at any one beacon can differ from it by a few metres.
Good enough to find a plot, check a boundary against the cadastre, or take a coordinate into QGIS. Not a substitute for a licensed surveyor when a beacon has to be set out or a boundary is in dispute.
Convert your coordinates
The Cassini converter is built into the Nairobi Maps toolbar and is free to use. It runs in both directions, takes metres or feet, drops a marker on the map so you can sanity-check the result, and exports a batch to CSV when you are working through a list of corner beacons.
Open the converterFree on every plan, including without an account. It will also name the survey block your point falls in — opening that block's scanned RIMS sheet needs Gold or Platinum.
Common questions
How do I convert Cassini coordinates to latitude and longitude?
Take the X and Y values from the survey plan, note whether they are in metres or feet, and run them through a converter that applies both the Cassini-Soldner projection on the Clarke 1880 ellipsoid and the Arc 1960 to WGS84 datum shift. Skipping the datum shift leaves the result about 317 metres from the true position in the Nairobi area. The Nairobi Maps Cassini converter does both steps and can drop a marker at the result so you can check it landed where you expected.
Why do Cassini and GPS coordinates disagree for the same point?
Because they are measured against different models of the earth. Kenyan survey records sit on the Arc 1960 datum using the Clarke 1880 ellipsoid, while GPS and web maps use WGS84. Around Nairobi the two differ by roughly 317 metres — about 91 metres west and 304 metres north. The same physical beacon therefore has two different sets of numbers, and neither is wrong; they are answers to different questions.
Are Kenyan Cassini coordinates in metres or feet?
Both, depending on the age of the record. Older Kenyan survey plans and deed plans are commonly quoted in feet, and newer ones in metres. The conversion factor is 0.3048 metres per international foot. Reading a figure in feet as though it were metres puts the result hundreds of kilometres away, so always check the units printed on the sheet before converting.
What is the Cassini projection used in Kenya?
Cassini-Soldner, computed on the Clarke 1880 ellipsoid as used by the Arc 1960 datum, with the central meridian at 37 degrees east. It is a transverse cylindrical projection that keeps distances true along the central meridian, which suited the local cadastral survey it was adopted for. It is still the grid printed on many Kenyan survey plans and deed plans.
Can I convert Cassini coordinates back from GPS?
Yes. The conversion runs in both directions — Cassini to WGS84 latitude/longitude, and latitude/longitude back to Cassini. Round-tripping a coordinate through both directions returns the original value to within a couple of millimetres, so the conversion itself contributes no meaningful error.
Related
- The map — parcels, Development Control Policy 2026 zoning, survey plans and historical maps of Nairobi.
- Coordinate Systems in the map's side menu — WGS84, UTM Arc 1960 and Cassini side by side.
- Plans and pricing — what each plan opens.