Four independent things can put a Ooru address away from where you meant. Three are bounded and small. The fourth will put you 200 m out and is entirely avoidable.
1. Quantisation — the dominant term
An address names a cell, not a point, and resolves to that cell’s centre. For a position uniformly distributed within a square cell of side s, the error between the true position and the cell centre has
| Level | Cell | Area | RMSE | Worst case |
|---|---|---|---|---|
| 1 · Region | 5 km | 25.00 km² | 2.0 km | 3.5 km |
| 2 · Locality | 250 m | 6.25 ha | 102.1 m | 176.8 m |
| 3 · Spot | 12.5 m | 156 m² | 5.10 m | 8.84 m |
| 4 · Pinpoint | 3.13 m | 9.77 m² | 1.28 m | 2.21 m |
2. Positional accuracy of the input
A consumer GNSS fix on a phone is typically 3–10 m in the open and considerably worse in the urban canyons of T. Nagar or George Town, where multipath can push it past 20 m.
3. Projection distortion
Across the whole extent the point scale factor varies by about 111 ppm. The practical consequence is that the largest and smallest 12.5 m cell in the metropolitan area differ in true ground edge by under 5 mm — three orders of magnitude below the GNSS term above, and therefore irrelevant in practice.
| Extent corner | Point scale factor k | Deviation | True edge of a 12.5 m cell |
|---|---|---|---|
| SW | 0.9997628 | -237 ppm | 12.5030 m |
| SE | 0.9996520 | -348 ppm | 12.5044 m |
| NW | 0.9997628 | -237 ppm | 12.5030 m |
| NE | 0.9996520 | -348 ppm | 12.5044 m |
4. The datum trap
Error budget, combined
How these claims are checked
The model ships with a test suite of 37 cases making roughly 155,000 assertions, run against the same code the site serves. Among them:
- projection round-trips to under 10⁻⁸ degrees over 500 random positions;
- encode/decode round-trips at every depth over 3,000 random positions, with the decoded centre always within half a cell diagonal;
- differential distortion across the extent asserted under 200 ppm, and the largest/smallest cell edge difference under 5 mm;
- exhaustive uniqueness over a 41 × 41 block of level-3 cells, and reversibility of every compass step including across parent boundaries;
- the single-character mutation test over the whole vocabulary described in what hierarchy costs.