Five systems you are likely to meet in Portugal, how they differ and why numbers from one can never be read in another.
In short: EPSG:20790 is the military grid (Datum Lisboa, false origin +200 km / +300 km); EPSG:5018 is the same grid without the false origin; EPSG:27493 is the Datum 73 grid; EPSG:3763 (PT-TM06 / ETRS89) is today’s official system; and WGS84 (EPSG:4326) is what GPS and Google Maps use.
At a glance
| System | EPSG | Datum / ellipsoid | Typical use |
|---|---|---|---|
| Datum Lisboa / Hayford-Gauss Militar | 20790 | Datum Lisboa / Hayford | Military maps (Carta Militar), older documents |
| ETRS89 / PT-TM06 | 3763 | ETRS89 / GRS80 | Official system for mainland Portugal since 2006 |
| Datum 73 / Hayford-Gauss | 27493 | Datum 73 / Hayford | Civil mapping and cadastre (1970s–2000s) |
| Datum Lisboa / Hayford-Gauss | 5018 | Datum Lisboa / Hayford | Civil mapping and cadastre on Datum Lisboa |
| WGS84 | 4326 | WGS84 | GPS, phones, Google Maps |
EPSG:20790 — the military grid
Transverse Mercator on Datum Lisboa with the projection centre at 39°40′ N, 8°07′54.862″ W, scale 1, and a false origin of X = +200 000 m, Y = +300 000 m. All mainland values are positive. Used by the Army's 1:25 000 maps and many older records.
EPSG:5018 — Datum Lisboa / Hayford-Gauss
Exactly the same datum and projection as the military grid, but with the origin at the projection centre, so values west or south of the centre are negative. For any point, X20790 = X5018 + 200 000 and Y20790 = Y5018 + 300 000.
EPSG:27493 — Datum 73 / Hayford-Gauss
Datum 73 is the result of the 1973 readjustment of the Portuguese geodetic network, with its origin at the Melriça vertex (the geodetic centre of Portugal). The grid uses the same projection centre with small offsets (X +180.598 m, Y −86.990 m). It was widely used by the former Instituto Português de Cartografia e Cadastro.
EPSG:3763 — ETRS89 / PT-TM06
The modern official system for mainland Portugal: Transverse Mercator on ETRS89 (GRS80 ellipsoid), origin at 39°40′05.73″ N, 8°07′59.19″ W, scale 1, no false origin. ETRS89 is the European realisation of the GPS reference frame, so PT-TM06 values convert to GPS without a datum shift (differences below about 1 m).
WGS84 — GPS latitude and longitude
The global system used by GPS. Coordinates are angles (degrees), not metres. Google Maps expects WGS84 decimal degrees in the order latitude, longitude.
Why the same numbers mean different places
For Lisbon (38.722252° N, 9.139337° W) the four Portuguese grids give, in metres:
| System | X | Y |
|---|---|---|
| EPSG:20790 (military) | 112 492.4 | 195 454.8 |
| EPSG:5018 (Datum Lisboa) | −87 507.6 | −104 545.2 |
| EPSG:27493 (Datum 73) | −87 509.3 | −104 543.4 |
| EPSG:3763 (PT-TM06) | −87 506.7 | −104 544.2 |
Two lessons follow. First, the military grid is shifted by hundreds of kilometres, so mistaking it for another system moves a point far away (often into Spain or the Atlantic) — an obvious error. Second, EPSG:5018, EPSG:27493 and EPSG:3763 differ by only a few metres. Confusing them produces a point that looks right but is wrong by several metres, which matters for boundaries and engineering. That is why this site never guesses the system.
Datum transformations and accuracy
Converting Datum Lisboa or Datum 73 to ETRS89/WGS84 requires a datum transformation. The Direção-Geral do Território publishes two kinds:
- NTv2 grids (
DLx_ETRS89_geoandD73_ETRS89_geo), which model regional distortions. Accuracy about 0.1 m. Used by default here. - 7-parameter Bursa-Wolf sets. Simpler, but over mainland Portugal they differ from the grids by a median of about 2 m (Datum Lisboa) or 0.6 m (Datum 73), and up to about 5 m.
If the grid cannot be loaded, or a point lies outside it, this site stops and asks you to choose the Bursa-Wolf method explicitly — it never substitutes it silently.
Madeira and the Azores
The islands have their own systems. The modern one is PTRA08 (ITRF93) with UTM zone 28N in Madeira (EPSG:5016), zone 26N in the central and eastern Azores (EPSG:5015) and zone 25N in the western Azores (EPSG:5014). Older maps use local datums — Base SE (Porto Santo) in Madeira, S. Braz (São Miguel), Base SW (Graciosa) and Observatório (Flores) in the Azores — converted here with the DGT Bursa-Wolf parameters (residuals 0.06–0.36 m), with island-specific sets available.