Short answers about Portuguese military coordinates, coordinate systems and GPS.
In short: select the coordinate system your values come from — usually EPSG:20790 for Portuguese military maps — and the converter returns WGS84 GPS coordinates and a Google Maps link, computed in your browser with the official DGT grids.
How do I convert Portuguese military coordinates to GPS?
Enter the X (M) and Y (P) values in the converter, select “Portuguese Military Grid — Datum Lisboa / Hayford-Gauss (EPSG:20790)” and press Convert Coordinates. The site applies the official DGT grid and returns WGS84 latitude and longitude, a map and a Google Maps link.
What do X and Y mean on a Portuguese military map?
They are distances in metres from the grid’s false origin: X (also called M) is measured eastwards and Y (also called P) northwards. In the military grid every mainland value is positive.
What is EPSG:20790?
EPSG:20790, “Lisbon (Lisbon) / Portuguese National Grid”, is the Portuguese military grid: a Transverse Mercator projection on Datum Lisboa (Hayford ellipsoid) with a false origin of 200 km west and 300 km south of the projection centre.
What is the difference between military coordinates and PT-TM06?
They use different datums (Datum Lisboa vs ETRS89), different ellipsoids (Hayford vs GRS80) and different origins. Military coordinates are about 200 km larger in X and 300 km larger in Y, and after removing that offset a further datum difference of a few metres remains.
Can I open military coordinates in Google Maps?
Not directly — Google Maps needs WGS84 latitude and longitude. Convert them here first, then use the “Open in Google Maps” button.
Why do different coordinate systems produce different results?
Each system has its own datum, ellipsoid, projection and origin. The same pair of numbers therefore describes a different place in each system — sometimes hundreds of kilometres apart, sometimes only a few metres.
How accurate are coordinate conversions?
With the official DGT NTv2 grids the conversion of Datum Lisboa and Datum 73 is accurate to about 0.1 m, and this site matches PROJ and the DGT service to within a few millimetres. The 7-parameter method can be several metres off. Usually the biggest error comes from how the original coordinates were measured or read from a map.
Can I use coordinates from old Portuguese military maps?
Yes. Check the map margin for the datum and grid. Older sheets use Datum Lisboa / Hayford-Gauss Militar; many also show a UTM grid. Values read from a 1:25 000 map are typically good to 10–25 m.
What if I do not know the original coordinate system?
Use “Help Me Identify My Coordinates”. It asks a few questions, lists plausible systems and shows where your values fall under each one. A plausible location is not proof, so confirm with the source of the coordinates.
Are my coordinates stored or sent anywhere?
No. The conversion runs entirely in your browser. Only the map tiles for the area you view are loaded from the map providers, and Google receives the coordinates only if you click “Open in Google Maps”.