Finnish cartographic grid and general sheet line system

The horizontal coordinate system

In Finland the basis for determining the horizontal coordinates is the National Grid Coordinate System. The Basic coordinate system (Peruskoordinaatisto) and the Uniform coordinate system (Yhtenäiskoordinaatisto) based on that grid system are represented on the topographic maps.

In the Basic coordinate system in Finland is devided to four projection zones of 3° wide.
The corresponding central meridians are 21°E (zone 1), 24°E (zone 2), 27°E (zone 3) and 30°E (zone 4).
The grid of the basic coordinate system is directed along the central meridian. The origin is the intersection of the equator and the central meridian.
Thus the x-coordinate (or northing) is distance from the equator and y-coordinate (or easting) is distance from the central meridian.
To avoid negative values for the y-coordinates the y-coordinate of the central meriadian is 500 000 m.
Y-coordinates of the different zones are distinguished by adding the ordinal number of the particular zone before the actual coordinate value.
So the values of the y-coordinates of the central meridians are 21°E = 1500 km, 24°E = 2500 km, 27°E = 3500 km and 30°E = 4500 km.
In the Uniform coordinate system Finland is represented in one projection zone. The central meridian is 27°E. Other details equal the Basic coordinate system.
On topographic maps the Basic coordinate system is represented in black colour and the Uniform coordinate system in red colour. In the third zone (27°E) coordinate systems are equal and markings are in red colour.

Marko Ollikainen (National Land Survey): THE FINNISH NATIONAL GRID COORDINATE SYSTEM

General sheet line system