Garmin eTrex GPS settings for use with Finnish maps
In order to use my Garmin eTrex GPS with the Finnish National Park maps (made by Genimap) I had to define a User Defined Grid on the eTrex. The eTrex does provide the Finnish KKJ27 grid, but this did not appear to work with the maps I had.
It took me a while to figure it out, but below is how it seems to work. Although I own a Garmin eTrex, most of it is generally applicable to any type of GPS.
Finland uses a National Grid called KKJ (Kartastokoordinaattijärjestelmä), which is based on the Transverse Mercator Projection. This grid is used to define two coordinate systems: the Basic coordinate system (Peruskoordinaatisto), KKJ-P and the Uniform coordinate system (Yhtenäiskoordinaatisto), KKJ-Y (which is what the eTrex KKJ27 grid is).
In the Basic coordinate system Finland is divided in 4 projection zones, each 3° wide around a central meridian. The Y-coordinate, or 'northing' is the distance from the equator. The X-coordinate or 'easting' is the distance from the central meridian. To prevent negative easting values an offset of 500 km is added. This means that the central meridian of a zone always has an easting value of 500 km. In addition, to be able to distinguish between the zones the zone number is prefixed onto the easting value, giving each zone unique easting values. This complete offset is called 'False Easting'. The northing value in the Finnish Grid has no offset, so the False Northing is 0. All zones use the same northing values, i.e. distance from the equator. The scale factor for the Finnish grids is 1.
The 4 zones are defined as follows:
The Uniform or KKJ-Y grid is actually identical to zone 3 of the Basic grid.
This is the KKJ27 grid provided by the eTrex, the '27' indicates the coordinate of the central meridian.
Basic system (KKJ-P) coordinates are usually printed on the map in black,
Uniform system (KKJ-Y) coordinates in red.
Zone 3 Basic coordinates are the same as Uniform coordinates and are printed in red.
Results for specific maps
Helvetinjärvi 1:30000, Karttakeskus
Nuuksio 1:15000, Karttakeskus
Some general background information on GPS, grids and map datums can be found here . It is in German, though...
For information on the eTrex and other Garmin GPS receivers, visit the Garmin website.
A wealth of general information about GPS can be found on gpsinformation.net .