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:

Zone	Central Meridian	False Easting
1	21E	1500 km
2	24E	2500 km
3	27E	3500 km
4	30E	4500 km

Table 1: Finnish Basic Grid zones

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.
Determine the appropriate zone for your map and enter the values from Table 2 below into the 'Define User Grid' page on your eTrex (Setup->Units->Position Format->User Grid).
In all cases select 'Finland Hayford' as the map datum (Setup->Units->Map Datum).

Zone	Longitude origin	Scale	False E	False N	Comment
1	E021°00.000'	1.0000000	1500000.0	0.0
2	E024°00.000'	1.0000000	2500000.0	0.0
3	E027°00.000'	1.0000000	3500000.0	0.0	Or use the Garmin 'Finnish KKJ27' grid
4	E030°00.000'	1.0000000	4500000.0	0.0

Table 2: Garmin eTrex User Defined Grid values for Finnish Basic Grid zones

Results for specific maps

Rukatunturi/Oulanka 1:40000, Genimap This map is in Zone 4 and has printed annotations at the grid lines. Using the above method matches all my eTrex waypoints perfectly. This map covers the Oulanka National Park, where the famous and beautiful Karhunkierros ("The Bear's Ring") trail runs. For more information, see my Karhunkierros page.

Helvetinjärvi 1:30000, Karttakeskus
This map has no printed annotations to its grid, making it not very practical to use with a GPS. However, from the waypoints I took on my eTrex it was not too hard to determine the map grid:
The grid lines on the map have a 5 km spacing, using the KKJ-P Basic coordinate system, Zone 2. The top edge of the map is a grid line with northing value 6885, the next horizontal grid lines are 6880 resp. 6875. The bottom edge of the map is not a grid line.
The left and right edge of the map are also not grid lines. The vertical grid lines visible on the map have easting values 2485, 2490 resp. 2495.
This grid matches my eTrex waypoints perfectly.

Nuuksio 1:15000, Karttakeskus
The Nuuksio 1:15000 (Karttakeskus) map uses an unannotated 1 km grid which seems to be unrelated to either the Uniform or the Basic grids. The map seems to have some reference points for the Basic and Uniform grids on it, I am still working out how to use them.

Eino Uikkanen's excellent page explains the KKJ coordinate system very thoroughly.
Eino has lots of other interesting stuff on geodesy and GPS on his homepage .

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 .