Comparing printed nautical charts to GPS coordinates

Question

I'm just getting my head wrapped around the various geodesic systems. I have some (printed) Finnish nautical charts I'm trying to match to GPS coordinates. Given the number of underwater obstacles, I'm trying to make sure I'm doing this correctly. The charts simply state "Longitude from Greenwich" for East-West and are published in 2002, but otherwise have no technical information on the coordinate system used. According to this post (http://www.kolumbus.fi/eino.uikkanen/geodocsgb/ficoords.htm) I'm suspecting that the maps are in fact using a "KKJ" system which is based on ED50.

So, just to sanity check my understanding, the fact that I'm looking at a map with deg Lat/Long does not mean that these actually match up to GPS coordinates? Of course I can buy a complete new set of maps for 39euros to find out, but wanted to at least see if I got all this right ...

Answer 1

Yes, you are right. Finnish nautical maps printed 2002 or before are all based on KKJ coordinated system. KKJ coordinates may differ from GPS-coordinates (WGS84) as much as 200 meters.

Starting from 2003 have all new nautical charts been published in EUREF-FIN (appr. WGS 84). These new nautical charts apply international INT chart symbols. Because shallow waters are printed in blue according the INT symbols, the new nautical charts are called "blue nautical charts" while the old nautical charts are called "green nautical charts" based on the green color of the land areas.

For more information about Finnish coordinate systems: http://www.kolumbus.fi/eino.uikkanen/geodocsgb/ficoords.htm