Mfcttrf

I have a shapefile of Finland in WGS84 (EPSG:4326), that I would like to convert to projected coordinate system, most commonly used in Finland.

But, here is my confusion:
I have found that from 2005, topographic maps should be in EUREF-FIN CS, as ETRS89 realization in Finland http://www.kolumbus.fi/eino.uikkanen/geodocsgb/ficoords.htm

But, here they also list ETRS-TM35FIN to use for country wide displays. So, what is the difference between EUREF-FIN CS and ETRS-TM35FIN?

In ArcGIS, I can find both of them, under different WKID code:

• 
  WKID: 3067: EUREF_FIN_TM35FIN


• 
  WKID: 5048: ETRS_1989_TM35FIN_NE

Checking using EPSG codes (https://epsg.io/), the same WKID codes represent the same CS name, but with switched Northing and Easting! (why would anyone do that?!)

• 
  EPSG: 3067: ETRS89 / TM35FIN(E,N)


• 
  EPSG: 5048: ETRS89 / TM35FIN(N,E)

Well, what is proper coordinate system to use in Finland? and how is it specified in ArcGIS/proj4 library?

As you found, EUREF-FIN is a realization of ETRS89 in Finland.

Some countries, when displaying coordinates or transmitting data, use the convention northing, easting order while other countries use easting, northing order. Most GIS software wants easting, northing so the EPSG registry (http://wwww.epsg-registry.org NOT epsg.io) often has both versions depending on what's been requested to add to the registry.

In ArcGIS, on the Coordinate Systems tab in ArcMap or the Coordinate System section in ArcGIS Pro, then browse the folders for Projected Coordinate Systems, National Grids, Finland.

As @Hornbydd said, you should set a geographic (datum) transformation, but this particular one has zeroes for the parameters so won't change the coordinate values.

I can't answer why they would flip N/E but if I had got data (e.g. OSM) in WGS84 and wanted to projected it in ArcMap into EUREF_FIN_TM35FIN then I would have set the transformation as shown below. Not sure my answer helps?