Is this
spTransform(Finland3, CRS("+proj=longlat +ellps=WGS84 +no_defs+ellps=WGS84 +towgs84=0,0,0 "))
the right way to project a shapefile with this projection
coord. ref. : +proj=merc +lon_0=0 +lat_ts=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0
to plot it in ggmap on top of a Google map raster?
therefore looking like
+proj=longlat +ellps=WGS84 +towgs84=0,0,0
Why not just use the gmap() functionality offered by the dismo package instead of ggmap? You can download Google maps of type 'satellite', 'terrain', 'roadmap' and 'hybrid' there as well. The advantage is that the thus aquired maps are instantly available as objects of class 'RasterLayer' or 'RasterStack', holding a defined spatial information.
Setting gmap(..., lonlat = TRUE), you can then use
spTransform(Finland3, CRS(proj4string(map)))
as mentioned above to reproject your Finland shapefile to EPSG:4326 and combine the resulting data sets in one plot.
KKJ / Finland Uniform Coordinate System <2393> +proj=tmerc +lat_0=0 +lon_0=27 +k=1 +x_0=3500000 +y_0=0 +ellps=intl +towgs84=-96.062,-82.428,-121.753,4.801,0.345,-1.376,1.496 +units=m +no_defs <>
Commented
Jul 1, 2014 at 14:22
As suggested by @user30184 the kapsi repository had what I needed, it can be found at this link