The definition of lakes should, in case of doubt, include permanent water surfaces. Bogs, marshes, mires and other wetlands constitute a separate category which is classified as land area. The inland water might be salt or not depending on the existence of an outflow. In coastal areas special cases may occur. There should normally be a difference in elevation between the sea and any waters classified as inland waters. This difference can be small (measured in decimetres) but normally the water flow in any outflow should be in one direction only, towards the sea. Where the tide fluctuations are high, inland waters should be defined as separate from the sea only if they are separate (the inland water having higher elevation) at mean high tide (cf. coastline definition below).
Eurostat, "Empfehlungen zur Festlegung eines einheitlichen Verfahrens für die Berechnung der Fläche von Gebietseinheiten - Ausgabe 1999; Recommendations for a harmonised definition of calculation of surface area of territorial units, 1999 edition; Recommandations pour une définition harmonisée du calcul de la superficie des unités territoriales - Édition 1999", Office for Official Publications of the European Communities, Luxembourg, 1999, p.14