The Finnish Uniform Coordinate System (in Finnish Yhtenäiskoordinaatisto, YKJ) has been used in biological observation mapping since the 1970s. Based on YKJ, Finland is divided in square-shaped areas, the size of which are determined according to the needs of the study. The area division used in national biomonitoring is 10 km x 10 km squares, but in some cases 1 km x 1 km and 100 m x 100 m YKJ squares are also used. This data set includes XY-lines that form square grid in four scales according to Unified Coordinate System (100 m - 100 km), with identifiers describing each square.