The coasts of Finland are fringed by vast and labyrinthine archipelagos, in which wooded islands with small hills obstruct visibility. This is a great hind-rance to topographical survey. On the other hand, conditions for carrying out hydrographic survey are excellent, as positions can be fixed on the basis of a rectangular lattice of lines run easily and accurately along the surface of the ice. There was in the past every encouragement to adopt this method of survey, as the coastal waters and the lakes in central Finland are ice-covered for some 5 months every year (from December to April) and in the northern parts of the country for an even longer period. It is true that the work is hindered by the cold, but in early spring (March and April) surveying can generally be carried out in satisfactory conditions.