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Definition: Minimum legible delineation

Land Use/Land Cover

The Minimum Legible Delineation (MLD) of a map is the minimum legible size of a line or a polygon on a paper map. A black line should be 0.05 mm thick, a coloured line 0.08-0.1 mm. Concerning the spatial dimensions of polygons on the map the width should be 0.3 mm, for coloured areas 1 mm². 

Thickness of a black line:    0.05 mm
Thickness of a coloured line: 0.08 - 0,1 mm
Distance between lines:  0.25 mm 
Dimensions of polygons  0.3 mm
Space in between filled polygons: 0.2 mm

Depending on the scale of the map, this unit represents variable surfaces in reality: at a scale of 1:1.000 the smallest surface to be represented is 1 m² (1 mm * 1 mm on the map), at a scale of 1:100.000 the same surface on the map represents 10.000 m² (=1ha). That means that precision and accuracy of terrestrial measurement of centimetres will be blurred if represented at a scale of 1:100.000 on a paper map: 1 mm on the map corresponds to 100 metres in reality. The same concept applies when surveying in the field: a line of a piece of paper represents a certain area depending on the scale of the map to be drawn. 
Sometimes the MLD is confused with 'Mapping Unit' of a map.
Eurostat, "Manual of concepts on land cover and land use information systems (2000 Edition)", Office for Official Publications of the European Communities, Luxembourg, 2001