Definition: Sampling units

Category: Land Use/Land Cover

Area frame sampling basically deals with the division of land into pieces (primary sampling units) out of which a set of representative samples is taken. Three main types of sampling units can be distinguished: 

- Points: The point is the simplest sampling unit, represented as a dot in a map or an aerial photography. The point can exactly be located by its geographical co-ordinates.
- Lines: Lines can also be used as sample units to select features along a transect, represented as a line in a map or an aerial photograph.
- Quadrates or polygons: Quadrates or areas are commonly used by ecologists in the field and they are of particular interest for geographical sampling of vegetation. In contrast to points or lines, quadrates are difficult to handle because the exact location of the borders may be difficult to recognise. Aerial photographs are used to overcome this problem. With regards to the shape various approaches can be found, having all their specific advantages and disadvantages: regular square or irregular, with or without recognisable physical boundaries etc.

The selected set of samples are surveyed, i.e. the variables of interest (area acreage of crops or land cover/land use) information are gathered and finally projected for the whole territory or reference unit.
Eurostat, "Manual of concepts on land cover and land use information systems (2000 Edition)", Office for Official Publications of the European Communities, Luxembourg, 2001