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Validation of GIS layers in the EU: getting adapted to available reference data

An optimal validation of a thematic map would ideally require in-situ observations of a large sample of units specifically conceived for the map under validation. This is often not possible due to budget limitations. The alternative can be using photo-interpretation of high or very high resolution images instead of in-situ observations or using available data sets that do not fully comply with the ideal characteristics: unit size, reference date or sampling plan. This paper illustrates some examples of use of available data in the European Union. For land cover maps, the best existing data set is probably Land Use/Cover Area-frame Survey (LUCAS) that has been conducted by Eurostat on four occasions since 2001. Because LUCAS is based on systematic sampling, advantages and limitations of systematic sampling are discussed. A fine-scale population density map is presented as an example of a situation in which reference data on a statistical sample cannot be collected.