COUNTRYSIDE SURVEY FROM GROUND AND SPACE - DIFFERENT PERSPECTIVES, COMPLEMENTARY RESULTS

There is a strong demand for maps and data on the state of the country side to support regional and national studies. 'Countryside Survey 199 0' is a national survey of the state of the rural environment in Brita in. If combines detailed field surveys of a stratified random sample o f 508 1-km squares with a generalized census, based on satellite remot e sensing using Landsat Thematic Mapper, to compile a unique record of the ecology, land cover and land use in Britain. Mapping any landscap e into discrete classes, though routinely practised, produces results that are both artificial and simplistic. It follows that the collectio n of so-called 'ground truth' data, as the single standard by which su rveys can be judged, is in itself an elusive aim. This paper therefore compares and contrasts the methods and results of the two surveys, at tempts to explain differences and examines the consequences for use of the data. There were clear differences between the two surveys in def initions of similarly named classes reflecting, particularly, differen t perceptions of land cover and land use. Landscape patterns, map scal es and survey resolutions had complex inter-relationships, such that t he 1:10 000 field cartography did not readily compare with the 25 m ra ster format of the satellite map. Spatial generalization, an artificia l but inherent characteristic of conventional cartography, affected th e field survey. The satellite imagery imposed an equally artificial 25 m grid on outputs. Geometric displacements were confounding factors, wrongly suggesting thematic errors. Other differences emphasized the d ifficulties of subdividing a continuously variable landscape. Spectral mis-classification caused most of the errors in the Land Cover Map, o ften due to enforced compromises over the date of image acquisition. H owever; field surveyors also had difficulties in classifying landscape features consistently Although evidence suggests that it is impossibl e to determine absolute 'accuracy: if has been possible to apportion e rrors to the surveys. Results suggest that the Land Cover Map is perha ps 79-84% 'accurate' with the field records about 90% 'correct'. Natio nal statistics derived from the map are based on a census, albeit inco rporating these inaccuracies. Although individual field-based observat ions may be intrinsically more accurate, national predictions are base d on statistical extrapolation that introduces errors dependent on the sample variances. Nevertheless, the accuracies achieved from both app roaches are sufficient for the maps and data to have been used very wi dely for taking stock of environmental resources, measuring change, un derstanding environmental processes and predicting and managing enviro nmental impacts. (C) 1998 Academic Press.