Between-observer variation in the application of a standard method of habitat mapping by environmental consultants in the UK.
Abstract
Six ecologists surveyed independently the same upland site (including coniferous woodland, deciduous woodland and rough grazing) in northern England. In pairwise comparisons between maps, spatial agreement was found to average 25.6% (with a range of 17.3-38.8%) of the area of the study site. The number of land cover types that were identified ranged from 13 to 21. Four or more surveyors agreed on the classification of 19% of the study site, while the area of land upon which all six agreed was only 7.9% of the study site. Spatial errors in the positioning of habitat boundaries occurred, but were a relatively minor source of the differences between maps. The majority of differences between maps were due to classification errors. Land cover types with similar species compositions were most frequently confused. Spatially referenced field 'target notes' giving additional information on the vegetation mapped in each survey varied in number between 18 and 56. The contents of target notes were inadequate to allow a retrospective assessment of mapping decisions. The total numbers of species listed in target notes varied between surveys from 25 to 145. Sorenson's similarity for species lists derived from pairs of surveys ranged from 18.8% to 63.7%, and was not related to spatial agreement between surveys. Time spent at the field site was not a correlate of any aspect of the results or cost of the survey. Three surveys conducted by members of a professional institute for ecologists were the most expensive, and also recorded larger numbers of target notes and species than the other surveys. However, their maps were no more similar than other pairs of maps. Analysis of the survey results and comparisons with other methods of vegetation mapping suggested that mapping precision could be increased by placing a greater emphasis on use of aerial photographs and other extant map data prior to (and during) field work; by making greater provision for mapping of mosaics and increasing the level of floristic information in habitat definitions; by recording a greater number of more detailed target notes in the field; and by providing office-based support to assist in the interpretation of aerial photographs, and the cross-checking of field surveyors' preliminary classifications against the contents of target notes and habitat definitions. It is considered that the current application of the Phase 1 approach by environmental consultants places too great a reliance on decision-making by the (frequently) unsupported lone surveyor whilst in the field.