Europe, 1972-1995. Data collection periods vary per country but all lie in the mentioned period. Hundreds of thousands, perhaps more than a million, hours of fieldwork are the basis of this Atlas.
In some countries, fieldwork for this Atlas and their national atlas was combined; in others data from an independent national atlas project were used to provide dfata for this Atlas. Methods used in the field ranged from single visits to systematic surveys based on point- or line-transects or territory mapping on plots.
It was planned that Atlas fieldwork should take place during 1985-1988. Nations organized their own fieldwork, however, and because of practical difficulties could not all meet that timescale.
Where coverage was incomplete for the survey period, earlier or later data were evaluated to determine whether coverage or quality could be improved by including them.
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