The Atlas had two coverage targets: to receive distribution records from all parts of Britain and Ireland, and to achieve a minimum level of TTV coverage in all 10-km squares in Britain, and in every second 10-km square in Ireland. We were highly successful in meeting these targets.
Records were received from the majority of the 10-km squares with land in Britain & Ireland. 3,844 (98.7%) of the 3,894 10-km squares in Britain & Ireland containing any land were visited at least once in both winter and the breeding season. The 50 10-km squares with coverage in neither, or only one, season contained only small amounts of coast, offshore sandbars and rocks and they amounted to only 0.02% of the land area of Britain & Ireland. The species richness recorded in individual 10-km squares was comparable to that in previous atlases in the majority of cases, although the actual species pool may have differed subtly.
The minimum TTV coverage (8 tetrads surveyed per 10-km square) was met in over 99% of British 10-km squares and in over 97% of Irish chequerboard 10-km squares. The few 10-km squares that failed to meet the target were mostly coastal and most missed the target by only one or two tetrads. In 99% of 10-km squares, TTV coverage exceeded the minimum needed for analyses. The map shows every tetrad with TTV coverage in the breeding season, illustrating the near-complete coverage in many areas of southeast and northwest England, and southeast Scotland, where local atlases were underway. In Ireland the chequerboard can be seen clearly, which helped to ensure representative coverage in a systematic manner across the island.