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A comparison of the National Inventory of Woodland and Trees (NIWT) and the National Forest Inventory (NFI)

Method NIWT NFI
Map Created once every 15 years from aerial photos as a ‘snapshot in time’. Some photography used for the last woodland map was dated (up to 17 years) due to the lack of availability of more recent imagery Created once and then continually updated using new photography (not older than 3–6 years) and information from grants and licenses data. Various other data sources will be used to improve map precision
Number of sample squares 35,000 – a 1% sample 15,000 – set by desired accuracy and funding limits
Survey design Clustered and stratified Systematic and random as per the statistical design
Sample Square type Temporary Permanent
Size of sample squares 1 hectare 1 hectare
Time taken to measure a sample square 1.5 squares per day 0.5–1 square per day (with more data obtained from each square)
Period over which taken 5 years 5 years
Reporting cycle 4–19 years, 19–27 years for NIWT1 (1924,1938,1947,1951,1965, 1980, 1999 (not all were full surveys) 5-yearly, with one or two interim reports for the samples and an annual or biennial map-based area report
Reporting levels GB, Country, Region, County GB, Country , Region for sample data, down to County for the map areas
Funding for plotting £780K per annum £540K year one, £440K per annum thereafter
Error budgeting employed to design the programme No (flat 1% sample employed in 1999, but some was employed in 1980) Yes
Error budgeting employed to design the programme No (flat 1% sample employed in 1999, but some was employed in 1980) Yes
Use of private sector data for aiding inventory None Yes (where available)
Use of private sector data for aiding building a management intent picture None Yes (where available)
Top-up option available to improve accuracy No Yes
Small woods survey Yes Yes