Table 1 Summary of the National Forest Inventories in Spain and Portugal

SNFI1

1965–1974

Estimation of forest areas using a systematic sampling grid, overlaid on aerial photographs (scale 1:30,000)

Stratified double sampling design with optimal allocation of plots; temporary plots

PTNFI1

North: panchromatic aerial photography from 1963 to 1965 (scale 1:15,000)

South: agricultural and forestry areas map

Stratified sampling strategy; temporary plots

PTNFI2

North: new aerial photography

South: new infrared aerial photography (scale 1:15,000)

Stratified sampling strategy: temporary plots

SNFI2

1986–1996

Forest areas and strata were identified from the existing agriculture and land use map.

Systematic 1-km × 1-km grid; permanent plots

PTNFI3

New aerial photography coverage of the country

For the regions analysis, a panchromatic aerial photography from 1986 was used.

Stratified sampling strategy; temporary plots

PTNFI4

Phase 1. 1990: aerial photography from 1990 on false colour infrared film to produce the land use cartography (scale 1:25,000). Phase 2. 1995: aerial photography production; 1996–1997: photo-interpretation and area evaluation

Stratified sampling strategy; temporary plots

SNFI3

1997–2007

Stratification was ‘a posteriori’. Land cover classification and forest area estimation were based on digital maps (scale 1:50000) and ortho-images.

Same systematic grid as SNFI2; permanent plots

PTNFI5

New aerial photography from 2004, 2005 and 2006 covering the whole country

Systematic 2 km × 2 km grid; permanent location plots

SNFI4

2008-ongoing

Stratification was ‘a posteriori’. Land cover classification and forest area estimation were based on new digital maps (scale 1:25000) and ortho-images.

Same systematic grid as SNFI3; permanent plots

PTNFI6

Digital ortho-photo maps of 1995, 2005 and 2010 (500 m × 500 m grid)

Same systematic grid as PTNFI5; permanent location plots