Mapping Woody Vegetation Cover over the State of Queensland using Landsat TM Imagery

The Statewide Landcover and Trees Study (SLATS) is mapping woody vegetation cover as it existed in 1991. The analysis is being carried out for 88 Landsat Thematic Mapper (TM) scenes covering the entire state of Queensland. This paper presents the methods being used by SLATS for this mapping. Radiometric corrections are applied to the imagery to ensure matching of adjacent scenes and to enable the use of multi-temporal data. The imagery is also geometrically corrected using ground control points measured by a Differential Global Positioning System (DGPS). Field sample sites are established and vegetation cover measurements of basal area and foliage projective cover (FPC) are taken for woody vegetation. The positions of these sites are also measured with DGPS. A function relating average overstorey and shrub FPC to tree basal area has been established for mature vegetation stands. FPC has been found to correlate with Landsat TM Band 5 (TM5, middle-infrared) and the normalised difference vegetation index (NDVI). The relationship is based on the observed increase in NDVI and decrease in TM5 with increasing woody vegetation cover. Field measurements of tree basal area and FPC are used to optimise the coefficients of an equation which estimates perennial woody FPC from NDVI and TM5. Tree basal areas are then calculated for mature woody vegetation stands. The 1991 imagery alone is used to delineate woody vegetation, while multi-temporal data is used to estimate woody vegetation cover and tree basal area in mha for the 1991 woody areas. Initial validation of tree basal area using 221 independent field measurements shows a modelling efficiency (EF) of 0.71 and average deviation of 3 mha. Introduction The Statewide Landcover and Trees Study (SLATS) was initiated by the Queensland Government to provide factual information on land cover and trends in land clearing, tree growth and regrowth. This data is important for use in policy and community planning decisions regarding sustainable land management and the National Greenhouse Gas Inventory (Danaher et al., 1998). Landsat TM satellite imagery for 1988, 1991, 1995 and 1997 are being analysed to detect change in tree cover across the entire State and provide baseline 1991 landcover data. The vegetation and land cover information produced will provide information on the extent and trend of woody vegetation in Queensland. The estimation of tree cover aids the assessment of impacts of tree clearing and woodland regrowth on Queensland’s greenhouse gas emission balance. Woody vegetation cover estimates can be combined with vegetation community mapping for improved woodland and forest inventory across the State. Change in woody vegetation cover is being calculated between the years 1988-91, 1991-95 and 1995-97. The SLATS methodology for detecting woody vegetation change has been documented elsewhere (Paudyal et al., 1997). Where possible, dry season imagery (winter & spring dates) were acquired to maximise the spectral separability of tree canopy and pasture. For the same reason, the baseline landcover mapping is based on 1991 imagery (rather than the National Greenhouse Gas Inventory baseline year of 1990), because 1991 was a drier year than average in Queensland. The SLATS landcover mapping involves several stages of automated and semi-automated image classification techniques, together with visual interpretation to delineate water, shadow, cropped areas (Kuhnell and Danaher, 1996), urban centres, bare areas, pasture and woodland. This paper describes the remote sensing methods used to map woody vegetation and estimate tree foliage cover within those wooded areas.