Pasture Land Cover in Eastern Australia from NOAA-AVHRR NDVI and Classified Landsat TM

Abstract

A method for constructing a pasture land cover classification for the high rainfall zone of eastern Australia from the U.S. National Oceanic and Atmospheric Administration’s advanced very high resolution radiometer normalized difference vegetation index (NOAA-AVHRR NDVI) data is described. The method uses an established classification of pasture growth potential from single date, springtime, Landsat thematic mapper (TM) data to provide measures of subpixel mixture composition in mosaicked classes in the absence of ground truth or control sites. A sequence of AVHRR data from 1993 was transformed into a vegetation index and then classified to define pasture classes with differing patterns of NDVI. High-resolution classifications of pasture were constructed for ten selected sites within the study area by using Landsat TM scenes. The study area was split into a northern and southern zone on the basis of the temporal pattern of moisture indices. The pasture land cover classes were described in terms of the shape of the NDVI profiles, their geographical location, and the subpixel composition from Landsat TM data. The classified NDVI data were combined with local government area (LGA) boundary data to allow the particular pasture state of each LGA to be estimated. The NDVI–Landsat TM procedure identified 21 and 22 classes in the northern and southern zones, respectively. These classes could be broadly grouped into eight types: sown perennial pastures, sown perennial pastures with woodland, sown annual pastures (southern zone only), mixed pasture and cropping, native pastures, native pastures with woodland, degraded or revegetated areas, and forest. This eight-class classification combining the two zones appeared to represent regional distribution of the major types quite well. The pasture land cover classification was evaluated for selected LGAs by using agricultural statistics and a specialist pasture survey. Local estimates of proportions of major pasture types were sometimes inaccurate owing to difficulties in distinguishing between perennial, annual, and native types where seasonal conditions caused rapid senescence or where open woodland confused profiles between improved and native pastures. The method is nevertheless useful where ground truth or definitive spectral signatures for cover types are unavailable and where description in terms of an average or predominant cover type within a landscape mosaic is acceptable.