Analysis of Landsat NDVI time series for detecting degradation of vegetation

Abstract

Calculation of vegetation indices, especially Normalized Difference Vegetation Index (NDVI), has become one of the most successful, popular and traditional attempts in biogeographical research methods, because NDVI has certain advantages over other vegetation indices or band combinations. The NDVI is less depending on soil properties of the research area as well as to the daytime illumination comparing to simple red-infrared bands combination [5]. NDVI is well adjusted specially for the analysis of vegetation properties, which can be indirectly interpreted from the objects colors, as shown on the raster image. However, the correlation of the results received from the satellite spectral signals may not necessarily precisely correlate with real data on biomass and vegetation quality which is caused by the local climatic settings and biogeochemical background of the research area. Methodology of this work consists in following. Producing vegetation indices has been done in this research using mathematic calculation of the channels: (NIR-VIS) / (NIR+VIS), or a ratio for channels: (Band4-Band3) / (Band4 +Band3). NDVI values lie in the range of 0 – 1 and never become negative or extend over 1, as NDVI is a linear algebraic function of these bands. The resulting image shows vegetation types in enhanced colors and sharpness, which can help to highlight most rapid changes while comparing two or more images taken in different time periods. Monitoring spatial distribution, presence and health conditions of the vegetation is traditionally being performed using calculation of the NDVI. To model the NDVI I used Map Calculation tool in command line of the Raster Operations menu in ILWIS GIS, which enables to perform spatial analysis and modeling by combination of queries, arithmetic expressions and overlays of selected raster images. The calculation of the NDVI index is read into the ILWIS spatial analysis menu using following equation: NDVI = (Band4 – Band3) / (Band4 + Band3), where Band 4 is DN values of spectral reflectance in NIR (near infra-red) and Band 3 is DN values of spectral reflectance in VIS. The results show decrease in overall NDVI values for the study area since 1988 to 2011, which can be caused by the environmental change and anthropogenic factors. The calculated NDVI indicated biomass and can be also used as indicator of “greenness” of the vegetation.