Crown closure estimation of oak savannah in a dry season with Landsat TM imagery: comparison of various indices through correlation analysis

In this paper, we assess the capability of Landsat Thematic Mapper (TM) for oakwood crown closure estimation in Tulare County, California. Measurements made from orthorectified aerial photographs for the same area were used as a reference. The linear relationship between crown closure and digital values of each band of the TM image was examined. TM Band 3 had the highest correlation (ρ = −0.828; R² = 0.687) with crown closure measurements. The simple ratio (SR) and the normalized difference vegetation index (NDVI) were generated for correlation analysis and only NDVI showed better correlation (ρ = 0.836; R² = 0.699) than use of single bands. An additional index (NIR^N-R^N)/(NIR^N + R^N), called NDVIN, was experimented, NDVISQ (N = 2) and NDVICUB (N = 3) showed some improvements over SR and NDVI (ρ = 0.855; R² = 0.732 for N = 3). Through multiple regression with all six bands, we found that there was a considerable amount of improvement in variability explanation over any individual band or index tested (R² = 0.803). NIR, red and blue bands were able to adequately model crown closure as using all the six TM bands (R² = 0.802). Principal component analysis (PCA) and Kauth-Thomas (K-T) transform were applied to reduce multi-collinearity among bands. The third principal component and greenness in K-T transform showed similar effects to those of NDVI. Transformation of digital numbers (DNs) to radiances kept the results of single band and multiple band estimation the same, and did not improve the index estimation very much. A simple radiometric correction of the TM image improved results for the NDVI (ρ = 0.840; R² = 0.705) and NDVISQ estimation (ρ = 0.861; R² = 0.741), but worsened estimation results of single band and multiple bands.