Monitoring the plant density of Cotton with remotely sensed data Where Na was PDC per hectare(x10'plantl hm).n (i=1,2,3)was the number of plants in the sampling unit. 2.3 Remote Sensing Images and Pre-processing LANDSAT-5/TM images in April 22.May 24h June 9,June 25 and July 11h 2007 were he geo-locat romati wave as the asic imag transferring mo del uses 6s model and the erosol ical was done u The atmospheric radia was inversed by the algorithm of dense dark vegetation.The pixel size was 28.5mx28.5m. 2.4 Vegetation Index EVI= P+C,x P-C+LXG Paur Pred (2 sw心 e:L is the C:were the canopy ba gain facto 110C60.C7 To further explore the suitable remotely sensed parameters.DEVI is the difference of EVI between the growth stage and the bared soil stage: DEVI=EVI-EVI 3 2.5 The Evaluation of Estimation Accuracy The Relative Extremenum Predication Error(REPE)is used to evaluate the estimation accuracy of PDC,REPE is calculated as RMSE REPE No max-No min g (N.p-N. RMSE (5) n-1 NN is the maximum and minimum value in the testing models,respec- tively:Np,Nt is the estimation and measured value,respectively. 3 Results 3.1 PDC Estimation with EVI PDC estimation baMonitoring the Plant Density of Cotton with Remotely Sensed Data 93 Where Nu was PDC per hectare(×105 plant hm-2), i n (i = 1, 2, 3) was the number of plants in the sampling unit. 2.3 Remote Sensing Images and Pre-processing LANDSAT-5/TM images in April 22th, May 24th, June 9th, June 25th, and July 11th, 2007 were used. The geo-location was done by ENVI4.2 with the geo-referenced panchromatic waveband image from LANDSAT-7 as the basic image. The atmos￾pheric correction was done using the AgRSIS software. The atmospheric radiation transferring model uses 6S model and the aerosol optical thickness was inversed by the algorithm of dense dark vegetation. The pixel size was 28.5m×28.5m. 2.4 Vegetation Index 1 2 nir red nir red blue EVI G CC L ρ ρ ρρ ρ − = × +× − × + (2) Where ρnir, ρred, ρbule was reflectance in near-infrared (NIR), red and blue waveband respectively. C1 and C2 were the correction coefficients of atmospheric resistance in red and blue; L is the canopy background brightness correction factor and G is the gain factor. The coefficients adopted in the EVI algorithm are, L=1.0, C1=6.0, C2=7.5 and G (gain factor) =2.5 (Huete et al., 1994; Huete et al., 1997). To further explore the suitable remotely sensed parameters, DEVI is the difference of EVI between the growth stage and the bared soil stage: DEVI EVI EVI = −i e (3) 2.5 The Evaluation of Estimation Accuracy The Relative Extremenum Predication Error (REPE) is used to evaluate the estimation accuracy of PDC, REPE is calculated as max min U U RMSE REPE N N = − (4) 2 1 ( ) 1 n u u i N p Nt RMSE n = − = − ∑ " (5) Nu max, Nu min is the maximum and minimum value in the testing models, respec￾tively; Nup, Nut is the estimation and measured value, respectively. 3 Results 3.1 PDC Estimation with EVI PDC estimation based on the data from three emergence stages. As shown in Fig.2, from May 24th to July 11th, EVI rose with the cotton growing. The average EVI