Provincial agricultural stratification method for crop area estimation by remote sensing

Abstract: In current studies on estimation of crop area using remote sensing on a stratified sampling, researchers tend to use the classified images directly as stratification index and do not consider the impacts of classification errors coming from the stratification procedure. In this study, we tried to improve the efficiency and accuracy of crop area estimation at provincial level for solving this problem. Jiangsu Province was chosen as study area, and the crop area was estimated by a stratified sampling. We developed two steps to make stratification. Firstly, taking account the categorization of terrestrial geomorphology and the situation of study area, standard deviation of DEM was used as a stratification index to categorize the whole study area into two different layers. The study area where its standard deviation of DEM is lower than 10 m is classified as flat terrain layer, while the others are classified as alpine terrain layer. Secondly, in each first layer we further stratified it into sub-layer (i.e. classification) by using 4 different indices individually, including crop scale, crop structure, cropland fragmentation, combination of crop structure and fragmentation. The stratification results showed that there were some problems to identify wheat area with directly using remote sensing method. There are low planting structure and fragmented planting areas located in large scale crop layer due to the effect of plant spatial distribution referred to various crops in the same growing period. In order to eliminate the problem of confusion caused by crops in the same growing period, the index of crop structure was introduced in this study. For the purpose of quantitative analysis of the heterogeneity of plant spatial distribution arisen by mixed pixels, the index of cropland fragmentation was introduced. In order to analyze the effect of stratification by different indexes, we calculated the coefficient of relative stratification efficiency, which was defined as the ratio of variance of random sampling to variance of a given stratification. The results showed that the relative efficiencies of above four stratified methods were better than unstratified simple random sampling. Among the single index, the relative stratification efficiency achieved the highest value of 4.96 by using the crop structure. As a whole, the relative stratification efficiency was the highest by using the combination of crop structure and cropland fragmentation as a stratification index. This index can reflect not only the fragmental characteristics of cropland but also the difference among different regions in our study area; therefore, it contributes to the improvement of the accuracy and efficiency of crop area estimation at provincial level.