## 1 Introduction  
In recent years, the increase in global grain production has reduced the proportion of hungry people in the world, which has increased the overall level of global food security. However, with the continued growth of global population and future consumption, global food security continues to face major challenges [1, 2]. The expansion of cultivated land has become one of the main human activities to surpass the planetary boundary [3, 4]. The yield growth rate per unit of major global cereals (wheat, corn, rice and soybean) is still lower than those required to achieve the food security goal [5; 6]. Therefore, it is considered that the first priority is to produce more food on the existing cultivated land by increasing the cropland productivity [7] and narrowing the gap in agricultural production, rather than expanding the scale of cultivated land [8; 9]. According to the China Economic Bulletin (2014) published by the World Bank, China's demand for grain is expected to be 700 million tons by 2030, when the population reaches its peak. According to the level of grain production in 2019 (668 million tons), grain output should increase by about 32 million tons in 2030. To ensure food security, China has set a red line of 1.8 billion mu (100 mu \(\approx\) 6.67 hectares) of cultivated land and ensured stability of the total area of cultivated land in the process of urbanization through a policy of balance between occupation and compensation of cultivated land [10; 11]. However, due to the lack of reserve cultivated land resources in China, it is difficult to supplement the good quality cultivated land in urbanization. In areas with unfavorable natural conditions, the cropland productivity occupied by urban expansion is 80% higher than that of newly added cultivated land [12]. Recent research shows that from 1990 to 2015, the redistribution of cultivated land \"occupying the good quality cultivated land and compensating the low quality cultivated land\" stabilized the total cultivated land area in China to 2-2.1 billion mu. However, the main area of cultivated land shifted by about 70 km to the northwest region, resulting in a 4.5% reduction in total grain production capacity [13]. The middle-low productive cultivated land in China makes up about 80% of the total cultivated land area [14]. Improving cropland productivity, rather than expanding marginal land, is key to ensuring food security and an important way to achieve the sustainable development of China's agriculture. Since 2008, the Chinese government has attached great importance to improving the cropland productivity with the transformation of middle-low productive farmland (MLPF) and the construction of high-standard farmland (HSF) as the main activities. The construction goal is to form basic farmland with supporting facilities, high and stable yields and strong disaster resistance through land regulation and construction, improving or eliminating the main limiting factors of grain production and breaking the bottleneck in grain production. According to the National High-Standard Farmland Construction Plan (2021-2030), China will build a total of 1.2 billion mu of high-standard farmland by 2030 [15]. China's MLPF transformation and HSF construction projects were officially implemented in 2010, and from 2011 to 2014, 2,330 project regions were built. However, it is not yet known whether the MLPF and HSF project regions have reached the expected grain production capacity and whether the construction of HSF has effectively achieved a stable and balanced yield increase. Therefore, the application of remote sensing technology for comprehensive monitoring and the assessment of capacity improvements of each project region has become a necessary measure to master food security. With the development of remote sensing technology, remote sensing images are widely used in the estimation and monitoring of cultivated land productivity.