# Validation of Regional-Scale Remote Sensing Products in China: From Site to Network
## 1 Introduction
Second, although ground instrumentation is typically assumed to deliver reliable direct measurements on small scales, whether in situ measurements have reasonable spatial representativeness remains questionable. Biased estimation of ground truth at the pixel scale may be obtained due to inherent heterogeneity and the mismatch of scales between ground measurements and satellite footprint(e.g., [40]). This situation can notably worsen if point-based measurement is compared against RSPs with coarse resolution, particularly over extreme environments, e.g., cold and arid regions, where in situ measurements are sparsely distributed. Even when point-scale ground measurements are aggregated, the yielded pixel scale value may still not be reliable. This can be attributed to a lack of dense or better-distributed ground sampling, and, moreover, to deficiency in terms of scaling methods that are unable to account for the various characteristics of different RSPs and land surface heterogeneity [39; 41]. As a result, this clearly limits the ability of obtaining trustworthy estimates of variables with strong spatial-temporal variability at the satellite pixel scale and often impedes the performance of direct validation between in situ measurements and RSPs. The abovementioned research suggests that a framework to develop, design, and perform reasonable validation schemes and activities to acquire ground truth at the pixel scale over heterogeneous land surfaces is urgently needed. We must first ask, how do we integrate and get the utmost out of various ground observations collected at multiple scales to validate different RSPs, especially for those variables with strong heterogeneity? Second, can scale approaches be further developed to aggregate point-scale ground measurements to the pixel scale, and can such approaches improve spatial and temporal representativeness of heterogeneous land surfaces? These research questions are not yet fully understood. With the support of a Chinese national validation initiative, these questions, together with validation-related theories, methods, metrics, and scale effects, are intensively investigated. The objective of this study is to present our research progress on the development of a validation strategy (VS) and some representative validation activities. The VS is composed of four main components, namely, general guidelines and technical specifications, site-based validation activities, a prototype of a Chinese national validation network, and an operational RSP validation system. Validation activities are conducted over reference sites dependent on multi-scale observations collected from multi-platform and multi-source experiments, such as HiWATER [31]. The remainder of the paper is organised as follows: Section 2 provides an introduction to the overall VS, which can be viewed as a general roadmap for the validation efforts we conceived and performed. In Section 3, the validation guidelines and technical specifications are elaborated in association with pertinent improvements in sampling design and scaling methods. Section 4 describes experimental validation exercises that have been undertaken over reference sites. Section 5 illustrates a networking validation attempt in China and an operational validation system before providing conclusions in the last section.