# Validation of Regional-Scale Remote Sensing Products in China: From Site to Network
## 6 Conclusions  
In this study, we introduce an overall validation strategy with a site-to-network concept. The VS comprises four main components: (1) general guidelines and technical specifications that can guide users to validate various land RSPs; (2) substantial validation activities to experimentally examine and improve the first Component; (3) a national validation network to allow comprehensive assessment of RSPs from site or regional scales to the national scale across various zones; and (4) an RSP evaluation system to implement operational validation applications. The proposed VS systematically focuses on various aspects of validation issues, including its measures, approaches, and real-world application. Validation guidelines and a series of technical specifications are proposed. The former provides general conditions and universal methodologies to validate either continuous or categorical RSPs both at the pixel scale level and at the product level. Validation technical specifications focus on validation protocols towards 16 land surface RSPs of interest. Research outcomes related to ground sampling design and scaling method demonstrated that these investigations have been beneficial in quantitatively calculating various statistical indicators for evaluating RSPs, optimising the spatial layout of sampling points, and obtaining unbiased and representative aggregated ground truth data at the pixel scale. Substantial intensive observations and field campaigns performed from site to local to regional scales greatly supported validation activities. The HiWATER experiment occurred in the HRB, as well as other experiments conducted in China, these studies successfully collected high-quality datasets at multiple scales using WSNs, field measurements with footprint dimensions, airborne remote sensing, and other intensive in situ measurements. The utilised sampling schemes and deployment of ground instrumentation, etc., are compliant with the first component of the VS. In this context, considerable research results have already been yielded, including validation efforts addressed soil moisture, LST, ET and vegetation index, etc. In addition, within this strategy, we are coordinating a national-scale validation network in China. The prototype of this network has been established; 12 anchor stations over seven representative regions were chosen as basic observatories. Over the past few years, various ground-based remote sensing experiments and intensive field campaigns were individually and/or jointly conducted at four core sites, in order to investigate remote sensing and validation mechanisms and theories. To facilitate the operational validation task, a web-based validation system (LAPVAS) is designed to use multi-scale in situ and remote sensing data for validating quantitative RSPs at a regional or global scale. This system is built with assembled modules and contains a validation database subsystem, an RSP accuracy evaluation subsystem, and an inter-external interface. Research progress related to each component of the proposed VS is rewarding, as it improves our understanding of challenging issues in validation, especially in answering the question of how to acquire reliable ground truth at the pixel scale over heterogeneous land surfaces for validating RSPs. Future research should first further explore the validation methodologies, including validation metrics, the quantitative assessment of land heterogeneity, and uncertainty occurring during the validation process. Great importance could be attached to scaling approaches, which could facilitate ground truth acquisition at the pixel scale and provide the essential information required for remote sensing retrieval and validation. Second, more reference sites could be included in the validation network in order to improve the network to be more applicable for large-scale validation practice.