# Validation of Regional-Scale Remote Sensing Products in China: From Site to Network
## 2 Validation Strategy  
As mentioned earlier, validation is not a straightforward task. In contrast, in addition to inherent heterogeneity of land surfaces, validation is generally recognised as a challenging issue because RSPs may not be spatially and/or temporally consistent. In addition, as is well known, validation commonly comprises several key components, ranging from in situ measurements collection, modelling, and retrieval of land surface variables to scale-related analyses [15]. All these factors complicate the validation issue, calling for a better understanding of validation practice, including quantitative evaluation of uncertainties within the validation process. Against this background, in 2011, the Ministry of Science and Technology of the People's Republic of China (MOST) launched an initiative focused on the development and validation of regional-scale RSPs for terrestrial hydrological and ecological applications. The core mission of this programme is to derive long-term regional-scale land RSPs (listed in Table 1) and to investigate the validity of these developed products based on integrated airborne, satellite-based and ground-based measurements. Under this circumstance, validation-related theories, methods, evaluation scores, and scale effects can therefore be substantially studied. Based on previous research achievements (e.g., [19; 20; 41; 42; 43]), an overall validation strategy (Figure 1) was identified. The VS aims at validating regional-scale RSPs with systematic site-to-network concept consisting of (1) general validation guidelines and technical specifications; (2) site-based field campaigns and intensive observations; (3) a national validation network; and (4) an operational validation system. (1) As shown in Figure 1, validation guidelines and technical specifications constitute the first component of the VS, which can be regarded as a top-layer design to drive the whole VS. The validation guidelines provide universal principles and methodologies that can be applied to steer validation activities for diverse land RSPs, in which sampling design and scaling methods are stressed. In contrast, validation technical specifications are a set of dedicated implementation protocols towards 16 RSPs/variables of interest (as shown in Table 1). Each individual variable has a unique technical specification. Pertinent details about validation guidelines and technical specifications will be given in Section 3.2. It is also worth mentioning that in this component, validation activity is essentially dependent on a priori knowledge of the reference site, such as its meteorology, topography, plant type, agricultural activity, and other site parameters. Then, we can better integrate and utilise diverse observations to further develop sampling design and scale methods. In this case, observations are mainly dependent on ground measurements acquired at conventional point scale or from a source of wireless sensor network (WSN) and footprint dimension as well as airborne remotely sensed imagery with high resolution (on the order of sub-meters and/or meters). Some research efforts related to this component will be highlighted in Section 3.3. (2) The validity of proposed guidelines and specifications needs to be experimentally evaluated. Therefore, Component 2, i.e., field campaigns and intensive observations, is subsequently proposed in the VS. As depicted in Figure 1, activities in this component are to validate RSPs over well-established observation fields/sites, which links theories and methodologies to real-world validation exercises. On the one hand, in situ measurements (e.g., WSN) can be deployed based on a sampling scheme proposed in Component 1 to collect ground data. On the other hand, upscaling these measurements to the pixel scale value also relies on scaling approaches.