# Validation of Regional-Scale Remote Sensing Products in China: From Site to Network
### Validation Guidelines and Technical Specifications  
#### 3.2.1 Validation Guidelines  
Validation guidelines and technical specifications are the cornerstones of the validation framework [161]. The latter focuses on detailed validation protocols towards each specific land surface RSPs of interest, as described in Section 3.2.2. In contrast, the former is a baseline document that provides general considerations and basic procedures/approaches used in validating various land surface RSPs, and the content is roughly shown as follows:  
* Validation criteria include accuracy, precision, completeness, temporal variation, spatial pattern, spatial and temporal consistency.
* Validation metrics, such as bias, mean absolute error (MAE), root mean square error (RMSE), mean absolute percent error (MAPE) and correlation coefficient R.
* Basic procedures and approaches used to validate either continuous or categorical RSPs are provided at the pixel scale level and at the product level, as illustrated in Figure 2.
* Basic procedures and approaches used to validate continuous RSPs at the pixel scale level. In this aspect, direct validation can capitalise on in situ measurements collected at the point scale and/or footprint scale (e.g., EC system). Because both the location and number of sampling points can directly affect the reliability of pixel scale representative estimates, ground sampling design is conducted based on variables characteristics with the aid of geostatistical methods, such as the mean of the surface with non-homogeneity (MSN) [162; 163]. In the upscaling process, ground truth at the pixel scale is derived through integrating in situ measurements, such as various kriging models. Then, the upscaled ground measurements can be used to compare against RSP.
* Basic procedures and approaches used to validate categorical land RSPs at the pixel scale level are similar to the ones validating continuous RSPs. Ground sampling design also plays an important role in this process. For instance, in direct validation, ground measurements acquired at sampling points can be used to compare against remote sensing estimates.
* Basic procedures and approaches used to validate land RSPs at the product level. The first is to select typical sites, pixels and time periods across different biomes and landscapes. Next, in temporal, product accuracy should be assessed via comparison against time series reference data at the pixel scale over each specific site. On this basis, in spatial, product accuracy should be further assessed over a widely distributed set of locations. Finally, overall evaluation of an RSP can be achieved by combining the assessed results acquired at both time and space. The spatial and temporal consistency and uncertainty of a product can then be well evaluated and quantified via independent measurements over wildly representative locations and time periods.
* General guidance on field site selection and corresponding ground instrumentation. This aspect suggests detailed criteria on how to choose or build a proper validation site and how to deploy various instruments to acquire high-quality ground measurements. Primarily, a site should be spatially representative of a given biome/ecosystem, considering national physiographic conditions. Basic observation facilities are also needed, including meteorology (e.g., precipitation and air temperature), eco-hydrorological elements (e.g., soil temperature and moisture) and flux measurements. Other selection principles should be considered, e.g., accessibility, site extent, ambient environment, and commitment to long-term scientific study.