# Remote Sensing Data for Digital Soil Mapping in French Research--A Review
## 3 Developments Related to the Extraction of Soil Properties from Remote Sensing Data
### Use of RS Data as a Substitute for Soil Properties' Measurement
The soil moisture retrieved by passive or active MW instruments corresponds to surface SM (1 to 5 cm in depth). Several studies have shown that root zone soil moisture (RZSM) can be inferred from time series of surface SM based on model assimilation or from simple temporal filtering methods [165]. Optical and thermal infrared (TIR) observations can also be used to map surface SM at high spatial resolution, but to date, the research is not sufficiently mature and no operational RS product exists [166; 45; 167; 168]. Surface temperature measured by TIR sensors is related to evapotranspiration and can be used to estimate RZSM [169; 170]. SM is an important parameter in itself, but it can also be used to estimate permanent soil properties. For example, temporal variations in SM can be used, through assimilation, to retrieve soil parameters such as soil hydraulic properties [134]. Soil roughness is an important characteristic that plays a key role in understanding/modeling different processes (e.g., slaking, runoff, infiltration, and erosion). It also has an important effect on the remote sensing signatures observed by satellites: it can be estimated because it is a parameter of interest in itself, or because its effects need to be corrected for a better estimation of other variables of interest (such as SM in the MW domain). Figure 1: Percentage of articles by soil properties published by French researchers (out of 2426 articles). This citation report graphic is derived from Clarivate Web of Science, Copyright Clarivate 2022. All rights reserved. We selected articles whose TITLE, TOPIC, KEYWORDS AND/OR ABSTRACT contained “REMOTE SENSING” AND “SOIL” and FRANCE in the KEYWORDS or TITLE or ABSTRACTS or at least in the address of an author. Note that we complemented this search with other expert-based searches, notably for authors who published mainly in French (not included in the WOS), or French researchers whose address was not in France at certain periods. Second, we manually excluded articles that were clearly out of the scope of our searches. Finally, we grouped together some similar keywords when their meaning was comparable. Soil roughness, or micro-topography, is an important influencing factor on the soil spectral response to a degree similar to that of \"chromophores\" [65], and is therefore a key parameter in modeling soil surface reflectance [56]. In addition, it is an indicator of the water infiltration potential, soil slaking, and erosion risk [98; 171]. Several methods have been designed to measure soil surface roughness. Their main disadvantages were the time-consuming and tedious nature of the measurement in the field. [PERSON] et al. [172] and later [PERSON] et al. [173] developed an easy-to-handle and fully automatic photogrammetric method to derive digital elevation models (DEMs) of the soil surface with millimeter resolution from multi-angular photographs captured in the field. In the active MW domain, soil roughness has a significant effect that must be corrected to estimate other parameters of interest, such as SM and VOD. In the passive MW domain, soil roughness contributes to increased MW emissions from the soil. It is related to both micro-scale (\(\sim\)1 cm) and macro-scale (\(\sim\)100 m) topographic effects. The first global map of the observed roughness parameter from SMOS was produced by [PERSON] et al. [174]. The new generation of freely available radar sensors (e.g., Sentinel-1 since 2015) allows the scientific community to collect images with a revisit time (\(<\)6 days) and spatial resolution (\(\sim\)10 m) suitable for hydrological and agronomic applications, on local or regional scales [164]. At these scales, soil roughness mapping is mainly performed on bare agricultural soils. The radar signal increases with the roughness (root mean surface height, RMSH) according to an exponential law and becomes constant after a certain roughness threshold.