# 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  
In many cases, RS data are directly used as a measure of soil properties, which [PERSON] et al. [109] called the \"primary soil data source\". Bare soil reflectance, for example, has been correlated with many topsoil properties measured in the lab. A model or a spectral index is built to predict certain properties, a given behavior, or a threat to the soil from its spectral reflectance at one or more wavelengths. Thus, many attempts have been made to determine soil moisture [121], soil carbon [122], soil clay content [123; 124], or soil salinity [125]. In many cases, therefore, RS is not incorporated into a full SCORPAN model, but directly used as a predictor of the soil property of interest through generalized linear models, partial least squares regression (PLSR), geostatistics, machine learning, or other prediction tools (e.g., [126; 127; 128; 129]). In this case, uncertainties and biases in the predictions should be noted, and care should be taken not to apply them outside their domain of validity. If the results are satisfactory, i.e., if RS data capture most of the variability of the soil attribute in a given area, it may not be necessary to incorporate them into a SCORPAN model (e.g., [123; 124; 130; 131; 132]). Note that these considerations do not claim that working on the field and taking samples are not essential. Working on the field is a crucial step in DSM and RS for several reasons:  
* Lab measurements performed on samples taken into the field are essential to calibrate RS and DSM predictions, and they must be representative from both feature and spatial domains (e.g., [12; 126; 127; 133; 134; 135; 136; 137]). * Using more high-resolution spatial covariates does not necessarily improve the prediction accuracy (e.g., [12; 136; 139; 132]). * Validation should involve appropriate sampling in the field (e.g., [127; 128; 129; 126; 127; 131; 132; 133; 137]). * The lack of field data is often one of the main limiting factors of RS and DSM prediction performances (e.g., [138; 139; 140; 141; 142; 143; 21]). * Soil knowledge acquired in the field is useful to improve and assess the RS and DSM prediction performances (e.g., [17; 129; 139; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178; 179; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 188; 189; 190; 191; 192; 193; 194; 195; 196; 197; 198; 199; 200; 201; 202; 203; 204; 205; 206; 207; 208; 209; 210; 211; 212; 213; 214; 215; 216; 217; 218; 219; 220; 221; 222; 223; 224; 225; 226; 227; 228; 229; 230; 231; 232; 233; 234; 235; 236; 237; 238; 239; 240; 241; 242; 243; 244; 245; 246; 247; 248; 249; 250; 251; 252; 253; 254; 255; 256; 257; 258; 259; 260; 252; 257; 259; 261; 262; 263; 264; 265; 266; 267; 268; 269; 270; 271; 272; 273; 274; 275; 276; 277; 278; 279; 280; 281; 282; 283; 284; 285; 286; 287; 288; 289; 290; 287; 288; 289; 289; 281; 285; 286; 287; 288; 289; 282; 289; 291; 288; 289; 280; 282; 283; 285; 287; 289; 286; 288; 287; 288; 289; 289; 292; 293; 294; 295; 296; 297; 298; 299; 300; 301; 302; 303; 304; 305; 306; 307; 308; 309; 310; 311; 312; 313; 314; 315; 316; 317; 317; 318; 319; 320; 321; 322; 324; 325; 326; 327; 328; 329; 333; 340; 341; 342; 343; 344; 345; 346; 347; 34 measurements must take into account complex extinction effects due to vegetation structure and complex scattering effects due to soil roughness. However, while the spatial resolution of passive MW radiometers is limited to 35-50 km, that of active SAR systems can reach a few meters. Thus, the SM products retrieved from passive MW with low spatial resolutions mainly have applications in hydrology, climate studies, and meteorology [153], while the SM products retrieved from active systems can also find applications in agriculture [164].