2.3.4. CIELAB

The colour spaces CIELAB, CIE-Yxy, and Munsell are suited for the analysis of iron oxide content in soils and sediments [57,58]. The CIELAB colour space shows a clear benefit regarding quantitative analyses and was used throughout the presented study, since CIELAB coordinates are often influenced by soil and sediment properties: CIELAB lightness L\* is often regarded to be related with organic matter, and CIELAB chromaticity coordinates a\* and b\* were proven to be related with different iron oxide concentrations (cf. [27,57,59]). While these relations are measurable, authors generally conclude that the specific relation is highly dependent on the material examined and often cannot be transferred between different soil types [27].

The recorded image data were therefore transformed into the CIELAB colour space using the convertColor() function for R. The CIE standard illuminant D65 was used as reference white, since its properties are close to real indirect daylight as present during fieldwork. The multispectral data were modified so that only the red, green, and blue bands were transformed. The produced raster layers were used as additional input for classification with the intent of adding a rough measurement of lightness and, more importantly, iron oxide content.
