2.4 - Spectral signature of an object

The reflectance values for different objects, over a range of wavelengths, may be plotted for comparison. Such plots are called “spectral response curves” or “spectral signatures.” Differences among spectral signatures of landscape features are used to help classifying remotely sensed scenes, since the spectral signatures of similar features have similar shapes.

Figure 2.10 shows the spectral signatures of 3 different materials: conifers (in green), water (blue) and soil (red).

Figure 2.10 – Spectral response curves for 3 different types of features on the surface of the Earth.

The previous figure implies that the mentioned feature types are normally spectrally separable. The degree of separation of landscape features depends, however, on the range of wavelengths we are considering. This means that two features may be indistinguishable for a certain spectral range and perfectly individualized for other spectral band.

Exercise: The reflectance dependency on the wavelength can be observed in the following images of the Earth, taken with MSG satellite, in TWO different spectral channels. From these figures, it is quite obvious that there is a clearer separation of land/sea in the image on the left. Taking into consideration the spectral signatures of water and soil materials (last figure), please answer to the next question.