**3. Comparison of satellite and radiometric data**

 To better evaluate the type of radiometric departures between in situ measurements, which sample small, specific locations at an observation station, and satellite determinations which necessarily involve averaging the radiometric responses of the various materials in each pixel, we performed a comparison between them. We used the least noisy spectrum for each station as well as the values of the Landsat 8 pixel that contains the corresponding station, obtaining the values of the digital numbers of each band: B1, B2, B3, B4, and B5; these values were converted to a 0–100% reflectance scale and plotted with the corresponding spectral signature (**Figures 15**–**17**). The SWIR bands do not overlap the radiometer's range.

#### **Figure 15.**

*Reflectance comparison between Landsat 8 pixels and radiometric stations. Stations 1, 2-1, 2-2, and 3. The band coverage is shown as colored, continuous lines, while dashed lines correspond to the radiometric determinations.* 

*Radiometric Mapping of Hydrothermal Alterations in Isla Isabel, Mexico DOI: http://dx.doi.org/10.5772/intechopen.80530* 

In **Figure 15**, the low reflectance obtained with the radiometer in Station 1 corresponds well with the values obtained from the pixel of the Landsat 8 image. In Station 2-2, the radiometer readings overshoot those of Landsat 8 but maintain the general trend. Stations 2-1 and 3 correspond well with the satellite radiometric responses. In **Figure 16**, Station 4 departs considerably from the satellite band behavior. Although Stations 5, 6, and 7 conform to the general trends of the satellite measurements, reflectance is considerably larger.

In **Figure 17**, radiometric Stations 8, 9, and 10-2 show a behavior similar to the Landsat 8 bands, although displaced and amplified. Station 10-1 departs from the satellite bands owing mostly to the dispersion effect clearly shown below 700 nm.

We conclude that there is a fair agreement between the satellite and the in situ measurements. With the ground truth of the radiometric stations, we can now proceed to analyze the satellite data.

**Figure 16.** 

*Reflectance comparison between Landsat 8 pixels and radiometric stations. Stations 4, 5, 6, and 7.* 

**Figure 17.** 

*Reflectance comparison between Landsat 8 pixels and radiometric stations. Stations 8, 9, 10-1, and 10-2.* 
