# The Combined ASTER MODIS Emissivity over Land (CAMEL) Part 2: Uncertainty and Validation
## 4 Validation with Laboratory Spectra  
### Field Sites  
Figure 3 shows the HSR and 13 hinge point CAMEL products overlaid with the UW BF, ASTER, and lab measurement emissivities for four validation sites. The total HSR uncertainty estimate for the CAMEL HSR product is overlaid in the top panel (plotted as red dotted lines), and the CAMEL 13 hinge point total uncertainty with its 3 components are shown separately in the bottom panel. The first site in Figure 3a shows emissivity spectra comparisons over the Namib desert during January 2007. The quartz doublet feature can clearly be seen at 8.5 and 12.5 \(\upmu\)m since the desert sands consists mostly of quartz and hematite. In the top panel, the CAMEL HSR product closely captures the depth of the quartz doublet within the 8-9 \(\upmu\)m region. The middle panel shows how the CAMEL emissivity is defined using ASTER bands within this 8-9.3 \(\upmu\)m window. The difference between the ASTER and UW BF product around 9 \(\upmu\)m is reflected in the CAMEL algorithm uncertainty shown in the bottom panel. With an exception for wavelengths less than \(\sim\)5 \(\upmu\)m where the temporal uncertainty is the largest component, the total uncertainty is dominated by the algorithmic uncertainty. The second site shown in Figure 3b is for Tucson, Arizona in May 2007. The spectrometer data for this case was obtained from AERI in situ measurements of a characteristic creosote site, where the Creosote bush is the dominant vegetation type [14]. The ASTER NDVI is used as a vegetation fraction to form a weighted emissivity measurement from bare and vegetated spectra. This site shows very close agreement between ASTER, UW BF, and CAMEL products. This close agreement, in addition to the lack of temporal variation of the surface characteristics for wavelengths under 11 \(\upmu\)m, produces a total CAMEL uncertainty that is mostly driven by the spatial variability of the region. While the CAMEL HSR product closely resembles the UW BF HSR emissivity across most of the spectrum, the CAMEL HSR product is more accurately able to capture the 8-9.3 \(\upmu\)m subtle quartz feature seen in the lab validation data. The third site in Figure 3c shows results for a site in Greenland, Summit Station, for January 2007. The top panel shows that the CAMEL HSR emissivity more accurately fits the lab data for smooth snow than does the UW BF, particularly for snow features in the 11-14 \(\upmu\)m region. Snow spectral features are better captured because the CAMEL HSR for snow is derived from a unique set of  
Figure 2: January 2007 CAMEL 8.6 um emissivity uncertainty (**a**) total estimate; (**b**) spatial component; (**c**) temporal component; (**d**) algorithm component; and (**e**) total uncertainty quality flag which is defined in Table 2.  
principal component (PC) coefficients derived from laboratory spectra of different snow grain sizes, ice, and water, more details of which can be found in Part 1. While the CAMEL HSR uncertainty is around 0.01 for wavelengths over 11 \(\upmu\)m, the 13 hinge point CAMEL uncertainty is confined well under 0.01 and is dominated by algorithm variability. The last case site example in Figure 3d is for Yemen in January 2007. Lab measurements are derived from the ASTER spectral library and illustrate the primary carbonate features that are seen over regions of Yemen and Oman at the southern tip of the Arabian Peninsula. The CAMEL 13 hinge point product is unable to resolve the narrow 6-7 \(\upmu\)m main carbonate feature because MODIS does not have any bands covering that region; however, the CAMEL HSR emissivity is able to capture the feature since a unique set of PC coefficients for carbonate minerals are used. The CAMEL is an improvement upon the UW BF HSR in this region since the UW BF uses only one set of PC coefficients for all surface types.