# The Combined ASTER MODIS Emissivity over Land (CAMEL) Part 2: Uncertainty and Validation
## 3 CAMEL Uncertainty Estimates  
### Method  
Because the inputs to the CAMEL emissivity product do not provide consistent error estimates, it is not possible to use error propagation to estimate the CAMEL uncertainty. When uncertainties are made available for the MODIS emissivity, which is intended for the future MOD21 version, it is planned to include these in combination with the currently available ASTER uncertainties in the CAMEL uncertainty estimates. The current CAMEL uncertainty formula is defined in the following paragraphs and uses coherence analyses to define various components of the total uncertainty. Due to the nature and inherent constraints of the input MODIS and ASTER datasets, CAMEL data is likely to be highly correlated for certain domains, for example where no MODIS data is available in regions of the long polar nighttime. The CAMEL uncertainties are estimated by combining three separate components--temporal, spatial, and algorithm. Each measure of uncertainty is provided for all 13 hinge points and for every pixel. The total uncertainty is calculated from the components as a root sum square as in Equation (1):  
\[\sigma_{total}=\sqrt{\sigma_{spatial}^{2}+\sigma_{temporal}^{2}+\sigma_{ algorithm}^{2}} \tag{1}\]  
The spatial uncertainty component is calculated as the standard deviation of the surrounding 5 \(\times\) 5 pixel emissivity, which is equivalent to a 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) latitude-longitude region. This uncertainty represents the variability of the surrounding landscape and is only provided where the CAMEL emissivity quality flag is non-zero (i.e., sea/ocean is not included). Temporal uncertainty, also defined on a pixel by pixel basis, is defined by the standard deviation of the three bracketing months(e.g., Oct. uncertainty = standard deviation (September, October, November)). Even if emissivity values are not available for all three months as in the case of the starting or ending month of the CAMEL record, an uncertainty is still reported from the closest month. The algorithm uncertainty is estimated primarily by the differences between the two CAMEL emissivity inputs: the ASTER GEDv4 and UW BF products. Thus, the algorithm uncertainty is a measure of bias in the CAMEL inputs and is not a propagation of the ASTER and UW BF product uncertainties. The intent is to warn the user when differences in the ASTER and UW BF products are comparable to uncertainties due to other sources. To convert the signed difference into a standard deviation, the absolute difference is divided by the square root of 3. This scaling gives the variance associated with the algorithm uncertainty approximately equal weight to the spatial or temporal uncertainties. Table 1 shows the CAMEL, ASTER, and UW BF channel wavelengths, the method for combining the ASTER and UW BF emissivity, and the method for determining the CAMEL algorithm uncertainty. For hinge points 6-9 and 11-13 where ASTER and UW BF report emissivities at nearby frequencies, differences between the ASTER and UW BF emissivity are used to define the algorithm uncertainty as shown in Table 1. Hinge point 10, at 10.8 \(\upmu\)m, uses a linear interpolation between the ASTER channels 4 and 5. For CAMEL hinge points 1-5 which cover 3.6 to 7.6 \(\upmu\)m, no ASTER data is available to use in the uncertainty calculation. CAMEL hinge points 1 and 2 use a fractional difference from hinge point 7 due to the correlations between these window regions. Hinge points 3-5 are located in the water vapor absorption region so emissivity does not change much. Thus, hinge points 3 and 4 assume that there is a 1% constant difference. For hinge point 5 at 7.6 \(\upmu\)m it is assumed that there is no variation. This is based upon a previous study [1] which found that 7.6 \(\upmu\)m was the channel that showed the least amount of variation in a set of 123 laboratory emissivity spectra.