*2.3. Validation*

## 2.3.1. WorldView-2 Imagery

High spatial resolution imagery provides a validation source for cover fractions over large portions of the fire area. WorldView-2 is a commercially operated very high spatial resolution, multispectral satellite imager. Twelve WorldView-2 images from 28 September 2013 that together captured about 97% of the Rim Fire burn scar (Figure 2) were purchased from Satellite Imaging Corp. The images had a pixel resolution of 2 m. After acquisition, images were converted into reflectance using Atmospheric and Topographic CORrection (ATCOR) as implemented in the PCI Geomatica software package (PCI Geomatics, Geomatica version 2014) using a LiDAR data set acquired by the National Center for Airborne Laser Mapping (NCALM-University of Houston).

In order to compare the AVIRIS-derived MESMA analysis and the WorldView-2 images, 120 square samples were randomly selected. The samples were 119.2 m on a side, or equivalent to an eight by eight square of AVIRIS pixels, this was chosen to reduce problems of differences in georeferencing between the images, as well as reduce the effect of pixels which are only partially within the sample area in the AVIRIS imagery. Areas within the sample squares were manually interpreted and digitized

from the WorldView-2 imagery into five cover classes: GV, NPV, char, soil, and shadow (see Figure 3). The spectral resolution of WorldView-2 typically allowed for easy discrimination of GV. The NPV, char, and soil classes were not always easy separable spectrally, but contextual clues such as shape and texture were often helpful in separating these classes. There were likely some pixels that were misclassified, but overall this was considered a minor source of error.

**Figure 3.** An example of a randomly selected manually classified WorldView-2 plot, the false color infrared WorldView-2 imagery uses band centers at 830 nm for red, 660 nm for green, and 545 nm for blue. The upper image displays just the WorldView-2 imagery and the sampling box, the lower box has the classified area. In the lower image blue represent char dominated pixels, green represents green vegetation dominated pixels, red represents non-photosynthetic vegetation dominated pixels, orange represents soil dominated pixels. An overview of the plot location is featured in the top left.

For both the eight MESMA unmixing results and the manually interpreted high-resolution imagery, the average cover for each cover class was calculated for each point, then the fractions were shade normalized, in which the shade cover fraction was removed by dividing each endmember by the total percent cover of all non-shade endmembers [56]. This allowed the relative abundance of the non-shade endmembers classes to be compared by partly correcting for the effects of differential solar geometry between the two images [13]. The quality of a linear fit for the relationship between each of the eight MESMA analysis and the manually identified imagery based on validation plots was assessed.

## 2.3.2. Field Plots

Thirty-four field plots of fire severity were collected in October 2013. The plots were located in the southeastern part of the fire and were, due to permitting restrictions, all taken within Yosemite National Park. Five severity was assessed in 30 by 30 m square shaped plots. The plot's center coordinates were recorded with a handheld Trimble GeoExplorer 6000 series GeoXH GPS device (these unit average approximately 1-m x and y error, see Figure 2 for plot locations). The plots were located no less than 200 m apart and were taken in areas of relatively homogeneous fuel type and fire severity. Four of the 34 original plots were removed from the analysis since they were located outside of AVIRIS images of 17 November 2013.

GeoCBI [57], a modified version of Composite Burn Index (CBI) [4], was measured in the field plots. GeoCBI divides the ecosystem into five different strata: (i) substrates; (ii) herbs, low shrubs and trees less than 1 m; (iii) tall shrubs and trees of 1 to 5 m; (iv) intermediate trees of 5 to 20 m; and (v) tall trees taller than 20 m. In the field form, 20 different factors can be rated (e.g., soil and rock cover/color change, percent leaf area index change, char height) but only those factors present and able to be reliably evaluated for that strata are considered. Ratings are given on a continuous scale between zero and three and the results are averaged per stratum. These stratum averages are weighted according to their fractional cover within each plot to compute a final GeoCBI rating. GeoCBI scores range from zero to three, with three representing the highest severity. In addition to GeoCBI, every dominant and co-dominant tree within each plot was identified. Trees were then assigned to three possible classes based on their needle color: green, brown, or black.

To compare the 30 by 30 m GeoCBI plots with the 14.8 m pixel size, a weighted plot level averaging of pixels was performed. Weights were assigned based on each pixel's percentage of area within a plot. A weighted average of all pixels within an individual plot was then calculated, and compared with the plot level data.
