## 2 Data and Methods
### Study Methods
#### 2.2.1 Park Falls Wisconsin--EO-1/ ALI NDVI vs. Landsat NDVI  
Park Falls, Wisconsin, was chosen as the study site for this experiment because of the density of EO-1 imagery available and the diversity of included land cover types. This site falls within the Worldwide Reference System (WRS) path 25, row 28. One ALI image per year from summers of 2001 to 2016 were selected, and a corresponding Landsat image was chosen to minimize the difference between the dates of acquisition (Table 2). For the twelve pairs of images, the cumulative delta days between all acquisitions were 19 days, averaging just 1.58 days/pair. Comparison of the spectral \begin{table}
\begin{tabular}{c c c} \hline \hline
**Site Name** & **Central Coordinates (Longitude, Latitude)** & **Dominant Land Cover** \\ \hline Park Falls, Wisconsin & 90.18\({}^{\circ}\)W, 45.58\({}^{\circ}\)N & Mixed hardwood forest \\ Howland Forest, Maine & 68.5\({}^{\circ}\)W, 45.21\({}^{\circ}\)N & Mixed confierous forest \\ BARC 1 , Maryland & 76.85\({}^{\circ}\)W, 39.03\({}^{\circ}\)N & Evergreen, Corn Field \\ RRVP 2 , Nevada PICS & 115.69\({}^{\circ}\)W, 38.5\({}^{\circ}\)N & Desert \\ Libya-4 PICS 3  & 24.40\({}^{\circ}\)E, 28.53\({}^{\circ}\)N & Desert \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dominant land cover types and Northern Hemisphere study site locations used in analyses. bandwidths (Table 3) of each sensor shows that all bands except the Near Infrared (NIR) have very similar wavelength ranges. Sun and view angle differences also play a part in NDVI calculation [27]. Solar zenith angle differences were minimalized as acquisitions of the comparitive scenes were all within a few days of one another, so solar zenith angles were all very close. When considering view angles, Landsat images are always nadir viewing, whereas EO-1 has the ability to view up to one neighboring WRS path on each side. With this in mind, the EO-1 images were selected to reduce impacts of view angle, and all but one had view angles less than 7.5 degrees. The 2016 ALI image had an off-nadir angle of 20 degrees, but it has been demonstrated that NDVI is not highly effected until this angle is greater than 25 [27]. Data for the EO-1 and Landsat sensors were downloaded from USGS in units of TOA reflectance and had radiometric and systematic geometric corrections applied to achieve a Level One Terrain corrected and ortho-rectified (L1T) image product. An NDVI product was obtained from ALI and Landsat scenes, as a normalized difference spectral index using Equation (1):  
\[NDVI=\frac{\rho_{NIR}-\rho_{red}}{\rho_{NIR}+\rho_{red}} \tag{1}\]  
Each Landsat image was subset to the boundaries of the EO-1 ALI imagery to have exactly the same region of interest for statistical calculations. A data cube was built with these stacked images. To establish land cover classes, the 2011 National Land Cover Database (NLCD) [28] was downloaded and overlayed onto the data stacks. \begin{table}
\begin{tabular}{c c c} \hline \hline
** Landsat Date** & ** ALI Date** & **Off Nadir** \\ \hline
8/17/2016 & 8/17/2016 & 20.10 \\
8/15/2015 & 8/13/2015 & \(-\)0.30 \\
8/12/2014 & 8/09/2014 & \(-\)5.70 \\
9/26/2013 & 9/27/2013 & 6.10 \\
8/06/2012 & 8/05/2012 & 4.50 \\
8/20/2011 & 8/27/2011 & \(-\)2.72 \\
8/17/2010 & 8/17/2010 & \(-\)2.50 \\
9/23/2009 & 9/21/2009 & \(-\)1.00 \\
9/12/2008 & 9/09/2008 & 5.60 \\
6/30/2007 & 6/30/2007 & \(-\)7.20 \\
8/11/2002 & 8/11/2002 & 3.50 \\
5/04/2001 & 5/04/2001 & 0.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Coupled image pairs used for the Park Falls, WI analysis.