## 2 Methodology
### Data Acquisition  
Figure 2 shows the location of the study sites within Alaska, U.S.A. Three surveys were completed at each site during the summers of 2012, 2013, and 2014. A summary of equipment used, estimated accuracies and typical point sampling intervals is provided in Table 1. This section will describe the details of each survey because the scope and equipment used varied between surveys. Figure 1: Flowchart of key steps of the methodology (BFP = Best Fit Plane). (**A**) Model creation; (**B**) Model analysis. Figure 2: Topographic relief map showing the location of Study Sites **A**, **B** and **C** in Alaska, U.S.A. (Shaded relief map courtesy of the Earth Systems Research Institute, ESRI). The 2012 survey (Figure 3) was completed using a mobile laser scan system with a few supplemental TLS setups in locations with tall cliffs. The mobile scan surveys were completed for two major roadways in Alaska: a 16 km segment of Glenn Highway (State Route 1) and a second 16 km segment of Parks Highway (State Route 3). Multiple passes were completed to increase point density as well as verify the accuracy and consistency of each pass since the canyon limits the line of sight to GPS satellites. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**Site A**} & \multicolumn{2}{c}{**Site B**} & \multicolumn{2}{c}{**Site C**} \\ \cline{3-8}
**Survey** & \multirow{2}{*}{**Equignment Used**} & \multirow{2}{*}{**Accuracy**} & \multirow{2}{*}{**Typical Point**} & \multirow{2}{*}{**Accuracy**} & \multirow{2}{*}{**Accuracy**} & \multirow{2}{*}{**Typical Point**} & \multirow{2}{*}{**Accuracy**} & \multirow{2}{*}{**Typical Point**} \\
**Date** & & & **(3D-RMS, m)** & & **(3D-RMS, m)** & & **Sampling** \\ \hline \multirow{4}{*}{September,} & Terrapoint Titan mobile & & & & & \\  & laser scan system, Leica & & & & & \\  & ScanStation 2, Leica & 0.10 & 0.05 & 0.10 & 0.05 & 0.09 & 0.08 \\  & GNSS1200 receivers, Leica 1201 total station & & & & & & \\ \hline \multirow{4}{*}{August, 2013} & Riegl VZ-400 TLS, Nikon & & & & & & \\  & D700 camera, and Trimble & 0.04 & 0.01 & 0.04 & 0.01 & 0.04 & 0.04 \\ \cline{1-1}  & R8 GNSS receiver & & & & & & \\ \hline \multirow{4}{*}{July, 2014} & Riegl VZ-400 TLS, Nikon & & & & & & \\ \cline{1-1}  & D700 camera, and Trimble & 0.04 & 0.01 & 0.04 & 0.01 & 0.04 & 0.04 \\ \cline{1-1}  & R8 GNSS Receiver & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of survey data collected including typical accuracies and point sampling intervals on the cliff surface. Figure 3: Perspective view of a sample point cloud for a 1 km section of the Parks Highway obtained with combined TLS and MLS data. The dashed line highlights Study Site C.  
The 2013 and 2014 scan surveys were completed for relatively shorter segments each encompassing approximately 15 km in total for both Parks and Glenn Highway. These scans were obtained using a stop-and-go scanning approach utilizing field procedures very similar to those presented in [27] where the scanner is mounted on a wagon rather than a tripod for efficient data collection. Inclination sensors are also included in the scanner to correct for the scanner being out of plumb when conducted from the wagon platform. A digital SLR camera and a survey grade GPS receiver were mounted above the scanner with calibrated transformation offsets (translations and rotations) to the scanner origin. Atmospheric conditions (temperature, pressure, and relative humidity) were recorded and input into the laser scanner during data acquisition to correct distance measurements. During the 2013 and 2014 data acquisition, scan positions were typically spaced at 50-80 m apart along the highway and covered a 360 degree horizontal field of view and a 100 degree vertical view (-30 degrees to +70 degrees from the horizontal plane).The resolution of the scans varied between 0.02 and 0.05 degree sampling increments. Each position was occupied for at least 20 min to enable sufficient time for Rapid Static GPS data logging.