## 3 Considering the ALS Workflow and Its Impact on the Efficacy of Data Products
### Planning and Data Collection
#### 3.1.1 Flight Planning and System Configuration
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Configuration** & **Rtr/Shot** & **Grn/Sht** & **Grn/Rtr** & **Shot/m2** & **Rtr/m2** & **GR/m2** \\ \hline Cansahcab, Mexico & & & & & & \\
70 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.74 & 35.7\% & 20.5\% & 4.41 & 7.68 & 1.58 \\
125 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.45 & 16.1\% & 11.0\% & 7.92 & 11.53 & 1.27 \\ \hline Mayapan, Mexico & & & & & \\
100 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.59 & 37.0\% & 23.3\% & 2.98 & 4.74 & 1.10 \\
100 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.25 mrad & 1.48 & 29.7\% & 20.1\% & 3.33 & 4.94 & 0.99 \\ \hline Calakmul, Mexico & & & & & & \\
70 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.98 & 26.9\% & 13.6\% & 2.59 & 5.13 & 0.70 \\
100 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.70 & 16.0\% & 9.4\% & 3.64 & 6.22 & 0.58 \\
125 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.53 & 9.4\% & 6.1\% & 6.19 & 9.52 & 0.58 \\ \hline Yaxnohach, Mexico & & & & & & \\
100 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.71 & 17.2\% & 10.0\% & 3.77 & 6.47 & 0.65 \\
125 kHz 20\({}^{\circ}\)\(\times\) 45 Hz 0.8 mrad & 1.47 & 10.2\% & 6.9\% & 4.33 & 6.41 & 0.44 \\ Uxbenka, Belize & & & & & & \\ \hline
50 kHz 20\({}^{\circ}\)\(\times\) 35 Hz 0.8 mrad & 1.74 & 9.0\% & 5.2\% & 2.01 & 3.50 & 0.18 \\
125 kHz 18\({}^{\circ}\)\(\times\) 55 Hz 0.8 mrad & 1.26 & 3.5\% & 2.8\% & 4.01 & 5.06 & 0.14 \\ Mosquito, Honduras & & & & & & \\ \hline
125 kHz 15\({}^{\circ}\)\(\times\) 60 Hz 0.8 mrad & 1.65 & 2.7\% & 1.6\% & 5.90 & 9.79 & 0.16 \\
125 kHz 15\({}^{\circ}\)\(\times\) 60 Hz 0.25 mrad & 1.47 & 2.3\% & 1.6\% & 6.00 & 8.86 & 0.14 \\
142 kHz 15\({}^{\circ}\)\(\times\) 60 Hz 0.8 mrad & 1.45 & 1.7\% & 1.1\% & 6.96 & 10.10 & 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of ALS canopy penetration experiments. Abbreviations used in the Table heading are Rtr for return, Sht for LASER shot and GR for ground return. coverage. Increasing the PRF increases the number of laser shots emitted per unit time, increasing the shot density [37]. It is important when assessing the results of a survey to not confuse the shot density with calculated return density. Because of vegetation and partial interception of a laser beam by solid objects, multiple returns can be obtained from a single shot, so in the presence of vegetation the return density may actually be higher than the shot density. It is also important to note that while return density is mainly determined by vegetation, in multistory canopies the number or returns per shot is also a function of system configuration; higher PRFs generally produce fewer returns per shot than lower PRFs due to the lower energy per pulse at higher PRFs (see Table 2). Some ALS systems have the option to select the laser beam divergence and thus the size of the projected laser footprint on the target, as the product of the divergence and the flying height AGL determines the diameter of the projected laser footprint [37]. Considering the laser shot density and distribution in conjunction with the laser footprint area allows to estimate the fraction of the surface area that will be illuminated. Figures 11 and 12 illustrate how two surveys with similar LASER shot density can produce different results based solely on the size of the beam footprint and thus the fraction of the surface that  
Figure 11: Comparison of laser shot density and fraction of surface illumination. Shaded relief images of DSMs produced from ALS data collected with a beam divergence of (**a**) 0.8 mrad and (**b**) 0.25 mrad, overlaid with the first LiDAR returns that were used to generate the DSMs. Parts (**c**) and (**d**) are zoomed-in illustrations of the laser shot distributions and their footprints. is illuminated. These figures were produced based on two test flight strips with identical sensor configuration and under similar flying parameters while only varying the beam divergence.