## 3 Considering the ALS Workflow and Its Impact on the Efficacy of Data Products
### Planning and Data Collection
#### 3.1.3 Discrete Return _versus_ Full Waveform Digitizing Systems
**(a)** Top view rendering of the PC; **(b)** shaded relief image of the DEM showing an apparent rectangular depression; **(c)** oblique aerial photo of the area; **(d)** cross section of the PC. Additional control was obtained by collecting the data over an experimental agricultural plantation with relatively flat terrain that at that time had adjacent plots with energy cane (_Saccharum complex_) and soybeans plants (_Glycine max_). The energy cane at the time of the collection had an average height of 3 m (marked as plot 1 in Figure 15) and the soybean plants an average height of 40 cm (marked as plot 2 in Figure 15). These two average vegetation heights mark the short and tall extremes of what can be contained within a laser pulse width and hence the combined return signal of ground and the vegetation cannot be separated due to the range resolution limitation. Figure 15: Experimental comparison of the capabilities of discrete and full waveform digitizing ALS systems to produce bare-earth data products from underneath low vegetation canopies. (**a**) Oblique aerial photo of the test area; (**b**) rendering of an intensity textured 3D model; (**c**) shaded relief map of DSM from discrete data; (**d**) image map of footprint-scale roughness derived from full waveform data; (**e**) shaded relief map of DSM from waveform corrected data; (**f**) elevation image map of DSM from discrete data; (**g**) elevation image map of DSM from waveform corrected data. Despite the range resolution limitation which does not allow for the separation of the individual returns there is a phenomenon that allows some room for estimation of the range separation of the targets. The observable phenomenon is because as the LASER energy propagates through the vegetation and eventually to the ground the resultant waveform is a time-stretched or broadened version of the original outgoing laser pulse. The amount of stretching is proportional to the vertical distribution or \"roughness\" of the vegetation. Figure 15d shows a footprint scale roughness map generated by calculating the pulse broadening for each laser return. As observed in the figure, roughness for vegetated areas is much larger than roughness for bare soil surfaces. Figure 15c shows a shaded relief map of the DEM generated from the first and only discrete ALS returns and Figure 15f is an image map of the same DEM. Because the FWD data is available, two approaches can be taken: the first is to apply a specialized algorithm to produce the returns that correspond to the lowest elevation detectable in the return waveforms, the second is to use the roughness information to compensate the discrete ALS data for estimated vegetation height and produce a \"ground\" return product. Herein the latter approach was used, with shaded relief and image maps created from the resultant bare-earth point cloud represented in Figure 15e,g respectively. From these figures it can be observed that the roughness approach was successful in producing bare-earth DEM under the soybean plants, however it was only partially successful under the cane. By studying Figure 15b-d it can be seen that the plant density and roughness of the energy cane is heterogeneous within the plot. In areas of low plant density the roughness is much higher (one of these areas is marked with a red arrow in Figure 15d,g). Conversely in areas where the plant density is high the roughness is low; one of these areas is marked with a black arrow. To measure return pulse broadening with FWD systems the LASER energy needs to propagate through the vegetation and back to the sensor. If the low vegetation is extremely dense then it will not be possible to record the pulse broadening even if a FWD system is used, in this case the resultant surface product will be similar to that from a discrete return system. More research and experimentation is needed using systems that have the capability of simultaneously recording discrete and FWD data to compare and contrast the capabilities and limitations of each in different archaeological contexts.