## 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  
The debate between discrete return and full waveform digitizing (FWD) ALS has been discussed since the widespread emergence of commercial waveform topographic LiDAR systems in the mid-2000s [32]. In certain applications such as forestry [40, 41] and bathymetry [42] FWD systems have distinct advantages over discrete return systems, although in forestry the discrete data still provides abundant information [43, 44]. Other uses such as the mapping of large scale topographic features (faults, landslides) have been successful for decades with discrete return systems even when the terrain features of interest are occluded by dense vegetation [45]. FWD systems can provide additional information and improved vertical resolution not available with discrete systems; it is a matter of analyzing the cost/benefit trade-off for a given application. In the case of archaeology, [PERSON] has written extensively on the potential and benefits of FWD for prospection in forested areas [19, 34]. However, [PERSON] also points out that there are examples of archeological prospection projects that have been successful using discrete returns systems--which he describes as \"conventional\" systems [19]. The use of FWD systems is advantageous in areas where short vegetation (shorter than the discrete ALS range resolution) is present. Depending on the density of the vegetation, the returns recorded by a discrete system will be from a region near the top of the vegetation (the vegetation density determines how far the laser energy penetrates) and not from the ground. While [PERSON] has illustratedarchaeological false negatives due to the limitations of discrete systems [19], cultural false positive are also possible due to the discrete limitations. For example, Figure 14 presents different visualizations of discrete return ALS data from an area in the Honduran Mosquitia along the banks of a small stream that has a mixed open and forested cover. A rendering of the point cloud highlights a peculiar quasi-rectangular vegetated area (Figure 14a). The shaded relief from the DEM also presents a rectangular looking depression (Figure 14b); a cross section of the point cloud also shows a \"sunken\" feature. By analyzing the shaded relief image and the point cloud alone the rectangular feature can be interpreted as a cultural structure like a water retention pond. Due to the remoteness of this area ground verification has not been possible and the exact nature of the feature is unknown. This uncertainty means that this feature might be a false positive--a data artifact that is generated by the relatively higher returns produced by the \"top\" of the short vegetation (grass, brush, _etc._) that surrounds the rectangular tree area. The top returns of the short vegetation were classified as ground probably due to its relatively low elevation difference from the surrounding ground classified points underneath the thick tall canopy. It is also possible that there is very little understory vegetation that will cause relatively lower elevation returns with respect to the short vegetation returns, thus causing an apparent depression. In theory, with the improved vertical resolution of a FWD system it might be possible to derive an approximate ground level from the short vegetation returns and thus make a more informed decision on the nature of this feature without field verification. However, as will be demonstrated below, FWD may not always make this discrimination possible. Figure 15 shows the results of a controlled experiment that compared DEMs generated from discrete and FWD data obtained with the same system at the same time. The Optech Gemini ALTM is a discrete ALS system that has an option (with additional hardware) to collect FWD data simultaneously. Figure 14: Potential data artifact (false positive) created by the limitations of discrete ALS systems with dense short vegetation surrounding thick tree canopies.