## 2 Understanding the Jargon: Equipment and Data Products
### Understanding ALS Technology
Even within the broad category of \"scanning\" LiDAR units there are significant differences, one of the more important being the expected size of the projected laser footprint over the target. In general \"large footprint\" systems illuminate areas several meters in diameter with each shot, while small footprint systems typically illuminate areas with diameters of a few decimeters or less. The projected footprint diameter is a product of the laser beam divergence and the range to the target, and the size of the footprint is in general inversely proportional to the vertical accuracy (this will be covered in more detail later). A third characteristic that creates a distinction between ALS systems has to do with how weak a return signal can be distinguished from the background noise level. In electrical engineering jargon this is called Signal-to-Noise-Ratio, or SNR. In terms of ALS, systems can be divided into two main classes: high SNR system and low SNR (as low as single photo-electron events), which are sometimes referred to as \"photon counting\" systems [30, 31]. In high SNR systems the energy of the outgoing laser pulse is relatively high (typically 50 microjoules or more per pulse) and the return signals generally have to be hundreds of photons per shot for the system to record a return. Most, if not all, of currently available commercial ALS systems are high SNR systems. Only the military and some large research institutions have access to low SNR or single photon counting systems [31]. A fourth trait to classify ALS systems has to do with the way the return signals are recorded and processed. In full waveform systems the entire return signal (waveform) is digitized and recorded for real-time or post-flight processing to determine the range to the target [32]. In discrete systems the return signal is analyzed in real time and a discrete number of return ranges (or stops) are determined and stored, discarding the rest of the waveform. Some systems can simultaneously record both full waveform and discrete returns, but the maximum number of points that can be recorded per second is generally significantly higher for discrete returns, and the amount of data that must be stored and processed is significantly lower. Discrete systems may be further described based on the number of stops that can be detected and stored per laser pulse, ranging from a single stop to systems that can detect multiple stops and record all or a fraction of them. For instance in a complex return waveform from which five discrete peaks (returns) can be identified, a three-stop system (1 st, 2 nd, and last) would detect all but only store the first, second and fifth returns, discarding the third and fourth. Having described the operational traits of ALS systems, it is important to mention that most of the systems that are currently available to the archeologist through academic research centers or commercial mapping/surveying companies are pulsed, high SNR, small footprint, scanning, discrete or full waveform systems. Currently the four main manufacturers of such ALS systems are: AHAB (Sweden), Leica Geosystems, Optech and RIEGL. Based on this we will continue the discussion taking into account ALS systems with these characteristics. Airborne LASER scanning is performed along two dimensions (see Figure 2), regardless of the airborne platform used to acquire the data (e.g. fixed-wing, rotorcraft, lighter than air). The first dimension is along the airplane flight direction and is achieved by the forward motion of the aircraft. The second dimension, generally perpendicular to the flight direction, is obtained using a scanning mechanism, with the most common designs in the commercial sector being the oscillating mirror, the rotating polygon and the Palmer scanner (nutating mirror). The combination of aircraft motion and optical scanning distributes the laser pulses over the ground in different types of patterns depending on the type of scanning mechanism used. The oscillating mirror produces a saw tooth pattern (Figure 3a).