## 2 Understanding the Jargon: Equipment and Data Products
### Understanding ALS Data Products
Figure 5: Visualization of the point cloud (PC) **(a)** PC colored by elevation; **(b)** triangulated irregular network (TIN) colored by elevation; **(c)** PC colored by return intensity; **(d)** TIN colored by elevation and intensity; **(e)** PC colored by class; **(f)** hybrid PC and TIN. The term digital elevation model (DEM) in its broadest application serves as an umbrella for any type of electronically accessible data that contains elevation information. In this broadest use the DEM includes elevation data that can be irregularly (TIN) or regularly spaced (rasters) and it can represent any surface (bare-earth, vegetation canopy, bathymetry, _etc._). However, being extremely specific in the American usage, DEM refers to the bare earth surface free of vegetation and manmade structures (DTM in the European usage [18]). This usually refers to modern manmade buildings, towers, bridges, power lines, _etc._ In the case of archaeology it gets more complicated, as it is hard to distinguish between some manmade or man-altered features such as mounds, and natural occurring terrain features. A digital surface model (DSM) usually refers to what is called the first surface, and it is a representation of the surface that produced the first return from each of the laser shots The DSM represents the first layer of vegetation (not necessarily the tree tops due to sampling spacing and footprint interception), buildings and structures, and the ground when it is not obstructed. A digital terrain model (DTM) is a specific kind of DEM which represents bare-earth but which is enhanced by the inclusion of special \"breaklines\" that clearly delineate abruptly and key elevation changes such as ridgelines and river channels. Without these breaklines, key terrain features may get lost in the DEM resolution. However, it is important to point out, that the need for these breaklines becomes less necessary as the DEM resolution increases (grid cells become smaller). Similar to the point clouds, the elevation rasters can be displayed in a variety of ways as image maps, contour maps, 3D surface models and many more to be described later. However, the most common and perhaps useful way to display these rasters is the shaded relief image also known as a hill shade. Figure 6 shows a high resolution (0.5 m cell spacing) DSM and DEM from Mayapan, Mexico represented as a shaded relief images. Figure 6: Shaded relief images from Mayapan, Mexico, generated from (**a**) A first return digital surface model (DSM); (**b**) A vegetation filtered digital elevation model (DEM). Comparing ALS Products to Satellite or Aerial Imagery  
To further understand key features of ALS data let us compare a LIDAR derived shaded relief image with something that is more familiar: a satellite image. Figure 7 shows side-by-side comparisons between high resolution (0.5 m) visible and visible-infrared satellite images and an ALS derived shaded relief image produced from a DSM and a DEM of the same area near the western boundary of the Honduran Mosquitia. While both sets of \"images\" are representations of the same reality and both are rasters with a similar spatial resolution, there are more differences than similarities between these data products. At this point it is assumed that the reader knows that there are two main technical differences between LiDAR data and imagery. The first is that LiDAR is an active remote sensing technology (_i.e._, meaning that it relies on its own illumination source) and thus there is knowledge on the amplitude, polarization, wavelength and phase of the electromagnetic energy produced by the source,  
Figure 7: **(a)** Shaded relief map produced from a DSM; **(b)** Visible Satellite Image; **(c)** Image map from laser return intensity; **(d)** Visible-IR Satellite Image; **(e)** Shaded relief map produced from a DEM; **(f)** Perspective view overlaid with contour lines. while airborne or satellite imagery is passive meaning that it depends on solar illumination.