## 4 ALS Data Visualization, Analysis and Quality/Accuracy Assessment
### Data Visualization  
DEMs or DSMs can then be represented in a variety of ways, including shaded relief maps (hill shades), image maps (images where the changes in elevation are represented by a color gradient), 3D models, and contour maps among the most traditional ways; and local relief model (LRM) [57], sky view factor (SVF) [61], openness plots [62] and \"fish-bone\" plots [63] among many new ways proposed for archaeological applications to highlight the natural and manmade features of the terrain. The shaded relief map [64] is perhaps the most common visualization technique for elevation rasters. Shaded relief maps are generated by illuminating a DEM at a particular angle and thus generating lighted areas and areas with shadows creating an impression of natural terrain. In addition to the illumination angle, the vertical dimensions of the terrain and overlaying features can be exaggerated with respect to the horizontal scale to make features more visible. Figure 22 shows shaded relief images generated from the same DEM illuminated from different angles. The red arrows in the figure illustrate the azimuth direction of the illumination source; all the images with the exception of the central image are generated with a vertical illumination angle of 45 degrees. The central image is generated by illuminating the DEM at the zenith. The general practice is to illuminate the DEM form the northwest at a vertical angle of 45 degrees. Figure 22 illustrates how different illumination angles can highlight certain features. It also illustrates the main limitation of shaded relief images. This is that features in the DEM that are lined  
up with the illumination source are not visible in that particular shaded relief image. This small section of the Honduras data set is centered on a nearly square plaza, which is surrounded by rectilinear mounds on all four sides. The mounds on all four sides of the plaza are visible in the shaded relief image that is illuminated from the zenith, in all other images one or more of the mounds are not visible. One alternative to overcome this limitation of single illumination shaded relief images is to produce images illuminated from several horizontal and vertical illumination directions and combine them into a multi-illuminated shaded relief image. Sometimes this type of image is called a 360 shaded relief image. Another alternative for visualization of elevation rasters is the Fishbone diagram that produces results similar to the 360 shaded relief image. Because the elevation rasters are in essence single band images, well established image processing techniques can be applied to highlight certain type of features with respect to the natural terrain. These techniques can vary from simple slope computations to advanced techniques such as spatial spectral analysis (Fourier and wavelet) [65] and principal component analysis [66]. Intermediate techniques include spatial operators to assess \"openness\" or sky-visibility factors. Figure 23 presents data from another section of the Honduras Mosquita that contains smaller less evident features than the ones presented in Figures 21 and 22, where several of the spatial analysis techniques have been applied and the results have been presented in both greyscale and color scale to illustrate the usefulness and limitations of these techniques. The first row shows a standard shaded relief image of the area with standard illumination and a simple maximum slope computation, where the slope is computed as the elevation difference  
Figure 23: Different visualizations of the same DEM. (**a**) Shaded relief image from DEM; (**b**) maximum slope from 1 st derivative; (**c**) maximum slope from eight directions and across 5 cells represented in grayscale and (**d**) in color ramp; (**e**) normalized sky-view factor in grayscale and (**f**) in color ramp (**g**); Maximum height anomaly in grayscale and (**h**) in color ramp. between adjacent grid nodes in the row and column directions with only the maximum slope obtained for each grid node stored.