The archaeological information that may be extracted from lidar is highly dependent on the methods applied to view the processed digital elevation model. Some of the more important techniques are summarized here, while more details can be found in the references listed at the end (Kokalj et al. 2011 and 2012). Some tools to calculate the visualizations are available at ZRC SAZU web page. A number of visualisation techniques can also be implemented using the LiVT software which was developed under the auspices of the Arcland project.
To allow direct comparison of the effect of the different techniques all the images (except one) are based on the same example site, a WWI trenches and caverns in the Kras region, Slovenia. 1 m lidar DEM, used with permission by ZRC SAZU.
Analytical shading is the most often used technique in archaeological interpretation of digital elevation models (DEMs). It is included in all GIS software, easy to understand, and provided by most, if not all, lidar data suppliers. The surface is illuminated by a direct light that is constant for the entire dataset. The main drawbacks are that no or very little detail can be perceived in dark shades and brightly lit areas and a single direction of the light beam also fails to unveil linear structures lying parallel to it. The technique is therefore considered inadequate to be used alone.
Sky-view factor estimates the portion of the sky visible above a certain point. Locally flat terrain, ridges, and hills which receive more illumination are highlighted, while depressions are dark because they receive less illumination. It is the most suitable technique in variable or rugged terrain and is particularly useful for complex features because it: (i) is orientation independent, (ii) has no saturations, (iii) offers a clear difference between protruding features and depressions, and (iv) helps with noisy data.
Note: Results by SAGA GIS and SVF standalone (ZRC SAZU) differ substantially. It is better to use the second because the results are not saturated.
Slope severity (gradient) is aspect independent. It can be calculated either as percentage of slope or degree of slope. It works especially well in combination with hill shading and the big advantage is that it works well on most types of terrain. The downside is that it retains saturated areas and additional information is needed to distinguish between convex (e.g. banks) and concave (e.g. ditches) features.
Trend removal is a procedure that separates local small-scale features from large-scale landscape forms. It produces a local relief model (LRM) that can be used as an input for further visualization using other methods. The trend can be assessed by generalizing a detailed DEM. The LRM is the difference between the original relief model and the assessed trend. The method works best on terrain with gradual slopes, while it can produce false artefacts such as artificial banks and ditches where relief is diverse and/or changing abruptly.
Solar insolation mapping estimates the amount of the solar energy received at the surface. Global solar insolation maps are best for archaeological interpretation. Direct insolation maps suffer from the same problems as hill-shading, while diffuse solar insolation maps are similar to sky-view factor images, but are more generalized. A global insolation map preserves a sense of general topography and shows suitability of land for different human activities. However, calculations are complex and time consuming and numerous options can confuse the user.
Elevation differentiation also referred to as colour shading, colour cast, or constrained colour ramps method, limits the range of values that are presented simultaneously. The values of interest are stretched to the whole histogram, enhancing the contrast between light and dark areas. This is the only technique that retains the original height values. It is orientation independent and very useful for flat landscapes but completely fails in rugged terrain.
Derivatives of hill-shading from different directions
The principal component analysis (PCA) 'summarizes' the information of hill-shadings from several directions by a mathematical transformation. The first three components can be viewed as a RGB composite. Alternatively, a combination of the first and second principal components is less colourful, but much easier to interpret. This technique removes redundancy but does not provide consistent results with different datasets.
Another way of removing redundancy is calculating a range of values of multiple shadings, for each pixel. In order to display the areas with a low range of values more clearly, the result can be square rooted.
Hill-shading and nDSM
On a composite image of a shaded relief and a normalised digital surface mode (nDSM), the height of vegetation covering the archaeological features can be perceived. This helps evaluate the environment of the artefacts, especially when covered by forest.
Orthophoto and nDSM
Kokalj, Å½., ZakÅ¡ek, K., OÅ¡tir, K., 2011. Application of Sky-View Factor for the Visualization of Historic Landscape Features in Lidar-Derived Relief Models. Antiquity 85, 327: 263-273.
Kokalj, Å½., ZakÅ¡ek, K., OÅ¡tir,. K., (in press). Visualizations of lidar derived relief models. In: Opitz, R., Cowley, D. (eds). Interpreting archaeological topography â€“ airborne laser scanning, aerial photographs and ground observation. Oxbow Books, Oxford.