Using Global Mapper to Find Previously Unseen Structures in Archaeological Sites
The remains of buildings and other sites can be almost impossible to spot from aerial imagery alone. Blame the ravages of time and nature, which can obscure ancient walls and foundations in trees, vegetation, water, and more. High-resolution DEMs (digital elevation models) created from lidar have become a very valuable part of the verification and prospecting of potential archaeological features across a landscape. Lidar, a method of light-based remote sensing, can see through vegetation, exposing features in the terrain such as burial mounds, linear and circular earthworks, sunken roads, agricultural terraces, ridge and furrow fields, and more.
Lidar is captured remotely and can be used in inaccessible locations, such as areas that may be too deep, too steep, or behind blocked access. It can penetrate the dense canopy of a rainforest, mapping the terrain with accuracy. When compared to an archaeological excavation, creating a DEM with LiDAR is more efficient and less costly.
Global Mapper Pro is popular with terrain professionals and archaeologists. In the background of TV shows about discovering hidden archeology, you can often see Global Mapper being used to display and manipulate data. Check out our YouTube channel for interviews with real Global Mapper users on Using Lidar for Archaeological Research or the Ancient Mayan Pyramid at Uxmal.
Example Study Site
The archaeological site used as an example in this scenario is a recently discovered Mayan ruin named Aguada Fénix in Tabasco, Mexico. More information about the site itself and a link to the publicly available lidar data can be found here. The data is a high-resolution lidar survey conducted by the National Center for Airborne Laser Mapping (NCALM) and a low-resolution lidar survey by the Instituto Nacional de Estadística y Geografía (INEGI). Try it yourself with a free two week trial license of Global Mapper Pro!
Global Mapper Pro is an easy-to-use GIS software that excels in the processing, creation, and analysis of elevation data. This scenario will give a few examples of how Global Mapper can be used to generate a bare ground layer from lidar and apply a variety of visualization techniques to assist archaeologists in the search for potential archaeological features. Once the data is loaded in Global Mapper, you will be able to see what the vegetation has been hiding.
Processing Lidar to Focus on Important Features
Lidar classification is the first step in cleaning and processing point clouds. This removes erroneous noise points and gives the computer context for what it’s looking at in the data. Global Mapper’s lidar classification tools will work on just about any point cloud, including aerial lidar, terrestrial lidar, or photogrammetric point clouds.
Seeing the Ground Through the Vegetation
As Archaeologists are primarily interested in the ground and not recent vegetation, then only the ground points will need to be classified. The ground classification tool can identify ground points apart from vegetation and other features. Within the tool, a Height Departure setting lets you control how thick the layer of ground points is. This setting is especially useful in areas with grass, as it assists the tool in separating short vegetation apart from ground points. For more information on classifying ground points in Global Mapper, check out this blog.
Thinning Point Clouds without Losing Data Integrity
Depending on your sensor, collected lidar point clouds can be very dense, making lidar processing resource and time intensive. Global Mapper’s 3D Thin tool can be used to intelligently thin the data in a way that removes redundant points to maintain the data’s integrity and high resolution in places of high elevation change. For example, in a dataset that includes a flat field and a farmhouse, the field will be more severely thinned than the farmhouse because there is very little variation in the elevation of the field, making for more redundant points per area.
Creating the Bare Ground Surface
Once the lidar has been classified, the Create Elevation Grid tool can be used to easily interpolate the points into a solid surface layer. Digital Elevation Models (DEMs) are raster layers that represent the ground, omitting vegetation and building features. It works by overlaying a grid where each cell inherits a single value to represent the lidar in the area, and choosing the lowest point to represent each cell. This helps to ensure that the ground layer is laid bare without the interference of vegetation.
Using Global Mapper to highlight patterns in the terrain
The detection of potential archaeological features can be done visually with the assistance of shaders and illumination models or delineated with the assistance of filters and breaklines. Filters can be applied to the DEM to smooth out vegetation residual points or emphasize small variations in the terrain. Here are some examples of the many tools included in Global Mapper Pro to bring out features in your data.
Apply Hillshade Effects and Vertical Exaggeration
Hillshading, sometimes referred to as relief shading, provides an illustration of variations in elevation using artificial shadows. Hillshading is an effect that can be applied directly to a terrain layer displayed in Global Mapper without the need to create a new layer. The depth and direction of shadows can be set in the Dynamic Hillshading tool. Vertical exaggeration can also be used to emphasize small details and patterns in the terrain without altering elevation data.
A slope shader changes the value of cells in a DEM so that the cells now display the degree of slope instead of the Z or elevation value. By coloring the values with an inverted grayscale scheme, the final layer retains the relief representation while still communicating slope, independent of aspect. Any analysis that is done to a DEM in Global Mapper with the slope shader applied will use the slope values, including the raster to vector tools and Raster Classification.
One of the many ways filters can be used is to highlight and detect variation in elevation layers. Unlike hillshade, filters don’t rely on a specific light source or angle. Some filters have been designed with edge detection in mind, such as Laplacian and Sobel. These filters highlight cells whose neighbors have a different elevation, varying with significance and therefore ignoring flat areas. The Laplacian filter was specifically designed for local edge detection, as it emphasizes sharp anomalies.
A Gaussian filter can be used as a method of trend removal as it produces a smoother transition between features in the landscape. It works by creating a generalized DEM. More information on Convolution filters and how they work can be found in the Knowledge Base.
Breaklines are similar to contour lines, but instead of delineating changes in elevation, they delineate changes in slope. Changes in elevation can be visually distinguished by adjusting shaders, but manually digitizing them can prove to be time-consuming. The breakline tool can be used as a form of edge detection and as a digitizing method. For a demonstration of how breaklines work, check out this blog.
Filtering out Modern Land Uses and Structures
Satellite imagery, along with current and historical maps, can be layered on top of the DEM to highlight modern infrastructure, farming, and other more modern land uses. OSM data can be found in Global Mapper through the Online Data tool. This data includes digitized modern features such as gravel roads, airport runways, wetlands, etc. To section out areas of the DEM that represent recent construction, such as roads, vector data can be easily loaded from OpenStreet Map via the Online Data tool. Satellite imagery can also be loaded and rendered slightly transparent to compare against the DEM.
Image Display and Overlay
Transparency can be added to the imagery layer to display the terrain underneath, but this can sacrifice color intensity. Another option is to use the Texture Map option, found in the imagery’s Raster Options settings. See how an Image and a street map can be overlaid together on top of shaded texture from an elevation layer. This lets you compare modern roads and land use to patterns in the terrain. Lowering the color intensity of a layer can balance the display against the other layers.
GIS software offers a wealth of tools for terrain and morphometric analyses to assist archaeologists in finding previously unseen objects or structures in archaeological sites that have already been combed over. Visually interpreting the data still requires a human eye and background knowledge, but Global Mapper can make it easier to find and discern features and structures hidden in the terrain.
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Inomata, T., Triadan, D., Vázquez López, V.A. et al. Monumental architecture at Aguada Fénix and the rise of Maya civilization. Nature 582, 530–533 (2020). https://doi.org/10.1038/s41586-020-2343-4
Štular, B., Kokalj, Ž., Oštir, K., & Nuninger, L. (2012). Visualization of LIDAR-derived relief models for detection of archaeological features. Journal of Archaeological Science, 39(11), 3354–3360. https://doi.org/10.1016/j.jas.2012.05.029