The Chilean government has historically used 2D surveying and mapping technology to inventory, monitor, and map archeological sites on Rapa Nui.
The process typically begins with taking a single point at each archeological site. All relevant attribute data is then associated with the point. Hand drawn sketches are also created, scanned, and archived. The points gathered in the field are then downloaded to ArcView where 2D digital maps are produced. By taking a single point per site, the numerous sites on the island can be surveyed and mapped quickly. However, the disadvantage of this method is that the sites can’t be modeled accurately—the maps produced are 2D, are only accurate to 1 meter in the x and y axes, and have much worse accuracy in the z axis.
To my good fortune, as you already know, local officials welcomed me to Rapa Nui to demonstrate the advantages that a 3D, survey grade (+/- 1cm) process could provide.
Topsoil, silt rich run off draining to the sea in Hanga Roa
Their need to monitor both the erosion of the land and the archeological sites requires 3D models. The topography, the ahu (platforms), and the moai (statues), etc. are all affected by the elements, time, and tourism. Using Autodesk Civil 3D 2007 and Trimble GPS gear, I created survey grade, georeferenced, 3D models of two sites, and then overlaid these models on the base map of the entire island that I also created. As additional models are gathered, they too may be overlaid over existing models to monitor change over time. LIDAR (Light Detection and Ranging) technology is capable of sub-millimeter accuracy and is well suited to this task. More on that in a later post.
3D DTMs (Digital Terrain Models) are commonly used to study drainage, erosion, and slope analysis. Furthermore, they are ideally suited for 3D mapping of sites and the artifacts they contain—a traditionally 2D, manual process.
Surveying Ahu Tepeu. Note the erosion to the fallen moai
Survey data may be seamlessly imported and exported from any GPS system. In a standardized environment where a survey field coding standard (a layer standard that determines colors, symbols, text styles, etc.) is used, linework, symbology, and attribute data can all be imported directly from the survey equipment and the drawing/models can create themselves automatically.
This 3D process has significant advantages over the less accurate 2D process. By its very nature, a 3D model provides visual clues that a 2D map cannot. By adding the 3rd dimension, items like profiles, cross sections, hydrology, slope and elevation analysis can all be completed.
3D, 1cm accurate DTM of Ahu Tepeu
Ahu Tepeu - Elevation Study in Civil 3D 2007. The darker hues indicate the highest elevations
3D capability meets several significant needs on Rapa Nui. Increasing population, tourism, agriculture, and land development in such a fragile and archeologically-rich environment must ultimately be monitored, regulated, and controlled. A process for locating and archiving sites and then analyzing the effects of time, weather, tourism, agriculture, livestock, and land development on these sites can only be effective by adopting 3D mapping technologies. Furthermore, this technology can be used to educate the Rapa Nuians on just what a unique and beautiful home they have, how important it is to study, and ultimately to preserve.
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