Orthomosaics and digital elevation model of the 'Bear Trap' - a Norse ruin in Northwest Greenland
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This dataset consists of a digital elevation model (DEM) and an orthomosaic of the ‘Bear Trap’ (also called ‘Bjørnefælden’ in Danish, and ‘Putdlagssuaq’ or ‘The Great Trap’ Greenlandic Kalaallisut), a Norse ruin at the western end of the Nuussuaq Peninsula in NW Greenland. Images comprise 1032 low-altitude aerial images acquired from an unoccupied aerial vehicle (DJI Phantom 3 Standard). These images were processed using Agisoft Metashape Pro (v1.7; Linux Ubuntu) following the USGS protocols for processing imagery in coastal areas. The locations of 8 ground control points (GCPs) were surveyed with a high accuracy global navigation satellite system (GNSS) receiver (Emlid Reach). The base station and rover data were processed using the Emlid distribution of the free RTKLib software (https://docs.emlid.com/reach/common/tutorials/gps-post-processing/). Geoid height was computed using the online UNAVCO Geoid Height Calculator (https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html). During the image alignment step in Metashape, the ‘High’ accuracy setting and key point and tie point limits of 60000 and 0 were used. Generic and reference preselection were not used. Gradual selection was used to remove tie points that exceeded thresholds for the projection accuracy, reconstruction uncertainty, and reprojection error and the intrinsic camera parameters were computed for each camera calibration group. GCPs were then imported and placed in each image. The dense point cloud was then computed using the ‘Ultra High’ quality setting, followed by the DEM and orthomosaic. The resolution of the orthomosaic is 1.83 cm/pixel. 5 cm resolution orthomosaic and DEM were also exported for use in QGIS. A complete file list is provided in the README file that accompanies this dataset. The image survey was conducted as part of the Vaigat Iceberg-Microbial Oil Degradation and Archaeological Heritage Investigation (VIMOA) project, which was funded by the Danish Centre for Marine Research and supported by the Arctic Research Centre at Aarhus University, in affiliation with the National Museum of Denmark, the Greenland Institute of Natural Resources, and The Greenland National Museum and Archives in Nuuk. Proper archaeological permits for the survey were obtained in advance from the Greenland National Museum and Archives in Nuuk. Walsh et al. (2020) provide an overview of the archaeological surveys conducted during the VIMOA project and Walsh et al. (in prep) provide further details specific to The Bear Trap and surrounding archaeological contexts observed during the 2019 survey. Walsh et al. (2020) The VIMOA project and archaeological heritage in the Nuussuaq Peninsula of north-west Greenland. <em>Antiquity</em> 94:e6 doi:10.15184/aqy.2019.230 Walsh, Matthew J., Daniel F. Carlson, Pelle Tejsner, and Steffen Thomsen. The Bear Trap: Reinvestigating a unique stone structure on the northwest tip of the Nuussuaq Peninsula, Greenland. Submitted to <em>Arctic Anthropology</em>.
本数据集包含数字高程模型 (digital elevation model, DEM) 与“熊陷阱”（丹麦语称“Bjørnefælden”，格陵兰语卡拉利苏特语称“Putdlagssuaq”或“大陷阱”）的正射镶嵌影像。该遗迹为位于格陵兰西北部努萨苏克半岛西端的北欧人废墟。 所用影像为来自大疆幻影3标准型（DJI Phantom 3 Standard）的1032张低空航拍图像。这些影像按照美国地质调查局 (United States Geological Survey, USGS) 的沿海地区影像处理规程，使用Agisoft Metashape Pro（v1.7；Linux Ubuntu系统）完成处理。 8个地面控制点 (ground control points, GCPs) 的位置通过高精度全球导航卫星系统 (global navigation satellite system, GNSS) 接收机（Emlid Reach）进行勘测。基站与流动站数据采用Emlid分发的免费RTKLib软件进行处理（https://docs.emlid.com/reach/common/tutorials/gps-post-processing/）。大地水准面高程则通过在线UNAVCO大地水准面高程计算器计算（https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html）。 在Metashape的影像对齐阶段，采用“高”精度设置，关键点与连接点限制分别设为60000和0，未启用通用预选与参考预选功能，而是通过渐进选择移除超出投影精度、重建不确定性与重投影误差阈值的连接点，并为每个相机校准组计算相机内参。随后导入地面控制点并将其匹配至各张影像中。 使用“超高”质量设置生成密集点云，进而构建数字高程模型与正射镶嵌影像。本正射镶嵌影像的分辨率为1.83厘米/像素，同时还导出了分辨率为5厘米的正射镶嵌影像与数字高程模型，以供QGIS使用。 数据集附带的README文件中提供了完整的文件列表。本次影像勘测是“Vaigat冰山-微生物石油降解与考古遗产调查”（Vaigat Iceberg-Microbial Oil Degradation and Archaeological Heritage Investigation, VIMOA）项目的组成部分，该项目由丹麦海洋研究中心资助，奥胡斯大学北极研究中心提供支持，合作单位包括丹麦国家博物馆、格陵兰自然资源研究所以及位于努克的格陵兰国家博物馆与档案馆。本次勘测已提前从努克的格陵兰国家博物馆与档案馆获得正式考古许可。 Walsh等人（2020）对VIMOA项目期间开展的考古勘测工作进行了综述，Walsh等人（待刊）则针对2019年勘测中发现的“熊陷阱”及周边考古背景提供了详细细节。相关文献如下： Walsh et al. (2020) The VIMOA project and archaeological heritage in the Nuussuaq Peninsula of north-west Greenland. <em>Antiquity</em> 94:e6 doi:10.15184/aqy.2019.230 Walsh, Matthew J., Daniel F. Carlson, Pelle Tejsner, and Steffen Thomsen. The Bear Trap: Reinvestigating a unique stone structure on the northwest tip of the Nuussuaq Peninsula, Greenland. Submitted to <em>Arctic Anthropology</em>.
