Orthomosaics and digital elevation model of the 'Bear Trap' - a Norse ruin in Northwest Greenland

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. Antiquity 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 Arctic Anthropology.

本数据集包含数字高程模型（digital elevation model, DEM）与正射影像镶嵌图（orthomosaic），所对应遗址为“熊阱”（丹麦语称“Bjørnefælden”，格陵兰语Kalaallisut称“Putdlagssuaq”或“大阱”），该遗址为格陵兰西北部努苏阿夸半岛西端的北欧人废墟。

本数据集所用影像为1032张由无人驾驶航空飞行器（DJI Phantom 3 Standard）采集的低空航拍图像。所有影像均遵循美国地质调查局（USGS）沿海地区影像处理规程，借助Agisoft Metashape Pro（v1.7；Linux Ubuntu系统）完成后期处理。

8个地面控制点（ground control points, GCPs）的位置由高精度全球导航卫星系统（global navigation satellite system, GNSS）接收机（Emlid Reach）实地测量确定。基站与流动站的观测数据通过Emlid发布的免费RTKLib软件完成后处理（处理教程详见：https://docs.emlid.com/reach/common/tutorials/gps-post-processing/）。大地水准面高程则通过UNAVCO在线大地水准面高程计算器计算得出（计算网址：https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html）。

在Metashape的影像对齐步骤中，本次处理采用“高精度”模式，将关键点与连接点的阈值分别设为60000与0，未启用通用预选与参考预选功能，而是通过渐进选择算法剔除超出投影精度、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对应影像中。

随后以“超高”质量模式生成密集点云，进而基于该点云生成数字高程模型与正射影像镶嵌图。本数据集生成的正射影像镶嵌图分辨率为1.83厘米/像素，同时还导出了分辨率为5厘米的正射影像镶嵌图与数字高程模型，以供QGIS软件调用。本数据集的配套README文件中提供了完整的文件清单。

本次影像测绘工作属于“Vaigat冰山-微生物石油降解与考古遗产调查”（VIMOA）项目的一部分，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，合作单位涵盖丹麦国家博物馆、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。本次测绘活动已提前从努克的格陵兰国家博物馆与档案馆获取正规考古许可。

Walsh等人（2020）对VIMOA项目期间开展的考古测绘工作进行了全面概述，Walsh等人（待刊）则详细介绍了2019年测绘中发现的“熊阱”遗址及周边考古遗存的具体细节。相关参考文献如下：
1. Walsh et al. (2020) The VIMOA project and archaeological heritage in the Nuussuaq Peninsula of north-west Greenland. Antiquity 94:e6 doi:10.15184/aqy.2019.230
2. 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 Arctic Anthropology.