Regular counts of walruses (Odobenus rosmarus) across their pan‐Arctic range are necessary to determine accurate population trends and in turn understand how current rapid changes in their habitat, such as sea ice loss, are impacting them. However, surveying a region as vast and remote as the Arctic with vessels or aircraft is a formidable logistical challenge, limiting the frequency and spatial coverage of field surveys. An alternative methodology involving very high‐resolution (VHR) satellite imagery has proven to be a useful tool to detect walruses, but the feasibility of accurately counting individuals has not been addressed. Here, we compare walrus counts obtained from a VHR WorldView‐3 satellite image, with a simultaneous ground count obtained using a remotely piloted aircraft system (RPAS). We estimated the accuracy of the walrus counts depending on (1) the spatial resolution of the VHR satellite imagery, providing the same WorldView‐3 image to assessors at three different spatial resolutions (i.e., 50, 30 and 15 cm per pixel) and (2) the level of expertise of the assessors (experts vs. a mixed level of experience – representative of citizen scientists). This latter aspect of the study is important to the efficiency and outcomes of the global assessment programme because there are citizen science campaigns inviting the public to count walruses in VHR satellite imagery. There were 73 walruses in our RPAS ‘control’ image. Our results show that walruses were under‐counted in VHR satellite imagery at all spatial resolutions and across all levels of assessor expertise. Counts from the VHR satellite imagery with 30 cm spatial resolution were the most accurate and least variable across levels of expertise. This was a successful first attempt at validating VHR counts with near‐simultaneous, in situ, data but further assessments are required for walrus aggregations with different densities and configurations, on different substrates.

中文翻译：

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太空中的海象：同步遥控飞机系统中的海象计数与超高分辨率卫星图像的比较


定期对泛北极地区的海象（Odobenus rosmarus）进行计数对于确定准确的种群趋势至关重要，进而了解海象栖息地当前的快速变化（例如海冰消失）如何影响它们。然而，利用船只或飞机对北极这样广阔而偏远的地区进行调查是一项艰巨的后勤挑战，限制了实地调查的频率和空间覆盖范围。事实证明，涉及超高分辨率（VHR）卫星图像的替代方法是检测海象的有用工具，但准确计数个体的可行性尚未得到解决。在这里，我们将从 VHR WorldView‐3 卫星图像获得的海象数量与使用遥控飞机系统 (RPAS) 获得的同步地面数量进行比较。我们估计海象计数的准确性取决于 (1) VHR 卫星图像的空间分辨率，以三种不同的空间分辨率（即每像素 50、30 和 15 厘米）向评估员提供相同的 WorldView-3 图像和（ 2) 评估员的专业水平（专家与混合经验水平——公民科学家的代表）。研究的后一个方面对于全球评估计划的效率和结果非常重要，因为有公民科学活动邀请公众在 VHR 卫星图像中计数海象。我们的 RPAS“对照”图像中有 73 头海象。我们的结果表明，在所有空间分辨率和所有评估人员专业知识水平的 VHR 卫星图像中，海象的数量都被低估了。具有 30 厘米空间分辨率的 VHR 卫星图像的计数在不同专业水平中是最准确且变化最小的。 这是使用近乎同步的原位数据验证 VHR 计数的成功首次尝试，但需要对不同基质上不同密度和配置的海象聚集进行进一步评估。