Great Bay Estuary Eelgrass - 1994

Note that yearly variations in collection methods exist. Following is a summary of methods that have been used. The dataset shows the locations where eelgrass was observed in the Great Bay Estuary based on low-altitude aerial surveillance. The study area includes the following waterbodies: Great Bay, the mouth of the Squamscott River, the tidal portion of the Lamprey River, the tidal portion of the Oyster River, the tidal portion of the Bellamy River, the Piscataqua River and Portsmouth Harbor and its associated creeks. Aerial photographs are taken of the study area at 3000 feet at low spring tide with roughly 60% overlap on a calm day without preceding rain events and when the sun is at a low angle to minimize reflection (between 7 am and 10 am). Photographs are taken in late summer, usually late August or early September, depending on tides and weather, to reflect the maximum eelgrass annual biomass. 35 mm film (ASA 200) or a digital camera is used to acquire the images. Typically 300 images are needed to cover the entire Great Bay Estuary. The orientation of the photographs is near-vertical. This is a slight deviation from the NOAA Coastal Change Analysis Program protocol, but follows a published method (Short and Burdick, 1986). The photographs, in the form of 35mm slides or digital computer images, are projected on a screen and the eelgrass images are transferred to a base map. These maps are then digitized using GIS software. The eelgrass habitat mapped from the aerial imagery is verified using the ground truthing data by placing the ground-truthing locations onto the digital image using GIS software. Ground-truthing is done from a small boat during the same season as the photographs are taken. Ground-truth observations are made at low tide. Samples are collected with an eelgrass sampling hook. Positions are determined using GPS. The ground-truth surveys assess 10-20% of the eelgrass beds mapped in the estuary. The aerial survey is completed yearly. The first survey in the series was completed in 1986.需注意，本数据集的采集方法存在年度差异。以下为已采用的采集方法概述。本数据集基于低空航空遥感监测结果，记录了大湾河口（Great Bay Estuary）内观测到鳗草（eelgrass）的分布点位。本次研究的覆盖水域包括：大湾、斯夸姆斯科特河（Squamscott River）河口、兰普里河（Lamprey River）感潮段、奥斯特河（Oyster River）感潮段、贝拉米河（Bellamy River）感潮段、皮斯卡塔夸河（Piscataqua River）、朴茨茅斯港（Portsmouth Harbor）及其附属溪流水域。研究区域的航空摄影作业将在高度3000英尺的空域下，于大潮低潮时段开展，要求拍摄当日天气晴朗、无前期降雨，且太阳高度角较低以削弱水面反射（拍摄时段为每日7时至10时），影像旁向重叠度约为60%。摄影作业定于夏末开展，通常为8月下旬或9月上旬，具体时间需根据潮汐与天气条件调整，以确保捕捉到鳗草年度生物量的峰值时段。影像采集可采用35毫米胶片（感光度ASA 200）或数码相机。覆盖整个大湾河口通常需要约300张影像。航空影像采用近垂直拍摄角度，该方式与美国国家海洋和大气管理局（NOAA）海岸变化分析项目的规范略有出入，但遵循了Short与Burdick于1986年发表的标准方法。采集得到的35毫米幻灯片或数字计算机影像将被投射至屏幕，随后将鳗草分布影像转绘至底图上。随后使用地理信息系统（Geographic Information System, GIS）软件对上述地图进行数字化处理。通过航空影像解译得到的鳗草栖息地分布数据，将借助地理信息系统软件将野外核查点位叠加至数字影像中，以此完成精度验证。野外核查作业将与航空摄影同期开展，作业方式为乘坐小型船只进行。野外核查观测将在低潮时段进行。鳗草样本将使用专用鳗草采样钩进行采集。点位坐标将通过全球定位系统（Global Positioning System, GPS）测定。野外核查调查将覆盖河口内已解译鳗草床的10%至20%。本次航空调查为年度开展项目，系列调查的首次作业于1986年完成。