Great Bay Estuary Eelgrass - 2008

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.请注意，数据采集方法存在年度差异。以下为所采用采集方法的概要说明。 本数据集基于低空航空监测结果，展示了大湾河口（Great Bay Estuary）内观测到鳗草（eelgrass）的分布点位。 研究区域涵盖以下水体：大湾、斯匡斯科特河（Squamscott River）河口、兰普雷河（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）以及朴次茅斯港（Portsmouth Harbor）及其附属溪流水域。 研究区域的航空摄影作业在春潮低潮时段开展，飞行高度设定为3000英尺，影像重叠度约为60%；作业需选择无风且无前期降雨的天气，且需控制太阳高度角较低（于每日7时至10时之间）以弱化水面反射。 航空摄影选在夏末时节，通常为8月下旬或9月上旬，具体时间需根据潮汐与天气情况调整，以确保捕获鳗草年度生物量的峰值时段。 影像采集可采用35毫米胶片（ASA 200）或数码相机完成。完整覆盖大湾河口通常需要约300张航空影像。 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程略有偏差，但遵循了已发表的研究方法（Short与Burdick，1986）。 采集得到的35毫米幻灯片或数字计算机格式影像将被投影至屏幕，随后将鳗草分布影像转绘至底图之上。随后利用地理信息系统（GIS）软件将转绘完成的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校核点位匹配至数字影像中，以此完成精度验证。野外校核作业与航空摄影同期开展，作业载体为小型船只，观测需在低潮时段进行。鳗草样本采用专用采样钩采集，点位坐标通过全球定位系统（GPS）测定。 野外校核调查覆盖河口内已解译鳗草床的10%至20%。本航空调查每年开展一次，系列调查的首次作业完成于1986年。