Great Bay Estuary Eelgrass - 1998

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，本数据集的采集方法存在年度差异。以下为所采用采集方法的汇总说明。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）的分布位置。研究区域涵盖以下水体：大湾、斯夸姆斯科特河（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）及其附属支流。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，拍摄时需确保天气晴朗且无前期降雨，太阳高度角较低以减少水面反射，拍摄时段为上午7时至10时之间，且影像重叠率约为60%。拍摄时段选在夏末，通常为8月下旬或9月上旬，具体依潮汐与天气条件而定，以确保捕捉到鳗草年度生物量的峰值。采用35毫米胶片（ASA 200感光度）或数码相机进行影像采集。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，该设置与美国国家海洋和大气管理局（NOAA）海岸变化分析项目规程略有偏差，但遵循了已发表的研究方法（Short与Burdick，1986）。将35毫米胶片幻灯片或数字计算机格式的影像投影至屏幕，将其中的鳗草分布区域转绘至底图，随后通过地理信息系统（GIS）软件将底图数字化。通过地理信息系统软件将地面验证点位叠加至数字化影像，以此对照航空影像绘制的鳗草生境分布，完成精度验证。实地验证工作与航空摄影同期开展，作业载体为小型船只，观测时段选择低潮时刻。采用鳗草采样钩采集样本，采样点位通过全球定位系统（GPS）确定。实地验证调查覆盖河口内已绘制鳗草床的10%至20%区域。该航空调查每年开展一次，系列调查的首次作业完成于1986年。