Great Bay Estuary Eelgrass - 1996

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。以下为所采用采集方法的概要说明。 本数据集记录了大湾河口（Great Bay Estuary）经低空航空监测发现的鳗草（eelgrass）分布点位。研究区域涵盖以下水体：大湾（Great Bay）、斯阔姆斯科特河（Squamscott River）河口、兰普雷河（Lamprey River）潮汐段、奥斯特河（Oyster River）潮汐段、贝拉米河（Bellamy River）潮汐段、皮斯卡塔夸河（Piscataqua River）以及朴次茅斯港（Portsmouth Harbor）及其附属支流。 研究区域的航空摄影作业将在大潮低潮时段、风力平稳且无前期降雨的条件下开展，拍摄时太阳高度角较低（7时至10时之间）以减少水面反射，拍摄高度为3000英尺，影像重叠率约为60%。摄影作业安排在夏末时段，通常为8月下旬或9月上旬，具体需根据潮汐与天气情况调整，以捕捉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感光度）或数码相机完成。通常需拍摄约300张影像以覆盖整个大湾河口区域。航空照片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差异，但遵循已发表的研究方法（Short与Burdick，1986）。 将35毫米幻灯片或数字计算机影像形式的航空照片投射至屏幕，将其中的鳗草影像转绘至底图，随后通过地理信息系统（GIS）软件将该底图数字化。通过航空影像绘制的鳗草生境分布图，将通过地面验证数据进行校核：利用GIS软件将地面验证点位叠加至数字影像中完成验证。 地面验证作业与航空摄影同期开展，依托小型船只完成。实地观测需在低潮时段进行，采用鳗草采样钩采集样本，点位坐标通过全球定位系统（GPS）测定。地面验证调查将覆盖河口内已绘制鳗草床的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。