Great Bay Estuary Eelgrass - 2014

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。以下为所用采集方法的汇总说明。 本数据集基于低空航空监测结果，标注了大湾河口（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、斯夸姆斯科特河河口、兰普雷河感潮河段、奥斯特河感潮河段、贝拉米河感潮河段、皮斯卡塔夸河以及朴次茅斯港及其附属支流。 航空摄影作业在大潮低潮时段开展，飞行高度设定为3000英尺，要求拍摄当日天气平稳、无前期降雨，且太阳角度较低（需在早7时至10时之间）以减少水面反光，相邻影像重叠率约为60%。摄影作业安排在夏末时节，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。覆盖整个大湾河口通常需要约300张影像。影像采用近垂直视角拍摄，该设置与美国国家海洋和大气管理局（NOAA）海岸变化分析项目的规程略有偏差，但遵循了已发表的研究方法（Short与Burdick，1986）。 以35毫米幻灯片或数字计算机影像形式存储的航拍照片将被投影至屏幕，随后将鳗草影像转绘至底图上。随后利用地理信息系统（GIS）软件对上述转绘图进行数字化处理。 通过地理信息系统软件将实地核查（ground truthing）点位叠加至数字化影像中，以此利用实地核查数据对航拍影像绘制的鳗草栖息地分布进行验证。实地核查作业与航拍作业同季开展，作业平台为小型船舶，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。 实地核查调查覆盖河口内已绘制鳗草床的10%至20%。本系列航空调查每年开展一次，首次作业完成于1986年。