Great Bay Estuary Eelgrass - 1997

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监测结果，标注了大湾河口（Great Bay Estuary）内观测到鳗草（eelgrass）的区域。研究区域涵盖以下水域：大湾、斯匡斯科特河（Squamscott River）河口、兰普雷河潮汐段、奥斯特河潮汐段、贝拉米河潮汐段、皮斯卡塔夸河以及朴茨茅斯港及其附属支流。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，拍摄当日需天气晴朗且无前期降雨，太阳高度角较低以降低水面反射（拍摄时段为上午7时至10时），影像重叠率约为60%。摄影作业选在夏末时节，通常为8月下旬或9月上旬，具体时间根据潮汐与天气情况调整，以捕捉鳗草年度生物量峰值时段的状态。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。摄影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（National Oceanic and Atmospheric Administration, NOAA）海岸变化分析项目规程略有差异，但遵循已发表的方法（Short与Burdick，1986）。将35毫米幻灯片或数字计算机格式的影像投射至屏幕，再将其中的鳗草影像转绘至底图上。随后通过地理信息系统（Geographic Information System, GIS）软件将上述底图数字化。通过地理信息系统软件将实地验证点位叠加至数字化影像，以此对航空影像解译得到的鳗草生境分布图进行验证。实地验证作业与航空摄影同期开展，作业方式为乘小型船只开展，且需在低潮时段进行观测。采样工作采用鳗草采样钩完成，点位坐标通过全球定位系统（Global Positioning System, GPS）获取。实地验证调查覆盖河口内已解译鳗草草床的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。