Great Bay Estuary Eelgrass - 1999

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。以下为所用采集方法的汇总说明。 本数据集基于低空航空监测结果，记录了大湾河口（Great Bay Estuary）内观测到的鳗草（eelgrass）分布点位。本研究的调查范围涵盖以下水域：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茨茅斯港及其附属支流。 航空摄影作业在3000英尺高度开展，拍摄时机选择在大潮低潮时段、天气晴朗且无前期降雨的条件下，同时需确保太阳角度较低以减少水面反射（拍摄时段为每日7时至10时），影像重叠率约为60%。摄影作业定于夏末开展，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峰值时段。 影像采集可采用ASA 200感光度的35mm胶片相机或数码相机。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）海岸变化分析项目的标准规程略有出入，但遵循已发表的研究方法（Short与Burdick，1986）。 采集的影像以35mm幻灯片或数字计算机图像形式存储，先投影至屏幕，再将鳗草影像转绘至纸质底图。随后借助地理信息系统（Geographic Information System, GIS）软件将底图数字化。通过航空影像绘制的鳗草生境分布图集，需借助GIS软件将地面验证（ground truthing）点位叠加至数字影像中，以此完成精度核验。 地面验证作业与航空摄影同期开展，作业载体为小型船只。地面验证观测需在低潮时段进行，样本采集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。地面验证调查覆盖河口内已绘制鳗草床的10%至20%。 航空监测每年开展一次，该系列调查的首次作业完成于1986年。