Great Bay Estuary Eelgrass - 2007

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）内观测到的鳗草（eelgrass）分布位置。研究区域涵盖以下水域：大湾、斯夸姆斯科特河河口、兰普里河潮汐段、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。研究区域的航空摄影作业通常在春潮低潮时段开展，飞行高度设定为3000英尺，相邻影像重叠率约为60%；作业需选择无风且无前期降雨的天气，并在太阳高度角较低的时段（7:00至10:00之间）进行，以尽可能降低水面反射干扰。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。影像采集可采用感光度为ASA 200的35mm胶片，或数码相机。通常需要约300张影像即可覆盖整个大湾河口区域。航空影像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，但遵循了已发表的研究方法（Short与Burdick，1986）。采集得到的35mm胶片幻灯片或数字影像会被投射至屏幕，随后将鳗草分布影像转绘至底图上。随后利用地理信息系统（GIS）软件对转绘后的地图进行数字化处理。利用地理信息系统软件将地面核查点位叠加至数字影像中，以此前获取的地面核查数据对航空影像解译得到的鳗草栖息地分布进行验证。地面核查作业与航空摄影作业在同一季节开展，作业载体为小型船舶。地面核查观测需在低潮时段进行，鳗草样本采用鳗草采样钩进行采集，点位坐标通过全球定位系统（GPS）测定。地面核查调查覆盖河口内已解译鳗草床的10%至20%。本航空调查为年度例行工作，该系列调查的首次作业完成于1986年。