Great Bay Estuary Eelgrass - 2011

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，现将所采用的采集方法概述如下：本数据集基于低空航空监测结果，记录了大湾河口（Great Bay Estuary）内鳗草（eelgrass）的观测分布点位。研究区域涵盖以下水域：大湾、斯夸姆斯科特河河口、兰普里河潮汐河段、牡蛎河潮汐河段、贝拉米河潮汐河段、皮斯卡塔夸河、朴茨茅斯港及其附属溪流水域。 研究区域的航空摄影作业均在大潮低潮时段开展，飞行高度设定为3000英尺，拍摄时要求天气晴朗且无前期降雨，太阳高度角较低以减少水面反射（拍摄时段为每日7时至10时之间），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，具体时间需结合潮汐与天气条件确定，以确保捕捉到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，或使用数码相机完成。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（National Oceanic and Atmospheric Administration, 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）。将35mm幻灯片或数字计算机格式的航空影像投射至屏幕，将其中的鳗草影像转绘至底图，随后使用地理信息系统（Geographic Information System, GIS）软件对该底图进行数字化处理。通过地理信息系统软件将地面校验（ground truthing）点位叠加至数字化影像中，利用地面校验数据对航空影像绘制的鳗草生境分布图进行验证。 地面校验作业与航空摄影同期开展，均在相同季节进行，作业平台为小型船舶，且观测需在低潮时段完成。采样工作采用鳗草采样钩完成，采样点位通过全球定位系统（Global Positioning System, GPS）确定。地面校验调查会覆盖河口内已绘制鳗草床的10%至20%区域。该航空监测每年开展一次，该系列的首次调查完成于1986年。