Image of the 4-m Sun-illuminated Topography of the Sea Floor off Eastern Cape Cod (CAPENORTHSUN_GEO4M_WGS84.TIF, Geographic, WGS84)

This data set includes sun-illuminated of the sea floor offshore of eastern Cape Cod, Massachusetts. The data were collected with a multibeam sea floor mapping system during USGS survey 98015, conducted November 9 - 25, 1998. The surveys were conducted using a Simrad EM 1000 multibeam echo sounder mounted aboard the Canadian Coast Guard vessel Frederick G. Creed. This multibeam system utilizes 60 electronically aimed receive beams spaced at intervals of 2.5 degrees that insonify a strip of sea floor up to 7.5 times the water depth (swath width of 100 to 200 m within the survey area). The horizontal resolution of the beam on the sea floor is approximately 10% of the water depth. Vertical resolution is approximately 1 percent of the water depth.
The sun-illuminated topographic (shaded relief) image has a 4-m pixel size and was created by vertically exaggerating the topography two times and then artificially illuminating the relief by a light source positioned 45 degrees above the horizon from an azimuth of 0 degrees. In the resulting image, topographic features are enhanced by strong illumination on the northwestward-facing slopes and by shadows cast on southeastern slopes. The image also accentuates small features (relief of a few meters) that could not be effectively shown as contours alone at this scale. Unnatural-looking features or patterns oriented parallel or perpendicular to survey tracklines are artifacts of data collection and environmental conditions.
The data have a weak striping that runs parallel to the ship's track. Some of the striping is the result of poor data return at nadir that appears as evenly-spaced thin speckled lines. Some striping is also due to critical angle effects, where the intensity of return varies as a function of the angle of incidence of the incoming sound on the seafloor (Hughes-Clark and others, 1997).

本数据集涵盖美国马萨诸塞州科德角以东近海海底的日照晕渲地形图像。
该数据由美国地质调查局（USGS）于1998年11月9日至25日开展的编号为98015的科考航次中，通过多波束海底测绘系统采集获取。本次航次搭载安装于加拿大海岸警卫队“弗雷德里克·G·克里德”号（Frederick G. Creed）科考船上的Simrad EM 1000型多波束回声测深仪完成数据采集。
该多波束系统配备60个电子定向接收波束，波束间距为2.5度，可声呐扫测宽度达7.5倍水深的海底条带（本航次调查区的条带宽度为100至200米）。海底波束的水平分辨率约为对应水深的10%，垂直分辨率约为对应水深的1%。

本次生成的日照晕渲地形图像像素尺寸为4米，制作时先将地形垂直夸大2倍，随后采用方位角为0°、位于地平线以上45°的人工光源对地形晕渲效果进行光照处理。在最终生成的图像中，朝向西北的坡面将被强光照亮，朝向东南的坡面则会形成阴影，以此强化地形特征的可视效果。该图像还可凸显出在此比例尺下仅靠等高线无法有效呈现的小型地形特征（高差仅为数米）。与测线平行或垂直分布的非自然特征或图案，均属于数据采集过程及环境条件引发的观测伪影。

该数据存在与航迹平行的弱条带噪声。部分条带噪声源于天底方向的低数据回波，表现为间距均匀的细斑点状线条。另有部分条带噪声由临界角效应导致：当入射声波在海底的入射角发生变化时，回波强度也会随之改变（Hughes-Clark等，1997）。