Drainage Lines (Loudoun County, VA)

Drainage is a base map data layer derived by automated processes and processed for cartographic representation at 1:2400 scale. The line features represent the any visible surface drainage feature, natural or manmade, as seen from above and are mapped to National Map Accuracy Standards (NMAS). Drainage arcs must logically follow topo contours downhill and are not disconnected when traveling through culverts, under bridges or representing the centerline of double-line streams and ponds. Drain arcs will be captured according to proper orientation or direction of flow. Photogrammetrically derived drainage DOES includes known underground culvert and pipe connections with stormwater infrastructure points and other visible surface outlets. This drainage layer DOES NOT include subsurface groundwater flow, fine soil drain patterns in swales, or any other historical surface drains.排水要素图层（Drainage）是通过自动化流程生成的基础地图数据图层，经处理后可用于1:2400比例尺的制图表达。该图层的线要素代表从空中视角可见的各类地表排水设施（包括天然与人工设施），并遵循《国家地图精度标准（National Map Accuracy Standards，NMAS）》进行制图。排水弧段（Drainage arcs）需严格沿地形等高线向下坡方向延伸，在穿越涵洞、桥下区域，或作为双线溪流与池塘的中心线时，不得出现逻辑断连。排水弧段的采集需符合正确的走向与水流方向。经摄影测量生成的排水要素确实包含已知的地下涵洞、与雨水基础设施节点相连的管线，以及其他可见的地表出水口。本排水图层不包含地下地下水流动、洼地内的细小土壤排水纹理，以及其他各类历史地表排水设施。