6k USGS Quadrangle Boundaries

Most USGS topographic maps use brown contours to show the shape and elevation of the terrain. Contour intervals vary, depending mainly on the type of terrain and the scale of the map. (Scale is the relationship between distance on the map and distance on the ground.) Quarter quadrangles cover an area measuring 3.75 minutes longitude by 3.75 minutes latitude. Area of each cell will vary depending on latitude and longitude.绝大多数美国地质调查局（United States Geological Survey, USGS）出品的地形图均采用棕色等高线来呈现地形的形态与海拔高程。等高距并非固定统一，其大小主要取决于地形类型与地图比例尺。（比例尺指地图上的距离与实地地面距离之间的比例关系。）四分之一图幅（quarter quadrangles）的覆盖范围为经度3.75角分 × 纬度3.75角分。每个栅格单元的面积会随经纬度位置的差异而有所不同。