When significant changes in the landscape have occurred, contours will be updated on an as-needed basis.
The 100-foot contours were derived from 3DEP (formerly National Elevation Dataset) one arc-second resolution data that was sub-sampled to a cell size of three arc-second. The 50-foot contours were also derived from one arc-second data. Large-scale contours were derived from 1/3 arc-second resolution data for US Topo products.
The seamless digital elevation models (DEMs) used to create the contour lines are enhanced as we collect lidar over the US through the 3DEP program.