What sources of elevation data were used to create contours?

The 500- and 100-foot contours were derived from the National Elevation Dataset (NED) one arc-second resolution data, which was sub-sampled to a cell size of three arc-second.  The 50-foot contours were also derived from NED one arc-second data.  Large scale contours were derived from the NED 1/3 arc-second resolution data for US Topo products.