Contours are not projected, but are provided in geographic coordinates (latitude and longitude) in units of decimal degrees, and horizontally referenced to the North American Datum of 1983.
The 100-foot contours were derived from 3DEP (formerly National Elevation Dataset) 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 one arc-second data.
Contours cover the conterminous United States at small (578K/289K), medium (144K/72K), and large (36K/18K) scales.
The 500-foot contours are shown at a scale of 578K in base maps, while 100-foot contours are visible at 289K/144K and 50-foot contours are visible at 72K.  Large scale contours from US Topo products are shown from 36K to 18K, and in dynamic base map se
Large scale contours from US Topo products are included in an ArcGIS REST service endpoint or
Contours will be updated on an as-needed basis. When significant changes in the landscape have occurred (such as mountain top removal mining in Kentucky) and are represented by new elevation data, only those areas will be replaced.
Yes, depression contours are identified with tick marks, but only in large scale contours from 36K to 18K.
A low-pass filter was applied, which provides the basis for obtaining smoother contours.
Contours with an elevation of zero were deleted, unless there were also contours with negative elevation values in the same area.
Contour Shapefiles have elevation data stored in the "CONTOURELE" attribute field. To preserve elevations, map the attribute to the elevation of the surface in Civil 3D through an "Alter Properties" query.