The Shuttle Radar Topography Mission (SRTM) digital elevation dataset was collected by a modified radar system that flew on board the Space Shuttle Endeavor in February 2000, using a technique known as Interferometric Synthetic Aperture Radar
The 3DEP DEMs are updated continuall
Yes, lidar data are represented as a cloud of variably spaced point locations (x,y,z) of any terrestrial target reflecting the laser pulse.
Specific data for average (or mean) county elevations for the United States are not presently available from the USGS. You can extract an elevation for the centroid of each county polygon using Esri ArcGIS software (see procedures below).
Specific data for current mountain peak elevations do not exist at USGS other than what was previously published in an Elevations and Distances in the United States booklet.
National Elevation Dataset (NED) Release Notes were produced through February 2014 to provide details about bimonthly releases of the National Elevation Dataset (now referred to as 3DEP seamless DEMs).
All 3DEP seamless DEMs are provided in geographic coordinates (longitude and latitude) in units of decimal degrees, horizontally referenced on the North American Datum of 1983.