Look here for information on National Map Elevation data.  To explore and download elevation data visit this webpage.
There are a number of Mac programs that will read elevation files. A few open source programs are:
The Point Query Service interpolates elevations from the 1/3 arc-second DEM. Interpreted spot elevations are not official and do not represent precisely measured ground surveyed values.
All 3DEP products available through The National Map are in the public domain and may be used without restriction. The USGS does request that proper credit be given when used for redistribution, resale, presentation or publication.
Yes. To maintain consistency, original seamless elevation data distributed from USGS are only produced on NAD83 with a geographic coordinate system.
The USGS distributes source DEMs, received after March 2014, in their original project resolution (OPR) and horizontal coordinate system.
Elevations are pixel-centered in raster datasets, which are used by most GIS applications.
There are frequently multiple sources of elevation data held by the USGS that cover a given geographic location.
Availability graphics for all 3DEP products can be found on the Highlights and Product Availability Web page.