Elevation

There are a number of Mac software packages that will read elevation files. Five that came up in a quick Web search are:
The NED Point Query Service uses the National Elevation Dataset (NED) 1/3 arc-second layer from the U.S. Geological Survey (USGS) as its source.
All elevation data from the 3D Elevation Program are in the public domain and may be used without restriction.
Yes. To maintain consistency, the NED data distributed from USGS are only produced on NAD83 with a geographic coordinate system. Users can project the NED data to a different datum or projection using most GIS software after downloading the data.
The USGS distributes source DEMs, received after March 2014, in their original project resolution (OPR) and horizontal coordinate system.
Elevations are centered to each pixel in raster datasets, which are used by most GIS applications.
There are frequently multiple sources of elevation data that cover a given location. Preference is given to that which is the most detailed and up-to-date, unless there are overriding quality concerns.
Areas which have been updated for all resolutions of the NED are viewable within the NED Updates layers of The National Map viewer.