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 data from the U.S. Geological Survey (USGS) as its source.
All NED data are in the public domain and may be used without restriction. We do request that proper credit be given when used for redistribution, resale, presentation or publication.
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.
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.
The Data Source Index map shows areas which have been updated for all resolutions of the NED and is viewable at the NED Data Source Index within The National Map Viewer.
The NED is derived from diverse source data that are processed to a common coordinate system and unit of vertical measure. All NED elevation values are in meters.