The USGS DEM standard is a geospatial file format developed by the United States Geological Survey for storing a raster-based digital elevation model.
Floating-point numbers are encoded using Fortran scientific notation, so C/C++ programs need to swap the "D" exponent-indicating character with "E" when parsing (and vice versa when writing).
One of the key items is the quadrangle, which is a set of four terrestrial coordinates describing the four-sided polygon enclosing the area of interest.
Since measurements within UTM employ fixed distances (e.g., 30 meters between elevation samples), the quadrangle must slightly distort to map such locations onto the spherical Earth.
This distortion usually manifests as a rotated square, hence the elevation columns near the east and west edges start more northward and contain fewer samples.