  Cosmic rays represent one of the most important energy transformation
processes of the universe. They bring information about the surrounding
universe, our galaxy, and very probably also the extragalactic space, at least
at the highest observed energies. More than one century after their discovery,
we have no definitive models yet about the origin, acceleration and propagation
processes of the radiation. The main reason is that there are still significant
discrepancies among the results obtained by different experiments located at
ground level, probably due to unknown systematic uncertainties affecting the
measurements. In this document, we will focus on the detection of galactic
cosmic rays from ground with air shower arrays up to 10$^{18}$ eV. The aim of
this paper is to discuss the conflicting results in the 10$^{15}$ eV energy
range and the perspectives to clarify the origin of the so-called `knee' in the
all-particle energy spectrum, crucial to give a solid basis for models up to
the end of the cosmic ray spectrum. We will provide elements useful to
understand the basic techniques used in reconstructing primary particle
characteristics (energy, mass, and arrival direction) from the ground, and to
show why indirect measurements are difficult and results are still conflicting.
