  Experimental diagnostics, analysis tools and simulations represent particle
distributions in various forms and coordinates. Algorithms to manage these data
are needed on platforms like the ITER Integrated Modelling & Analysis Suite
(IMAS), performing tasks such as archiving, modeling, conversion and
visualization. A method that accomplishes some of the required tasks for
distributions of charged particles with arbitrarily large magnetic drifts in
axisymmetric tokamak geometry is described here. Given a magnetic
configuration, we first construct a database of guiding center orbits, which
serves as a basis for representing particle distributions. The orbit database
contains the geometric information needed to perform conversions between
arbitrary coordinates, modeling tasks, and resonance analyses. Using that
database, an imported or newly modeled distribution is mapped to an exact
equilibrium, where the dimensionality is reduced to three constants of motion
(CoM). The orbit weight is uniquely given when the input is a true
distribution: one that measures the number of physical particles per unit of
phase space volume. Less ideal inputs, such as distributions estimated without
drifts, or models of particle sources, can also be processed. As an application
example, we reconstruct the drift-induced features of a distribution of
fusion-born alpha particles in a large tokamak, given only a birth profile,
which is not a function of the alpha's CoM. Repeated back-and-forth
transformations between CoM space and energy-pitch-cylinder coordinates are
performed for verification and as a proof of principle for IMAS.
