  We show that measuring the trajectories of charged particles to finite
accuracy leads to the commutation relations needed for the derivation of the
free space Maxwell equations using the {\it discrete ordered calculus} (DOC).
We note that the finite step length derivation of the discrete difference
version of the single particle Dirac equation implies the discrete version of
the p,q commutation relations for a free particle. We speculate that a careful
operational analysis of the change in momenta occurring in a step-wise
continuous solution of the discrete Dirac equation could supply the missing
source-sink terms in the DOC derivation of the Maxwell equations, and lead to a
finite and discrete (``renormalized'') quantum electrodynamics (QED).
