  We lay out a general framework for calculating the variation of a set of
cosmological observables, down the past null cone of an arbitrarily placed
observer, in a given arbitrary inhomogeneous metric. The observables include
redshift, proper motions, area distance and redshift-space density. Of
particular interest are observables that are zero in the spherically symmetric
case, such as proper motions. The algorithm is based on the null geodesic
equation and the geodesic deviation equation, and it is tailored to creating a
practical numerical implementation. The algorithm provides a method for
tracking which light rays connect moving objects to the observer at successive
times. Our algorithm is applied to the particular case of the Szekeres metric.
A numerical implementation has been created and some results will be presented
in a subsequent paper. Future work will explore the range of possibilities.
