This paper contains the technical foundations from stochastic differential geometry for the construction of geometrically intrinsic nonlinear recursive filters. A diffusion X on a manifold N is run for a time interval T, with a random initial condition. There is a single observation consisting of a nonlinear function of X(T), corrupted by noise, and with values in another manifold M. The noise covariance of X and the observation covariance themselves induce geometries on M and N, respectively. Using these geometries we compute approximate but coordinate-free formulas for the ``best estimate'' of X(T), given the observation, and its conditional variance. Calculations are based on use of Jacobi fields and of ``intrinsic location parameters'', a notion derived from the heat flow of harmonic mappings. When any nonlinearity is present, the resulting formulas are not the same as those for the continuous-discrete Extended Kalman Filter. A subsidiary result is a formula for computing approximately the ``exponential barycenter'' of a random variable S on a manifold, i.e. a point z such that the inverse image of S under the exponential map at z has mean zero in the tangent space at z.