The algorithm of construction of the operator of a trend for acyclic function of two variables is offered. For such function its periodic continuation on each variable can be discontinuous function on a plane. The algorithm is used by the operator polynomial Hermite interlineation on the sides of a rectangle. The periodic component of analyzed function is offered to be approached operators Fourier or Hartley, exact on trigonometrical polynomials given the order. It allows to increase efficiency of processing of experimental data.