Abstract: In this paper, two kinds of 3-point equations of computing divergence, usually used in synoptic diagnoses or derivation of physical quantity, are shown to produce equivalent results. Values of divergence given by the 3-point methods may be accompanied with significant errors with the same order of magnitude as observed in mesoscale systems. They are contributed by non-linear distribution of triangular domains of flow fields computed. It is inferred that the errors generate, disappear or displace themselves with corresponding change in the non-linear distribution.