Abstract: Based on data of daily air temperature and relative Humidity from 1961 to 1980 over Jianghusi Plain, the structure functions of the two elements have been calculated, and from these the relationships between the error of linear interpolation and distance have been established. According to the principle that the standard error of interpolation should not exceed the slanderd error of observation, the maximum admissible spacings between stations of the second group meteorological networks have been estimated over this area. Finally, it has been found that replacing interpolation along a segment by interpolation in a plane will increase the accuracy of interpolation.