After analyzing the difference between the geomagnetic total field F recorded by manual mode and automatic mode instruments, and comparing the difference between the geomagnetic total field F which recorded by two automatic magnetic instruments, the significant annual variation can be found in the difference in same observatory. The similar characteristic also can be found when the analysis applied to other observatories. These mean that the difference of geomagnetic field between the two different points in the same area changes with time. Also, the same methods were applied to the hour mean value, the annual variation also can be found, but this feature did not change a lot with the hour increasing in one day. This implies that this difference can not be reduced by choosing the time of the absolute observation. Therefore, the geomagnetic total field F with manual mode is very necessary during the absolute observation.