The relationship of geomagnetic field at the earth's surface with S
qe current systems is analysed. It is pointed out that there exsist some differences between the true focus of the S
qe current system and the "apparent focus" determined by using S
qe variations in the geomagnetic field at the earth's surface. Practical approaches to estimating S
qe current focus by means of geomagnetic data are discussed and examined by using geomagnetic data during the period of IGY/C.