Abstract: 1024 magnetopause crossing data from 1963 to 1979 are used to calculate the ellipsoid of the dayside magnetopause surface.An attempt is made to search for the effects of the interplanetary magnetic field and solar wind thermal pressure on the position and shape of the dayside magnetopause.The uncertainties in the determination of the parameters of the ellipsoid from various sources are analyzed.Both numerical results and theoretical analyses show that the accuracy of the data available is not adequate enough to see the effects.Some suggestions are proposed for further work.Observations of the average values of the size,shape and orientation of the dayside magnetopause agree quite well with the theoretical prediction.