A comparison is given of the latitude of the last closed field line and its dependence on various parameters, as determined from the Mead-Fairfield model and from particle measurements on the Isis 2 satellite. The analysis involves the determination of the latitude at which, on the average, electron energies are a minimum and the identification of this latitude with the average position of the high-latitude limit of closed field lines. At local noon the closed field line limit as given by the model is in good agreement with the measurements except for large negative values of the tilt angle of the earth's axis, and here the measured tilt angle dependence is smaller than is predicted by the model. However, the measurements show a significant dependence of the latitude of the last closed field line on local time, which the model does not allow for.