Global ionospheric mapping (GIM) is a new and emerging technique for determining global ionospheric TEC (total electron content) based on measurements from a worldwide network of Global Positioning System (GPS) receivers. In this study, GIM accuracy in specifying TEC is investigated by comparison with direct ionospheric measurements from the TOPEX altimeter. A climatological model (Bent model) is also used to compare with the TOPEX altimeter data. We find that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model. The difference between GIM and TOPEX in TEC measurements is very small (less than 1.5 TEC units (TECU)) within a 1500-km range from a reference GPS station. The RMS gradually increases with increasing distance from the station, while the Bent model shows a constant large RMS, unrelated to any station location. Within a 1000-km distance of a GPS site (elevation angle >25¿), GIM has a good correlation (R>0.93) to TOPEX with respect to TEC measurements. The slope of the linear fitting line to the data set from two TOPEX cycles is 44.5¿ (near the ideal 45¿). In the northern hemispheric regions, ionospheric specification by GIM appears to be accurate to within 3--10 TECU up to 2000+km away from nearest GPS station (corresponding to ~1¿ elevation angle cutoff). Beyond 2000 km, GIM accuracy, on average, is reduced to the Bent model levels. In the equatorial region, the Bent model predictions are systematically lower (~5.0 TECU) than TOPEX values and often show a saturation at large TEC values. During ionospheric disturbed periods, GIM sometimes shows differences from TOPEX values due to transient variations of the ionosphere. Such problems may be improved by the continuous addition of new GPS stations in data-sparse regions. Thus, over a GPS station's measurement realm (up to 2000 km in radius), GIM can produce generally accurate TEC values. Through a spatial and temporal extrapolation of GPS-derived TEC measurements, the GIM technique provides a powerful tool for monitoring global ionospheric features in near real time.¿ 1997 American Geophysical Union