Discrepancies exist between vertically measured ionospheric total electron content (TEC) and slant measurements of TEC that are converted to vertical with the use of a mapping function. Vertical measurements of TEC that are determined by the TOPEX altimeter are compared with equivalent vertical TEC values that are derived from the Global Positioning System (GPS) constellation at latitudes -40¿ to +40¿ and longitudes 180¿ to 360¿ during periods in 1993, 1994, and 1995. Also, comparisons are made with the Phillips Laboratory parameterized ionospheric model (PIM) predictions of vertical and equivalent vertical TEC from the same observation points. A trend of disagreement in maximum and minimum TEC values is observed between TOPEX and GPS passes that involve measurements within 20¿ to the south and to the north of the geomagnetic equator. PIM model predictions, although not exact in value, are consistent in configuration with these observations of overestimation as well as underestimation of TEC. It is shown that the errors are dependent on not only elevation angle but also azimuth of the line-of-sight direction. The elevation mapping function that relates the line-of-sight TEC to vertical TEC and other assumptions that are made in the application of the ionospheric shell model may be contributing factors to the slant-to-vertical conversion errors.¿ 1997 American Geophysical Union