Tomographic techniques are applied to relative total electron content (TEC) data obtained using radio signals transmitted by the polar orbiting Navy Navigation Satellite System (NNSS) and received at a chain of six stations located at approximately 121¿E longitude. Images reconstructed for each satellite passage provide a picture of electron density over a 25¿ latitude range and to an altitude of about 1000 km at a longitude of 121¿E. Over 350 ionospheric images of the northern equatorial anomaly region have been reconstructed for October/November 1994. The daytime equatorial anomaly crest is a prominent feature of the reconstructed images. The rms difference between foF2 computed from tomographic images and foF2 measured by two ionosondes is found to be less than 12%. Causes for the differences are discussed. We show how the crest develops on average days in October and November and discuss the nature of seasonal variations. The fully developed anomaly core is aligned approximately along geomagnetic field lines, resulting in the existence of strong and directional gradients in the anomaly region. As a consequence, the crest latitude and the maximum vertical TEC (VTEC) latitude are noncoincident. The tilt of the anomaly core also causes the slant TEC integrated through the crest to be highly directional. In fact, the use of spherical stratification in predicting slant TEC based on the knowledge of the vertical TEC may give an error as large as 30 to 50% in some directions. Implications for GPS ranging measurements are discussed.