In this paper we assess the accuracy of the SMART-tomography technique. The approach involves the generation of synthetic oblique ionograms and a comparison of these with oblique incidence (OI) ionograms. The OIs were recorded at close to the time when total electron content data was collected on six UK paths which intersect the tomographic meridian plane. The OIs were generated by the "Improved Radio Ionospheric Sounder" (IRIS). This instrument incorporates GPS timing synchronisation to permit the measurement of absolute rather than relative group path delays.