This paper describes approaches to measuring small- and medium-scale spatial rate of vertical TEC from GNSS observations. The horizontal scale size measured by small-scale TEC spatial rate (STSR, in TECU/km), including its latitude and longitude components, ranges from about 1.8 km to 15 km at 450 km altitude. The medium-scale TEC gradient components can be derived from global ionospheric map (GIM) based TEC gradient (GBTG) over about 110 km horizontally at low and middle latitudes, with smaller scale lengths in the longitude component in the polar region. Our analyses of GPS data show that the latitude component of STSR and GBTG is much larger than the longitude component. The spatial rates in most regions of the globe are relatively small, and their absolute mean values are mostly under 0.03 TECU/km. However, STSR is substantially larger, often greater than 0.05 TECU/km, in the equatorial ionospheric anomaly (EIA) region than in other regions, or when ionospheric disturbances occur. The disturbances include ionospheric irregularities, significant changes of regional ionospheric state during space weather events, and traveling ionospheric disturbances. Combined global TEC spatial rate (CGTR) (also named combined global TEC gradient, CGTG) with STSR and GBTG is also shown in this paper as snapshots using data from networks of more than three thousand GNSS receivers. The comparisons between the mean STSR and GBTG in each bin indicate that their regional variation patterns are consistent though there are subtle differences between them particularly in the equatorial anomaly region or during ionospheric disturbances particularly during space weather events. The difference is at least partially attributed to the fact that the GBTG smooths out small-scale variations particularly in regions where spatial variations are large.
