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Helmert transform (HT) and singular spectrum analysis (SSA) in one-dimension variant were used by author and his colleagues in their works [1, 2, 3, 4, 5]. This article closes the series. Beside the TEC map analysis the article contains the comparison of Helmert transform (HT) and two-dimensional SSA (2dSSA). It should be noted that latter of the methods needs much more calculations than the rst one. That is why determination of the optimal values of 2dSSA window parameters as of independent significance. All les containing TEC maps in IONEX format have been taken from NASA server1 for six successive solstices and equinoxes: 22/09/2014, 22/12/2014, 21/03/2015, 20/06/2015, 22/09/2015, 22/12/2015. Very short information about the data sources is given in Table 1. Di erent les contain di erent amount of maps calculated for di erent time moments inside the day. The only maps equally distributed during the day with two hour interval were used. It means 13 maps per day per le. All the maps have identical dimensions: 71 rows (latitude step is 2.5◦) and 72 columns (longitude step is 5◦). The second column of Table 1 contains six symbols. Each of them is to be used for the single date from the just speci ed date list. Their meanings are: (+) the maps of the le were used for comparison, (−) the maps were rejected during steps of comparison, (0) there was no le for the date. Each data processing centre uses di erent methods to build TEC map: di erent amount of satellites, stations, smoothing, combination, forecasting or just present raw data. None of those di erences were taken into account during the analysis. We just used the maps as they were provided. That is why in Table 1 we have presented the data from le headers only with minimum comments. It should be taken in mind that combined solutions Igrg and Igsg for di erent dates were created with di erent composing source maps.

Formal estimation of the random component in global maps of total electron content