In order to achieve a coherent and homogeneous Umkehr data set, several controls were performed on the data deposited at the WOUDC (World Ozone and UV Data Centre) archive in Toronto:
In the Umkehr method, the measurement correspond to the variation of the ratio of two zenith-sky UV intensities at selected wavelengths with solar zenith angle (SZA) changing from 60 degrees to 90 degrees. A thorough review of the individual readings from the Dobson instruments for all the SZA was performed, with in parallel, the re- examination of the calibration records of all Dobson stations made during past inter-comparisons and total ozone re-evaluations. A number of "shifts" in the individual SZA data were identified and suggestions for homogenisation of the original raw data accounting for the various "shifts" were made.
In addition, the total ozone records were re-evaluated and a rigid quality control of the Umkehr ozone profiles was applied: from over 60,000 Umkehr profiles deposited initially in the WOUDC archive, 40,000 were finally selected for the REVUE statistical and long-term analysis.