Corrected hourly values from Eskdalemuir observatory and the implications for studies on the long term changes in geomagnetic activity

The UK observatory hourly values published in yearbooks were digitised in the 1970s, using data entry by groups of prisoners. These were made available for research purposes via the WDC for geomagnetism. It has recently been discovered [1] that the hourly values from Eskdalemuir (ESK) observatory for years 1911 to 1931 are 2-hour running means, instead of the original hourly values published in the yearbooks (see Fig 2). Unfortunately there was no record of the original digitised data. This filtering was carried out to centre the data at 30 minutes past each hour instead of at 0 minutes, for homogeneity with the data from 1932 onwards. It may also have been the belief at the time, that the raw data from the yearbooks were spot values rather than estimated mean values, and averaging would have been appropriate. In any case the original raw digitised data should also have been stored. Following this discovery a campaign to re-digitise the ESK yearbook data began.