Svalgaard & Schatten 2015, "backbone" group number : description.
New group number reconstruction based on the "backbone observer" method. This series is the very first revision of the original sunspot group series.
In order to avoid the continuous daisy chaining of k personal coefficients used in the original group number series, successive long-duration observers were used as "backbone" references, to which all overlapping observers were scaled, both forward and backward in time. The partly-overlapping backbone sub-series were then scaled relative to each other. The unit scale is provided by the numbers from A.Wolfer (1877-1925). Moreover, in order to avoid the bias induced by the Greenwich photographic data, this reference was replaced by multiple visual observers, keeping a consistent calibration scheme from 1749 to 2015. The extension from 1995 to 2014 mainly used the group counts contained in the SILSO database.
In addition to the main 1749-2015 backbone reconstruction, the scale factor for the late 18th century (Staudacher) was simply applied to all early group counts before 1749, thus assuming that this scale factor remains valid, although the sparse data do not allow to determine it directly. On the other hand, all values after 1946 were uniformly reduced by 7%, based on the assumption that all observers changed their group splitting practices following the introduction of the standard Zürich morphological classification of sunspot groups. Some aspects of this new series may thus be revised based on deeper validation of the included hypotheses.
Given the adopted calculation method (yearly averages), only yearly mean values were obtained in this case.