![]() In Greenwich, London (UK) an initial dedicated manual observation programme was set up in 1838, with continuous recording instrumentation on photo-sensitive paper beginning less than a decade later (Brooke & Airy, 1847). A legacy of this crusade was the establishment of many permanent geomagnetic observatories, some of which have lasted in one form or another for almost two centuries. In 1832, Carl Frederik Gauss invented a method of measuring total field intensity (Garland, 1979) and in this era of exploration, a great ‘magnetic crusade’ was embarked upon to understand the Earth's magnetic field by making measurements around the world (Cawood, 1979 Collier, 2014 Sabine, 1849). Prior to the 1830's there were no absolute measurements of the strength of the field though relative variations between sites could be deduced (Enebakk, 2014). ![]() Declination measurements from the 16th century exist in some locations (Alexandrescu et al., 1996 Barraclough et al., 2000 Malin et al., 1981) and from the era of exploration and sail, between the 15th and 20th centuries, high-quality ship navigation records have been used to constrain the shape of the Earth's magnetic field back to 1590 (Jackson et al., 2000). As with climate records, geomagnetism has a very long history of observations available. Campaigns to digitize temperature or climate-related measurements have been very successful especially with the recruitment of keen citizen scientists to help manually extract numbers from old or distressed paper records where optical character recognition technology struggles (Ryan et al., 2021 Skrynyk et al., 2021). The conversion of analogue records to digital values is highly advantageous for example, allowing modern computational techniques and analysis to be applied. In many areas of geophysical study, long time series of measurements exist in analogue form on photographic paper, in journals or as published tables. These include cross-checking the final digitized values with the recorded hourly mean values from observatory year books and comparing several observatory records for the same storm to catch errors such as sign inversions or incorrect ‘wrap-around’ of data on the paper records. We discuss our approach to digitizing the traces from large geomagnetic storms and highlight some of the issues to be aware of when capturing magnetic information from analogue measurements. However, converting the traces to digital values is difficult and time consuming as the magnetograms can have over-lapping lines, low quality recordings and obscure metadata for conversion to SI units. ![]() ![]() Around 350,000 magnetograms have been digitally photographed at high resolution. Over the past two centuries, eight observatories have existed in the United Kingdom, which measured the daily field variations using light-sensitive photographic paper to produce analogue magnetograms. Continuous geomagnetic records of the strength and direction of the Earth's field at the surface extend back to the 1840s. ![]()
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