INTRODUCTIONIn the past, geomagnetic field observations were recorded in analog form on photographic paper (Jankowski & Sucksdorff, 1996) with a silver bromide emulsion, known as bromide paper. Up to now, these analog magnetograms have been read only by hand scaling with low time resolution. Many observatories have analog records covering long observation periods (Iyemori, Nose, McCreadie, Odagi, Takeda, Kamei et al., 2005). At Kakioka Magnetic Observatory (KMO) in Japan, records on photographic paper of geomagnetic observations go back to 1924. However, most of the numerical data available from these magnetograms are hourly values that were digitized by hand scaling. Conversion of these analog records to high-resolution numerical data would make them very useful in investigations of past geomagnetic activity. We therefore developed a method for converting the analog magnetograms into digital data with high time and amplitude resolutions and then examined the conversion accuracy. OUTLINE OF THE CONVERSION METHODThe first step was to obtain high-resolution scans of the photographic paper records and to store them as graphic files. We then used an image processing program that we developed to distinguish lines and curves in the graphic files. The identified pixels were converted into numerical time and geomagnetic field data. Each step of the conversion is described in detail below. Photographic paper record and scan specificationsEach record is a sheet of photographic paper about 510 mm long by 195 mm wide (Figure 1 In our digitization method, we scanned the photographic records at a relative resolution of 600 pixels per inch, which corresponds to about 4,600 by 12,000 pixels per sheet. This resolution is equivalent to a time resolution of about 7.5 s per pixel and an amplitude resolution of about 0.1 nT (H and Z) and 0.01 minutes (D) per pixel. The scanned photographic records are stored as 24-bit color bitmap images so that each pixel can be weighted according to its luminosity during image processing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.