The agonic line, characterized by zero values of geomagnetic declination, has had a westward drift during the last centuries, crossing the location of the Real Observatorio de Madrid at the end of the year 2021. This fact, which was monitored by the Instituto Geográfico Nacional, moves us to study the evolution of the magnetic declination in this emblematic emplacement between the last two crosses of the agonic line. Our results point out that the current westward drift started around the year 1810 and, before this period, the agonic line moved from west to east, crossing the location of the Real Observatorio around 1650–1675.

The agonic line, representing geomagnetic declinations of 0°, recently crossed the Royal Observatory of Madrid (ROM) in December 2021, causing a shift in declination values from west to east. This event constitutes a notable milestone for this significant place, where the first geomagnetic observation series in Spain commenced around 1855. Consequently, a thorough investigation into the historical evolution of the declination has been undertaken to decipher prior occurrences of the agonic line crossing at the ROM. Despite the ROM hosted the first series of geomagnetic measurements in Spain, the present lack of geomagnetic measurements in this observatory makes necessary to extend the declination measurements to other observatories distributed throughout the Iberian Peninsula to better define the passage of the agonic line since 1855 up to the present. For periods prior to 1855, a bibliographic search for declination measurements conducted in the Iberian Peninsula has been carried out, complemented by historical data from the HISTMAG database. As a result, a time-continuous curve of geomagnetic declination is generated from 1590 to 2021 at the ROM coordinates. The declination curve reveals that the agonic line also crossed the ROM 400 years ago (around 1600) passing from west to east declination values.

The agonic line, which represents geomagnetic declinations of 0°, recently crossed the Royal Observatory of Madrid (ROM) in December 2021, causing a shift in declination values from west to east. This event constitutes a notable milestone for this significant observatory, where the first geomagnetic observation series commenced around 1855 in Spain. In this work, taking advantage of the occurrence of this event, a detailed study has been conducted to investigate the historical evolution of the magnetic declination at the ROM to decipher prior occurrences of the agonic line crossing this place. Despite the ROM having hosted the first series of geomagnetic measurements in Spain, the present lack of geomagnetic measurements in this observatory makes it necessary to extend the declination measurements to other observatories distributed throughout the Iberian Peninsula to better define the passage of the agonic line since 1855 up to the present. For epochs prior to 1855, a bibliographic search for declination measurements conducted in the Iberian Peninsula has been carried out, complemented by historical data from the HISTMAG database. As a result, a time-continuous curve of geomagnetic declination is generated from 1590 to 2022 at the ROM coordinates. The declination curve reveals that the agonic line also crossed the ROM 400 years ago (around 1600), passing from western to eastern declination values.

In 1870, the newly created Instituto Geográfico of Spain, the precursor of the current Instituto Geográfico Nacional (IGN), began to carry out work for the preparation of the National Topographic Map of Spain, a major project that would take almost 100 years to be completed. This work began with the measurement of geodetic bases throughout the national territory. Subsequently, the necessary topographic surveys were conducted to delimit the boundaries of municipalities, and to represent all the planimetric elements. As a part of this, surveys were carried out with topographic compasses, which allowed work to be performed at a good pace and with sufficient accuracy for cartographic purposes. The current IGN keeps in its archives all the documentation generated in the work related to the completion of this major project. The objective of this study is to extract information from this documentation on the magnetic declination measured at that time, and to evaluate it as a possible source of historic geomagnetic information for use in future works. To achieve this, we compared the recovered declination dataset with those generated for the same locations and dates using two independent sources: the Cov-Obs.x2 geomagnetic field model, which spans the last two centuries, and the declination data used to produce the first Spanish declination chart developed by the IGN at the beginning of the 20th century. The results show a clear agreement between the recovered dataset and both sources of independent declination data, suggesting that this dataset is valuable for improving our understanding of the recent geomagnetic field history and for refining main field models for the last centuries.