ITALIAN MAGNETIC NETWORK AND GEOMAGNETIC FIELD
MAPS OF ITALY AT YEAR 2010.0
Authors….
G. Dominici, A. Meloni,…….
M. Pierozzi, M. Sperti……
Summary.- in the frame of a collaboration between Istituto Nazionale di Geofisica e Vulcanologia and Istituto
Geografico Militare, a survey of 131 repeat stations of the Italian Magnetic Network (including 2 Observatories, 11
stations in Albania, 3 stations in Corsica and 1in Malta ) was carried out between 2009 and 2010 with the main purpose
of elaborating magnetic cartography. In this paper we describe the characteristics of magnetic first and second order
network, the magnetic measurements and reduction procedure. We report the new repeat station data list reduced at
hour 02UT for the day of measure and at 2010.0; we determined, for all elements, the coefficients for normal fields at
2010.0 and for secular variations in the period 2005-2010. The new maps for Italy, for D, F, H and Z at the year 2010.0,
drawn with graphic contouring programs shortly explains, are encloses to the paper.
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1) INTRODUCTION
A long tradition of magnetic measurements, allowed Italy to acquire a rich patrimony of data in the
discipline of terrestrial magnetism (see for example Cafarella et al., 1992a, 1992b). If we exclude
measurements undertaken at sea, for navigation purposes, the first documented magnetic
measurements in Italy can date back to the beginning of XVII century. Those measurements refer to
the city of Rome and were made by Hartmann. If we limit ourselves only to those observations
undertaken on networks and conceived for the knowledge of spatial distribution of the magnetic
field, we can refer to Table 1 in which a brief synthesis of all magnetic network surveys undertaken
in Italy since 1640 is shown. In Table 1 all measured elements are reported and it clearly appears
that for the most ancient measurements, magnetic declination is the only measurable magnetic
quantity. The number of measured stations appears with the names of the corresponding survey
authors, in parenthesis the measurements on second order networks (see below), are also shown.
The role of magnetic networks is, in the first place, to allow the realization of magnetic cartography
in which the values of the field, as expressed by the field elements values and their temporal
variation, identified with the term secular variation, are reported on a map. Starting from the end of
nineteen hundred 80s, the National institute of Geophysics and Volcanology (INGV) has
systematically undertaken the task of making measurements of the Earth’s magnetic field in Italy.
These measurements are repeated regularly, every 5 years, on a series of points called repeat
stations of the Italian Magnetic Network. Repeat stations are distributed on the national territory,
not only to determine the spatial variation of the elements of the field, but also to define the secular
variation of the field in the different geographical zones. The latest modern magnetic surveys made
in Italy are reported in Molina et al., 1985, Meloni et al., 1988, Meloni et al., 1994, Coticchia et al.,
2002.
Survey results are collected and used in the international community by scientific organizations
such as IAGA (International Association of Geomagnetism and Aeronomy) for the determination of
the geomagnetic field at the global level and for its analytical synthesis in the form of spherical
harmonic analysis (IGRF), the so called International Geomagnetic Reference Field. The field has
in fact origin in the Earth’s fluid core and shows, at the Earth’s surface, wavelengths greater than
3000 km; for this reason only a global study allows to fully understand its physical characteristics.
The magnetic modelling allows also to infer the characteristics of the complex system of electric
currents that flow in the core generating the field. Data coming from national network surveys also
allow to determine the global secular variation; this normally shows spatial characteristics larger
than national dimensions (at least on the European scale). The secular variation pattern on the
global scale is attributed to the variability of the current systems flowing in the Earth’s core. The
IGRF set of coefficients is updated every five years, the last revision, the tenth, is reported in
McMillan and Maus (2005). In the national cartography aspects of local relevance can emerge, as
for instance the space-time variations of small wavelength, that cannot be brought up in detail on
the global scale. These features are generally not related to the Earth’s core currents and can be due,
for instance, to local anomalies of the subsoil electric conductivity, or to time variations of the rocks
magnetization. For a general information on the Earth’s magnetic field and related scientific matters,
the reader can consult, for example the following texts: Merrill et al, 1996, Campbell, 2003, Lanza
and Meloni, 2006.
At the national and local scales, magnetic measurements can be addressed to the determination of
the so called normal field, an analytical expression in latitude and longitude that best interpolates,
for the region of interest, the longest wavelength part of the measured geomagnetic field. Such a
normal field doesn't have a well defined physical meaning, since it can contain deep crustal origin
contributions superimposed to the core origin field; it has however the general purpose to furnish an
analytical level, to which, for difference, the local magnetic anomalies can be derived. The
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magnetic anomalies are used to resolve the crustal contribution to the magnetic field. Recently new
elaborations of the crustal total field anomalies were made for Italy; they can be found in Chiappini
et al (2000), for what concerns sea level anomalies, and in Caratori Tontini (2002) et al., for what
concerns aeromagnetic measurements.
In the international community the coordination of all magnetic operations is very important for
sharing operational standards, defining common times of realization and to exchange scientific
improvements. A coordination of European networks was recently created, MAGNET (acronym of
Magnetic Network in Europe), to which Italy has adhered putting its results in common with other
European nations.
2) ITALIAN MAGNETIC FIRST AND SECOND ORDER NETWORKS
The characteristics of a national magnetic network, as for instance the average distance among its
repeat stations, reflects the need to represent, in an accurate way, the spatial variation and the
secular variation of the magnetic field. In Italy, both for tradition and to preserve a detail that can
tag along the peculiar geometric shape of our peninsula and islands, a grid of around 110 points
called the first order repeat stations, with an average spacing around 55-60 km, is in operation.
The magnetic characteristics of the Italian territory to a detail that allowed the compilation of an
appropriate scale cartography for the identification of the crustal magnetic contribution, were the
object of an effort that dates back to the end of the 80s of nineteen hundred. Within the Finalized
Project Geodinamica (PFG-CNR), the measurement of horizontal H, vertical Z and total field F, on
2552 stations distributed in homogeneous way, was completed. These stations, constituting the socalled second order network, has been since then, the basis of the national magnetic cartography.
For what concerns magnetic declination D, the second order network was born earlier than the PFGCNR started. 1529 points of measurement were in fact determined in the 30s of nineteenth hundred
by the Military Geographical Institute (IGM). For military demands, in fact, a network was
established for declination; information on this can be found in Talamo, 1975, in which the latest
survey completely realized by the IGM is reported.
Because of secular variation, a regular updating of all magnetic elements is necessary. New
measurements are made on repeat stations every 5 years, on average. These measurements are
however repeated only on repeat stations, i.e, the first order network. This means that not all points
of the of the second order network are reoccupied, but in practice only a selection of those points.
This is possible, without loosing accuracy, since the secular variation wavelengths proceed on
scales of a very large extent, much larger than what is required for the definition of the crustal field.
The second order network points are therefore updated on the basis of the values of the secular
variation pattern, as derived by the latest measurements on the repeat stations only. Determination
of secular variation is obtained by taking the difference between results from successive repeat
station occupations. The recent 2005 survey measured values allow the determination of secular
variation, that in turn, is used for updating all second order network points for finally realizing the
new maps of all the magnetic elements. In figures 1 and 2 all grid points of measurement for the
second order network is reported.
The 2005 survey, to which in this publication we explicitly refer, started in September 2004 and
was concluded in November 2005. In this period of time the INGV has made measurements on the
national repeat station network and also repeated some measurements on 11 repeat stations in
Albania, in collaboration with the National academy of the Sciences of Albania and with the
university of Tirana, on 3 repeat stations in Corsica, thanks to the information furnished by the
Institute de Physique du globe de Paris (IPGP), and made 2 new measurements in the islands of
Elba and Caprera. For the 1994 first survey in Albania please refer to Chiappini et al 1997. All
measurements have been included in the various calculations, for the secular variation computation,
the cartography, updating and for the definition of the normal fields (see later), so contributing to a
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best characterization particularly of the Ionian and Tyrrhenian areas. The resultant average density
of repeat stations for the 2005 magnetic network is of around 1 station on 2800 km2, with an
average distance among the stations of around 55 km. Such stations density can be considered
elevated in comparison to the other European magnetic networks, nevertheless the number and the
distribution of the stations is also finalized to deepen several scientific aspects, such as the
geodynamical evolution of the Mediterranean.
Survey operation in magnetic repeat stations every 5 years, can let some problems to arise.
Sometimes a magnetic repeat station could not be located anymore in a suitable magnetically quite
area, or, for example, simply the magnetic mark that enables the operator to relocate the station is
not found. When this, or other, occurs a new station closely located to old one must be realized. The
choice of the location of the new stations requires to go after the following criterions internationally
defined:
- the station must be located in a magnetically quite place, far away from electrified railway lines,
lines of high tension, urban installations and generally man-made iron rich artefacts;
- the magnetic gradient in the area must be smaller than 3 nT/m; this requires generally a check
made with various magnetic profiles in the area of interest, also carrying out measures of the
vertical gradient;
-The identification of geodetic azimuths must carefully be made for the computation of declination,
choosing reference marks that are expected to be clearly visible also in the future.
For every new station a monograph is realized. In the monograph the site is described, instructions
are given to easily reach it, what land marks are useful for the approach, what azimuths are visible
and all those information that could be useful for identification. For other details on site selections
please refer to Newitt et al. 1996.
3) MAGNETIC MEASUREMENTS
Also in this 2005.0 survey a DIM (Declination Inclination Magnetometer), a portable theodolite
fitted with a fluxgate magnetometer, was used for the angular measurements of the Earth’s
magnetic field (therefore the declination D and the inclination I) and a nuclear precession
magnetometer for the measurement of the field intensity F. Al other elements can be derived. The
same instruments are regularly used for the measurements in the Observatory and they have,
therefore, high intrinsic precisions. We recall however that, because of field measurement
operations, the precision on the final values has an error greater than what possible to obtain in the
Observatory. Sources of error can be a consequence of ample variations of temperature in the period
of measurement, or because in the field the theodolite is positioned on a tripod, rather than on a
more solid pillar, as in the ordinary procedure in observatory, etc. Newitt et al., (1996) have
appraised that, for the averages latitudes, the typical errors for the elements observed in magnetic
networks are: 20" for D, 1.5 nT for H, 1.0 nT for Z, 0.5 nT for F. Also for the results of this 2005
Italian survey therefore we can refer to the reported values.
For the azimuth determination in newly constituted stations, as well as on those in which the
visibility of the old azimuth marks is limited or absent, a gyroscopic theodolite was used. This
instrument was preferred Global Positioning System (GPS) instruments, since it can also be used
for measurements in galleries or roof covered places, or even where, in presence of bushes of tall
stem, the GPS could show problems for the satellites limited visibility. Moreover the two
instruments show, on the overall, comparable precisions of tenth of minutes.
As a rule measurements are performed in series of 10, of which 5 in the morning 5 in the afternoon,
far, therefore, from the maximum magnetic filed elements diurnal variation. Moreover in order to
reduce the error due to the diurnal variation of the field, that normally has the effect to vary the
ordinary level of the field around the nighttimes average value, the 02UT (Universal Time or time
taken to the meridian of longitude 0 of Greenwich) hours was selected as the time to which all data
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are finally reduced to. For what concerns, in general, the knowledge of the more rapid time
variations, both for the diurnal variation and for possible irregular perturbations, the two
Observatories of the INGV, L’Aquila, in central Italy, and Castello Tesino, in northeast Italy are
used. Moreover during the survey for selected areas, other temporary magnetic time recording
stations, favourably displaced for having nearly a total coverage of the Italian territory, were
installed for a needed time. In fact time recording stations far away from quoted permanent
observatories were sensing a slight different rapid time variation.
The position of the Italian magnetic repeat stations are represented in figure 3 together with the
Observatories and temporary stations. In this image the areas of 'competence' of the observatory or
temporary station, located in the centre of the single circle, is marked to indicate the area considered
suitable for the time variation corrections.
For what concerns the data reduction procedures the following was used. Magnetic elements
observed at the repeat stations are reduced firstly to 02UT for the diurnal variation correction. The
value of element E (i.e., D, I or F) at station s, reduced i.e., at 02UT of day of measure d, was
calculated following the:
Es (02UT ) d  Evar (02UT ) d  Es (t ) d  Evar (t ) d 
Where:
E s (02UT ) d = Value of E at station s reduced at time 02UT of day of measure d
E var (02UT ) d = Value of E at variometer station s at time 02UT of day of measure d
E s (t ) d = Value of element E observed at station s at time t at that day d
E var (t ) d = Value of element E at variometer station at time t at that day d
In Table 2 all repeat stations data and all Earth’s magnetic field element values at 02UT of the
measurement day are reported. The Element E computed at epoch 2005.0 (January 1st 2005) is
obtained following the:
E s (2005.0)  Eobs (2005.0)  E s (02UT ) d  Eobs (02UT ) d 
Where:
E s (2005.0)  Value of E at station s reduced at epoch 2005.0
Eobs (2005.0) = average Value of E at epoch 2005.0
Eobs (02UT ) d = Value of E at Observatory at time 02UT of day of measure d
In Table 3 all magnetic values for all elements at January 1st 2005 (2005.0) for all measured repeat
stations, are reported. This table is the final survey result.
4) NORMAL FIELDS AND CARTOGRAPHY
For what concerns the normal field computation for the Italian region, an analytical expression of
second order degree polynomial type, in latitude and longitude, was chosen. For a more general
information on the regional normal fields, see for instance Haines, 1990 and for the Italian area see
also De Santis et al, 1997. For the concept of equivalent degree of Bullard (see Bullard, 1967) and
for the dimensions of the Italian region, the proposed normal field can represent the characteristics
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of the field up to 700 km wave-lengths, allowing us to consider that the analytical expression
reproduces with good approximation the main field with origin in the Earth’s core.
A generic magnetic element is therefore calculated through the following expression:
E  a 0  a1  a 2   a3 2  a 4 2  a5
The a0 , a1 , a 2 , a3 , a4 , a5 coefficients have been determined through the least squares method on the
basis of the repeat stations values reduced to the epoch 2005.0 and to the sea level. The reduction to
the sea level has been made considering only the Earth’s centred dipole contribution, according to
the equation:
E 
3Eh
R
where:
E= generic geomagnetic element
h = station elevation a.s.l.
R = average Erath radius
An iteration procedure was made to eliminate the stations located in anomalous areas from the
coefficient computation and the Chauvenet ‘principle of exclusion’ has been used. The obtained
normal field has been denominated INGRF2005 (ING Reference Field for 2005)
Such normal field, although of easy use, following what above said, doesn't have a physical
meaning but it can only be considered mathematically equivalent to other field synthesis, like
spherical harmonic analyses, for limited regions. For geophysical applications Molina and De
Santis, 1987 introduced a different normal field synthesis. The approach is very similar, a
polynomial expression is calculated now on the basis of grid data in the interested region using the
IGRF value on a 12°x11° grid centered on Italy. The time variation is determined by the annual
averages of the L’Aquila observatory; the model is called ITGRF2005 (ITalian Geomagnetic
Reference Field, 2005). Comparing the two fields, it emerges that the differences among the two
models, for the various components, are not particularly relevant (fig.4).
A second degree polynomial in latitude and longitude has been used for computing secular variation
for the period 2000-2005; by means of this model all the second order network points were
adjourned and reduced to 2005.0.
In the enclosure, four maps of the Earth’s magnetic field to the epoch 2005.0, for the total field
horizontal and vertical components and declination can be found. Maps were made by means of
contour programs on the updated 2552 measurements for F,H and Z and 1529 measurements for D.
Following an agreement between INGV and IGM also in this edition the map version is proposed to
the scale 1/2,000,000 for all magnetic elements.
5 – GEOMAGNETIC FIELD INFORMATION SYSTEM
6 –AKNOWLEDGEMENTS
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Also for this 2005.0 edition of Magnetic maps for Italy, we would like to express our sincere
gratitude to Prof Boschi, President of INGV for his help, suggestions and encouragement…
7- TABLE AND FIGURE CAPTIONS
Table 1, History of Magnetic Surveys in Italy with the magnetic elements together with the number
of repeat stations (in parenthesis the II order stations) and the staff and institutions that have made
the observations: UCM=Ufficio Centrale Meteorologia, IGM=Istituto Geografico Militare,
ING=Istituto Nazionale di Geofisica, PFG=Progetto Finalizzato Geodinamica, INGV=Istituto
Nazionale di Geofisica e Vulcanologia.
Table 2, Names and coordinates of repeat stations.
Table 3, Magnetic element values of repeat stations at time 02UT of the measure day and at 2005.0
epoch (see text)
Table 4, Coefficients of normal fields for epochs 1979 to 2005 for all magnetic elements. Latitude
 and longitude  are referred to 42° N and 12° E respectively, with values expressed in minutes
of arc.
Table 5, Coefficients of secular variation normal fields for referred epochs for all magnetic
elements. Latitude  and longitude  are referred to 42° N and 12° E respectively, with values
expressed in minutes of arc.
Fig.1, Italian II order network undertaken by CNR-PFG for elements F,H and Z: the measurements
have been made in years 1977-1981.
Fig. 2, Italian II order network undertaken by IGM for element D: the measurements have been
made in years 1932-1938.
Fig. 3, Italian repeat station network. Red crosses indicate repeat stations, filled circles indicate
Observatories and diamonds indicate variometer stations; large circles indicate coverage areas for
rapid time variations as recorder at Observatories and variometer stations.
Fig. 4, Difference between reference fields ITGRF and INGRF (see text) at year 2005.0 for total
intensity F expressed in nT.
8- REFERENCES
Bullard, E.C., The removal of trend from magnetic surveys, Earth Planet. Sci. Lett., 2, 293-300,
1967.
Cafarella L., De Santis A., Meloni A., Secular variation from historical geomagnetic field
measurements, Phys. Earth Planet. Inter., 73, 206-221, 1992a.
Cafarella L., De Santis A. Meloni A., Il Catalogo geomagnetico storico italiano, Ist. Naz. Geofis., 7,
160, 1992b.
7
Campbell, W.H., 2003. Introduction to Geomagnetic Fields, Cambridge University Press, pp 337.
Jankowsky J., Sucsdorff, C., 1996. IAGA Guide for Magnetic Measurements and Observatory
Practice, Warsaw , 235 pp.
Chiappini, M., O. Battelli, S. Bushati, G. Dominici, B. Duka and A. MELONI, The Albanian
Geomagnetic Repeat Station Network at 1994.75, Journal of Geomagnetism and Geoelectricity, 49,
701-707, 1997.
Chiappini M., A. Meloni, E. Boschi, O. Faggioni, N. Beverini, C. Carmisciano and I. Marson,
Shaded relief anomaly map of Itlay and surrounding marine areas, Annali di Geofisica, 43, 5, 983989, 2000.
Coticchia A., A De Santis, A. Di Ponzio, G. Dominici, A. Meloni, M. Pierozzi, M. Sperti, La Rete
Magnetica Italiana e la carta magnetic d’Italia al 2000.0, Italian Magnetic Network and
Geomagnetic Field Maps of Italy, Bollettino di Geodesia e scienze Affini, 4, 261-291, 2001.
De Santis A., M. Chiappini, G. Dominici, A. MELONI, Regional geomagnetic field modeling: the
contribution of the Istituto Nazionale di Geofisica, Annali di Geofisica, 40, 1161-1169, 1997.
Haines, G.V., Regional magnetic field modelling: a review, J. Geomag. Geoel.,42, 1001-1018, 1990.
Meloni A., O. Battelli, G. Dominici, S. Arca and A. Marchetta, Italian magnetic network at 1985.0,
Bollettino di Geodesia e scienze Affini, 4, 339-350, 1988.
Meloni, A., Battelli, O., De Santis, A. and Dominici, G., The 1990.0 magnetic repeat station survey
and normal reference fields for Italy, Annali di Geofisica, Vol. XXXVII, N. 5, 949-967, 1994.
Molina F., E. Armando, R. Balia., O. Battelli, E. Bozzo, G. Budetta, G. Caneva, M. Ciminale, N.
De Florentiis, A. De Santis, G. Dominici, M. Donnaloia, A. Elena, V. Iliceto, R. Lanza, M. Loddo,
A. MELONI, E. Pinna, G. Santarato, R. Zambrano, Geomagnetic Survey of Italy. Repeat Stations
Network and Magnetic Maps: a Short Report. Annales Geophysicae, 3, 3, 365-368, 1985.
Molina, F. and De Santis A.,Considerations and proposal for a best utilization of IGRF over areas
including a geomagnetic Observatory, Physics Earth Plan. Inter., 48, 379-385, 1987.
McMillan and Maus, International geomagnetic Rernce Field the 10th generation, Earths Planets
and Space, 57, 1135-1140, 2005.
Merril, R.T., McElhinny, M.W., McFadden, P.L., 1996. The magnetic field of the Earth:
Paleomagnetism, the Core and the Deep Mantle, Academic Press, San Diego, California, 531 pp.
Newitt, L.R., Barton C.E. and J. Bitterly, Guide for magnetic repeat station surveys, IAGA Working
Group, 1996.
Talamo R. Le carte magnetiche d’ Italia delle isodinamiche della H e delle isogone dell’Istituto
Militare e loro aggiornamento al 1973.0, Bollettino di Geodesia e Scienze Affini, 34, (1), 1975.
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