Is a Compass Needle a Permanent Magnet

Magnetic Compass

If a magnetic compass needle is weighted so as to swing horizontally, it takes up a definite direction at each place and its deviation from geographical or true north is called the declination (or magnetic variation), D. In geomagnetic studies D is reckoned positive or negative according as the deviation is east or west of true north.

From: International Geophysics , 2000

Paleomagnetism

In International Geophysics, 2000

1.1.2 Main Features of the Geomagnetic Field

If a magnetic compass needle is weighted so as to swing horizontally, it takes up a definite direction at each place and its deviation from geographical or true north is called the declination (or magnetic variation), D. In geomagnetic studies D is reckoned positive or negative according as the deviation is east or west of true north. In paleomagnetic studies D is always measured clockwise (eastwards) from the present geographic north and consequently takes on any angle between 0° and 360°. The direction to which the needle points is called magnetic north and the vertical plane through this direction is called the magnetic meridian. A needle perfectly balanced about a horizontal axis (before being magnetized), so placed that it can swing freely in the plane of the magnetic meridian, is called a dip needle. After magnetization it takes up a position inclined to the horizontal by an angle called the inclination (or dip), I. The inclination is reckoned positive when the north-seeking end of the needle points downwards (as in the northern hemisphere) or negative when it points upwards (as in the southern hemisphere).

The main elements of the geomagnetic field are illustrated in Fig. 1.1. The total intensity F, declination D, and inclination I, completely define the field at any point. The horizontal and vertical components of F are denoted by H and Z. Z is reckoned positive downwards as for I. The horizontal component can be resolved into two components, X (northwards) and Y (eastwards). The various components are related by the equations:

(1.1.1) $\begin{matrix} H = F cos I, & Z = F sin I, & tan I = Z / H \end{matrix};$

(1.1.2) $\begin{matrix} X = H cos D, & Y = H sin D, & tan D = Y / X \end{matrix};$

(1.1.3) $F^{2} = H^{2} + Z^{2} = X^{2} + Y^{2} + Z^{2} .$

Variations in the geomagnetic field over the Earth's surface are illustrated by isomagnetic maps. An example is shown in Fig. 1.2, which gives the variation of inclination over the surface of the Earth for the year 1995. A complete set of isomagnetic maps for this epoch is given in Merrill et al. (1996). The path along which the inclination is zero is called the magnetic equator, and the magnetic poles (or dip poles) are the principal points where the inclination is vertical, i.e. ±90°. The north magnetic pole is situated where I = +90°, and the south magnetic pole where I = −90°. The strength, or intensity, of the Earth's magnetic field is commonly expressed in Tesla (T) in the SI system of units (see §2.1.1 for discussion of magnetic fields). The maximum value of the Earth's magnetic field at the surface is currently about 70 μT in the region of the south magnetic pole. Small variations are measured in nanotesla (1 nT = 10⁻⁹ T).

Gilbert's observation that the Earth is a great magnet, producing a magnetic field similar to a uniformly magnetized sphere, was first put to mathematical analysis by Gauss (1839) (see §1.1.3). The best-fit geocentric dipole to the Earth's magnetic field is inclined at 10½° to the Earth's axis of rotation. If the axis of this geocentric dipole is extended, it intersects the Earth's surface at two points that in 1995 were situated at 79.3°N, 71.4°W (in northwest Greenland) and 79.3°S, 108.6°E (in Antarctica). These points are called the geomagnetic poles (boreal and austral, or north and south respectively) and must be carefully distinguished from the magnetic poles (see preceding paragraph). The great circle on the Earth's surface coaxial with the dipole axis and midway between the geomagnetic poles is called the geomagnetic equator and is different from the magnetic equator (which is not in any case a circle). Figure 1.3 distinguishes between the magnetic elements (which are those actually observed at each point) and the geomagnetic elements (which are those related to the best fitting geocentric dipole).

In 1634, Gellibrand discovered that the magnetic inclination at any place changed with time. He noted that whereas Borough in 1580 had measured a value of 11.3°E for the declination at London, his own measurements in 1634 gave only 4.1°E. The difference was far greater than possible experimental error. The gradual change in magnetic field with time is called the secular variation and is observed in all the magnetic elements. The secular variation of the direction of the geomagnetic field at London and Hobart since about 1580 is shown in Fig. 1.4. At London the changes in declination have been quite large, from 11½°F in 1576 to 24°W in 1823, before turning eastward again. For a similar time interval the declination changes in Hobart have been less extreme.

The distribution of the secular variation over the Earth's surface can be represented by maps on which lines called isopors are drawn, joining points that show the same annual change in a magnetic element. These isoporic maps show that there are several regions on the Earth's surface in which the isoporic lines form closed loops centered around foci where the secular changes are the most rapid. For example, there are several foci on the Earth's surface where the total intensity of the geomagnetic field is currently changing rapidly, with changes of up to about 120 nT yr⁻¹ (from −117 nT yr⁻¹ at 48.0°S, 1.8°E to +56 nT yr⁻¹ at 22.5°S, 70.8°E). Isoporic foci are not permanent but move on the Earth's surface and grow and decay, with lifetimes on the order of 100 years. The movements are not altogether random but have shown a westward drifting component in historic times. Because declination is the most important element for navigation, records of it have been kept by navigators since the early part of the 16^th century. These records show that the point of zero declination on the equator, now situated in northeast Brazil, was in Africa four centuries ago.

Spherical harmonic analysis of the geomagnetic field (§1.1.3), first undertaken by Gauss in 1839, has been repeated several times since for succeeding and earlier epochs. When the field of the best fitting geocentric dipole (the main dipole) is subtracted from that observed over the surface of the Earth, the residual is termed the nondipole field, the vertical component of which is illustrated in Fig 1.5 for epoch 1995. The magnetic moment of the main dipole has decreased at the rate of about 6.5% per century since the time of Gauss' first analysis (§1.1.4). However, the largest (percentage) changes in the geomagnetic field are associated with the nondipole part of the field. Bullard et al. (1950) analyzed geomagnetic data between 1907 and 1945 and determined the average velocity of the nondipole field to be 0.18° per year westward, the so-called westward drift of the nondipole field. Bloxham and Gubbins (1985, 1986) used the records of ancient mariners to extend the spherical harmonic analyses back to 1715. The general view is that the westward drift is really only a recent phenomenon and has been decreasing up to the present time. The dominant feature of secular variation is, in fact, growth and decay.

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Geomagnetism

M. Kono , in Treatise on Geophysics (Second Edition), 2015

5.01.1.1.3 Magnetic compass in European documents

It is not clear when the knowledge of the magnetic compass reached Europe and when it was first used in navigation. Gilbert wrote that it was brought to Europe by the Venetian Marco Polo, but there is evidence that the compass was used well before his return to Europe in 1295. It is often thought that the knowledge of the compass came from China through the intermediary of the Islam civilization. There is no written evidence, however, and the appearance of the compass is earlier in European documents than in Islamic ones (Mitchell, 1932; Needham, 1962).

The earliest record of the north- and south-seeking property of the compass in Europe appears to be that of Alexander Neckam (1157–1217) of St. Albans, England. In two treatises, De Utensibibus (On Instruments) and De Rerum Naturis (On the Nature of Things) written about 1190, he described the use of the magnetic needle in navigation to indicate north and that the needle is put on a pivot that may be the form of a primitive compass (Mitchell, 1932). Guyot de Provins of France (1184–1210) wrote a poem called La Bible around 1205, in which he described a floating compass. Jacques de Vitry of the Kingdom of Jerusalem (1165–1240) left a similar document (c. 1218). These people were all monks or priests, and they only referred to the compass as having the noble property (to point always to the same direction). It is therefore natural to think that the properties of the compass were known to mariners well before it became popular so that the priests could use it for allegory in these writings (Mitchell, 1932).

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Technology, Instrumentation

P. Collar , G. Griffiths , in Encyclopedia of Ocean Sciences (Third Edition), 2019

Directional Measurement

The directional reference for measurement of current is invariably supplied by a magnetic compass, two main types of which are in common use. The first type is the traditional bar magnet, often mounted on an optically read encoded disk. The entire assembly is mounted on jeweled bearings, with arrangements for damping and gimballing. In the fluxgate compass, the second type of sensor, a soft magnetic core is driven into saturation by an a.c. signal. Orthogonal secondary windings detect the out-of-balance harmonic signals caused by the polarizing effect of the Earth's field and, from an appropriately summed output, the orientation of the sensor relative to the Earth's field can be determined. In current meters a gimballed two-component system may be used, but as in the case of the magnet compass, this does require that the system will respond correctly to any rotational and translational motions arising from mooring or platform motion.

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Tectonic and Structural Framework of the Zagros Fold-Thrust Belt

J. Vergés , ... P. Skott , in Developments in Structural Geology and Tectonics, 2019

Sampling Strategy

Samples were collected in the field with a portable gas-powered drill and oriented in situ with a magnetic compass coupled to a core-orienting fixture. Favorable outcrop conditions and an abundance of suitable lithologies allowed sampling of the Afrineh syncline section at regular intervals of ∼10 m. A total of 128 sites were drilled along 1200 m of the fine-grained Agha Jari-Bakhtyari red bed succession (Fig. 3.4). In comparison, sampling of the Chaman Goli succession, which is located at altitudes above 2000 m, was more complicated because of vegetation cover and a rough topography and climate. Structural complexity also added difficulties in selecting the best transect to sample the Agha Jari Formation. Sampling was carried out along two different transects with 107 sites along a 1300 m thick succession. Abundance of conglomerate intervals caused sampling gaps of tens of meters in the upper part of the section (Fig. 3.6). The average sampling interval of the Chaman Goli section is 12.5 m.

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Magnetic stratigraphy

Neil D. Opdyke , James E.T. Channell , in International Geophysics, 1996

2.1 Introduction

The magnetic field of the Earth has fascinated human beings for well over 2 millennia. The Chinese invented the magnetic compass in the second century B.C. ( Needham, 1962) and knowledge of the magnetic compass reached Western Europe over a thousand years later in the twelfth century A.D. The first truly scientific paper on geomagnetism was written in 1262 by Petrus Peregrinus and entitled "Epistola de Magnete" (Smith, 1970). In a series of experiments on lodestone (magnetite) spheres, Peregrinus defined the concept of polarity and defined the dipolar nature of a magnet stating that like poles repel and unlike poles attract; however, the treatise was not published until 1558. The work by Peregrinus undoubtedly influenced the later work of William Gilbert, who published the book "De Magnete" in 1600. Gilbert studied the variation of the angle of inclination (or magnetic dip) over lodestone cut into the shape of a sphere, perhaps one of the greatest model experiments ever made in Earth science. His conclusion was that "the Earth itself is a great magnet." From the 14th century to the present, compasses were widely used as a navigation aid on both land and sea and were carried by explorers throughout the world. The angular distance between the polar star, which does not move in the heavens, and the direction of the north-seeking end of the compass needle was often faithfully recorded. This magnetic deviation from true north is called the magnetic declination (D). In midlatitudes, from 40°N to 40°S, the declination of the present geomagnetic field can vary up to 40° from true north. In polar regions, near the magnetic poles, declination anomalies of up to 180° are observed (Fig. 2.1).

If a magnetized needle is suspended on a fiber and allowed to swing freely, the north-seeking end of the needle will not only point north, it will also point down in the northern hemisphere and up in the southern hemisphere. This property of the earth's field, its inclination, was discovered by George Hartmann in 1544 (Smith, 1968). If a dip needle which measures the angle of magnetic inclination is carried from high northern latitudes to high southern latitudes, it will be seen that the inclination of the field varies systematically from vertical down at the north magnetic pole, to the horizontal at some point at low latitude (magnetic equator), and vertical once more (but with the north-seeking end of the needle pointing vertically upward) at the south magnetic pole (Fig. 2.2). It should be noted that the north and south magnetic poles are not 180° apart and that the magnetic equator is not equidistant from the two magnetic poles.

If the geomagnetic field is analogous to that of the magnetized spheres studied by William Gilbert, the intensity of the field, as well as the inclination and declination, would vary systematically over the Earth. After Gauss invented the deflection magnetometer, this was found to be the case, the magnetic field strength at the magnetic poles being almost twice that at the magnetic equator.

The field at any point on the Earth's surface is a vector (F) which possesses a component in the horizontal plane called the horizontal component (H) which makes an angle (D) with the geographical meridian (Fig. 2.3). The declination (D) is an angle from north measured eastward ranging from 0° to 360°. The inclination (I) is the angle made by the magnetic vector with the horizontal. By convention, it is positive if the north-seeking vector points below the horizontal or negative if it points above. The horizontal component (H) (Fig. 2.3) has two components, one to the north X and one to the east Y. The following equations relate the various quantities:

(2.1) $H = F cos I Z = F sin I Tan I = Z / H$

(2.2) $X = H cos D Y = sin D Tan D = Y / X$

(2.3) $F^{2} = H^{2} + Z^{2} = X^{2} + Y^{2} + Z^{2} .$

The dipole nature of the geomagnetic field, originally suggested by Gilbert, was first put to a mathematical analysis by Gauss (1838), who used measurements of declination, inclination, and intensity from 84 widely spaced locations to derive the first 24 coefficients in the spherical harmonic expansion of the geomagnetic field. The results of this analysis indicated that, within the uncertainties of the data, the geomagnetic field had no sources external to the Earth and that it had a dominant dipole component. Since 1835, when Gauss first determined the intensity of the dipole moment, the calculations have been repeated many times, and this value has been decreasing steadily over the last century and a half. During this century, the decay rate has increased, and during the last 30 years has reached a rate of 5.8%/century (see Barton, 1989). If the trend were to continue, the main dipole field would disappear by the year 4000 A.D., a possible but unlikely event.

Modern spherical harmonic analyses confirm Gauss' findings and lead to the conclusion that: (1) the geomagnetic field is almost entirely of internal origin, (2) ~ 90% of the field observed on the surface can be explained by a dipole inclined to the Earth's axis of rotation by 11.5° (Fig. 2.4), (3) the magnitude of the dipole moment is 7.8 × 10²² Am², (4) the axis of the centered dipole (geomagnetic pole) emerges in the northern hemisphere between Greenland and Ellesmere Island at 79.0°N, 70.9°W. The great circle midway between the geomagnetic poles is called the geomagnetic equator (Fig. 2.4).

Magnetic observations of the elements of the geomagnetic field were begun as early as the 16th century in London and the 17th century in Paris. It was realized early on that the declination and inclination of the geomagnetic field change with time (Fig. 2.5). This relatively rapid change of the magnetic field with time is called secular variation. The declination of the field at London has been changing at a rate of about 14° per century from 0° in 1650 to 24°W in 1800 and is now 5°W. The change in the magnetic elements is not constant over the Earth's surface and, for example, changes less rapidly over the Pacific Ocean than in, say, South America. Global secular variation is often displayed as "isoporic" charts, which are contour maps of equal annual change of a particular element of the field, such as the vertical component. From isoporic charts constructed from observations dating back about 200 years, isoporic centers have drifted westward at rates of up to about 0.28°/yr over this time interval. This observation is a manifestation of the westward drift of dipole, quadrupole, and octupole components of the field. Drift rates of these components can be calculated directly from the spherical harmonic coefficients and their changes with time. Changes in the rate of westward drift and the growth and decay of isoporic features occur on a decadal time scale. The westward drift is generally attributed to differential angular velocity of Earth's lithosphere and outer core, where the nondipole components of the field are thought to originate. It is important to note that although westward drift is dominant, not all secular variation features drift westward; some are more or less stationary and some are moving slowly eastward.

The geomagnetic field varies over a wide range of time scales (Table 2.1). The longer term behaviors (4-7, Table 2.1) are produced in the Earth's interior, and the short-term behaviors (1-3, Table 2.1) are atmospheric or ionospheric in origin. Secular variations of the field, first observed in observatory records, can be monitored further back in time using paleomagnetic data from marine and lake sediments with high accumulation rates. The longer term variations (5–7, Table 2.1) have also been documented by paleomagnetic study of rocks and sediments. Since some lavas and archaeological artifacts can become permanently magnetized in a matter of hours, it is theoretically possible to detect ancient field behavior on this time scale from paleomagnetic data.

Table 2.1. Scales of Geomagnetic Variability

Geomagnetic behavior	Duration
1. Pulsations or short-term fluctuation	minutes
2. Daily magnetic variations	hours
3. Magnetic storms	hours to days
4. Secular variations	10²–10³ yr
5. Magnetic excursions	10³–10⁴ yr
6. Reversal transition	10³–10⁴ yr
7. Interval between reversals	10⁵–10⁶ yr

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Paleomagnetism

In International Geophysics, 2000

2.1.1 Magnetic Fields, Remanent and Induced Magnetism

The study of magnetism originated from observation of the behavior of natural permanent magnets, the earliest known of which were used as magnetic compass needles. A permanent magnet is usually described by its "north" and "south" magnetic "poles", imagined to reside at the opposite ends of the magnet. The concept of magnetic poles has been of considerable use in analyzing the behavior of magnets, but since the discovery by Oersted in 1820 that an electric current flowing in a wire deflected a compass needle placed near it, it has been recognized that all magnetic effects are appropriately described in terms of electric currents. An immediate consequence of this is that there are no isolated magnetic poles, so the "magnetic poles" at the end of a magnet are just convenient fictions for the purpose of simple analysis. An electron in orbit around a nucleus is, in essence, a current flowing in a loop and the magnetic effects of materials can all be described in terms of such elementary current loops.

Magnetic fields are specified in terms of the two vectors H, called the magnetic field, and B, the magnetic induction. B includes the effects of the macroscopic magnetization M, defined as the dipole moment per unit volume, according to the relation

(2.1.1) $B = μ_{0} (H + M),$

where μ₀ is the permeability of free space and has the value of 4π × 10⁻⁷ Hm⁻¹ (Henry per metre). Outside any magnetic materials M = 0 and B and H are parallel and in this case B = μ₀H. In geomagnetism and paleomagnetism the magnetic field of interest is almost always external to a magnetic medium, so that B and H are parallel and it is of no consequence which one is used. However, numerical conversion between the old cgs system and the current SI system of units is trivial for B but involves a factor of 4π (from μ₀) in the case of H, so magnetic fields are generally specified in terms of the magnetic induction B rather than the magnetic field H. Furthermore, B is typically referred to in an informal manner as the magnetic field B, a usage that will often be followed in this book. Naturally, when considering magnetic fields inside a magnetic material M ≠ 0, so it becomes important to distinguish between B and H because they will not be equivalent, will not always be parallel, and can even have opposite signs. Under such circumstances it becomes important to use the magnetic field H (see §4.1.2, in which such a situation occurs).

A permanent magnetic and electric current loop both have a magnetic dipole moment, m, associated with them. When placed in a magnetic field B (Fig. 2.1) each will experience a torque L = m × B (i.e., L = mBsinθ, where θ is the angle between the long axis of the magnet and B or the angle between the axis drawn through the center of the current loop at right angles to the plane of the loop). Hence, the torque attempts to rotate the dipole moment into alignment with B. In the case of the current loop the dipole moment m = iA (current times the area of the loop), and in the case of the bar magnet it is m = pI, where p is the "pole strength" of the imaginary poles at each end of the magnet and l is the distance between the poles. Thus dipole moment is measured in units of Am² and the magnetization M (dipole moment per unit volume) is measured in Am⁻¹. Table 2.1 summarizes the basic magnetic quantities together with typical values that arise in geomagnetism and paleomagnetism.

Table 2.1. Various Magnetic Quantities Used in Geomagnetism and Paleomagnetism with Typical Values

Quantity	Units	Typical values
Magnetic induction, B	T(tesla)	Earth's field 10⁻⁴ T (0.1 mT or 100 μT)
Magnetic field, H	Am⁻¹	Earth's field 10³/4π = 79.6 Am⁻¹
Dipole moment, m	Am²	Orbital electron 10⁻²³ Am²;
		Bar magnet 1 Am²
Magnetization, M	Am⁻¹	Sediments ≈ 10⁻³ Am⁻¹; Volcanics ≈ 1 Am⁻¹
Magnetic susceptibility, χ	Dimensionless	Magnetite ≈ 2.5
Permeability of free space, μ₀	Hm⁻¹	4π × 10⁻⁷ Hm⁻¹

The magnetization of any material is generally made up of two components: the remanent magnetization (or simply remanence), which is that remaining in the absence of an applied field; and the induced magnetization, which is that induced by an applied field but which disappears after removal of that field. When dealing with rocks, the total magnetization M is made up of the vector sum of the remanence M_n and the magnetization M_i induced by the Earth's magnetic field, where

(2.1.2) $M = M_{n} + M_{i}$

In isotropic materials the induced magnetization M_i lies along the direction of the applied field H (i.e., B) and is proportional to the magnitude of that field, that is

(2.1.3) $M_{i} = χ H = \frac{χ B}{μ_{0}},$

Where χ is a constant of proportionality called the magnetic susceptibility. Since H and M_i have the same dimensions (2.1.1 and Table 2.1), χ is a dimensionless number. Some banded sediments, layered intrusions, and foliated metamorphic rocks are magnetically anisotropic and have greater susceptibility in the plane of layering. These are special cases and most rocks used in paleomagnetism, such as basalts, dolerites, redbeds, and limestones, are magnetically isotropic or nearly so (see also §2.3.9).

The Koenigsberger ratio (0 has been defined as the ratio of the remanent to induced magnetization and is given by

(2.1.4) $Q_{n} = \frac{μ_{0} M_{n}}{χ B}$

(2.1.5) $Q_{t} = \frac{μ_{0} M_{t}}{χ B} .$

Q _n is the ratio of the remanence (M _n) to that induced by the Earth's magnetic field at the sampling site, whereas Q _t is the ratio of the thermoremanent magnetization (M _t) acquired in the magnetic field B to the magnetization induced by the same field at room temperature.

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Layered (Stratified) Rocks and Topography

Graham Borradaile , in Understanding Geology Through Maps, 2014

How Does the Field Geologist Measure the Orientation of a Plane?

The dip-and-strike symbols reported on published maps are largely determined in the field by observation and measurement with a specially designed magnetic compass ( Figure 5.6). The "geological compass" takes several forms and costs several hundred dollars at current prices. In order of decreasing frequency, one finds models that encompass the following features above the basic requirement of a compass needle that points to North.

1.: All contain a scale, compass needle, and a separate nonmagnetic free-swinging needle that permits dip angles to be determined; this is called an inclinometer or clinometer.
2.: One or more bubble gauges may be fitted to ensure that the dip angle is indeed measured with respect to the horizontal.
3.: A lid or edge that may be placed along the strike of a plane, to facilitate in measuring the strike's azimuth. An appropriately placed bubble gauge assists here.
4.: The lid may itself be an inclinometer. Thus, the lid may be placed on a sloping surface and the dip and direction of dip of the surface may be measured in one action. (Most compasses require strike and dip to be measured in two separate actions, and most geologists prefer to record orientations as strike and dip rather than dip and direction of dip.) These compasses are called stratum compasses (Figure 5.6(a)).

Furthermore, some of the following features related to the compass needle and its reading may be present.

5.: An adjustment for magnetic declination of the compass needle (the azimuth of true or geographic north differs from magnetic north almost everywhere).
6.: A compass scale that reads anticlockwise from north. This is shown by the azimuth numbers but also most disturbingly for the beginner by the exchange of the symbols "E" and "W" for east and west! As the compass is turned to make its edge parallel to some significant geological direction, the compass needle then becomes direct reading, pointing to the azimuth number appropriate for the geological direction.
7.: Some device that permits the compass to be used for "sighting" or "reading a bearing" to some prominent topographic feature such as a cairn. This may take the form of a device like a "gun sight" which folds out from the compass or a hole in the lid of the compass; a mirror may be used to simultaneously sight on the object and read the compass needle bearing (analogous to the structure of a sextant). Compasses which show this feature are sometimes called "transit style" compasses since they are modified from land surveyor's compasses. They also often take the name of the most prominent manufacturer, "Brunton style" (see Figure 5.6(b and c)).
8.: An adjustment to ensure the compass needle remains horizontal at different latitudes. (Northern hemisphere geologists who travel for work in the southern hemisphere find that the needle of their compass points upward so steeply that it is trapped by the glass dial and will not rotate if the compass is held in its correct horizontal orientation!)
9.: The presence of an eddy current damping conductor around the edge of the compass ring suppresses unnecessary oscillations as the compass comes to rest at its north-pointing position. Traditionally, as in old marine compasses, this damping was achieved with low viscosity oil within the compass housing. However, it is not favored in geologists' compasses due to the added weight and risk of loss during the rugged handling required of a geologist's compass.

Digital electronic enthusiasts, please note that global positioning system (GPS) units do not provide a reliable means of measuring orientations of field structures! Also, attempts to market magnetic compasses with digital display have failed because the underlying mechanism relies on mechanical and magnetic interactions anyway. Such digital compasses give a false sense of accuracy, are dependent on batteries, and require careful construction so that electrical currents do not interfere with the operation of the magnetic needle. In comparison, the traditional analog compass needle leaves one with no false impressions of precision; directions are rarely measured to better than ±3° with a handheld compass and dip angles may be measured no better with its inclinometer. Indeed, most geological surfaces are so rough that it is not reasonable to expect better precision and commonly precision is much worse than ±3°. For many rough surfaces, it may be wise to lay a clipboard on the surface, which acts to average out its roughness, and then measure the orientation of the clipboard.

Of course, to locate position on a map with a small handheld device, the use of the magnetic compass has been superseded. An inexpensive GPS locates position at least within meters. In contrast, the geologists' small handheld transit compass to take bearings on topographic features and triangulating to fix one's position within a triangle of error was often inaccurate and rarely had a precision better than ±5 m when mapping at a 1:10,000 scale. Rarely, of course, GPS units do not work. For example, in areas of severe topographic relief, where the landforms obscure the line of sight to one of the Earth-orbiting GPS satellites. This unavailability may depend upon time of day due to satellite positions above the Earth. Finally, near some military installations, GPS service fails altogether for security reasons.

The beginner to cartography and geology should not worry about the accuracy of published topographic base maps. Features are just as precisely located upon them as if the best GPS system was used. Geologists routinely use topographic base maps made in the 1800s, with confidence. The original map surveyors located topographic features with precisions of centimeters using massive precision scientific instruments called theodolites; they determined positions relative to other topographic features. Latitude was determined precisely with respects to stars using large sextants and longitude was fixed with precise chronometers. Land surveyors of the prescientific age used instruments much more substantial than the equivalent models used on ships. The largest theodolites used by the British Ordnance Survey in the nineteenth century used theodolites weighing in at several tons and required a team of horses for transport!

On a final note, we should be aware that for the determination of elevation, the small handheld GPS unit still cannot always compete with the aneroid barometer. The aneroid is a disc-like chamber made of thin metal from which all air has been evacuated. As air pressure changes, so the surface of the disc flexes like the surface of a drum. The small motion caused by pressure changes is easily amplified by levers to read on a scale. The aneroid barometer was invented in 1843, and, like mercury tube barometers, it was designed primarily to record atmospheric pressure. However, atmospheric pressure varies due to two factors, obviously weather but less obviously altitude. If the atmospheric conditions are constant, the barometer responds to lower pressures as one carries the barometer ascends to greater altitudes. Even small, handheld barometers accurately detect elevation changes of <1 m. (Hold one in your hand and walk upstairs!) The aneroid must be read at some base station of known altitude and then the change in reading will permit one to determine the altitude of an outcrop. Unfortunately, in some climates and in certain seasons, atmospheric pressure may change significantly during the course of the day so that the aneroid should be used to determine relative elevations from frequently checked "base stations". (I took a lunch break on a Spanish mountain and watched the aneroid barometer apparently report that I had climbed 10 m while sitting.)

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The Role of Environmental Monitoring in Pollution Science

J.F. Artiola , M.L. Brusseau , in Environmental and Pollution Science (Third Edition), 2019

10.4.1 Maps

Until recently, drawn maps and aerial photographs were the only means at the disposal of environmental scientist to locate places and land features and to navigate (with a magnetic compass). Today, the use of geographic positioning system (GPS) of satellites makes it much easier to perform these tasks. However, since maps are small portable representations of portions of our environment, we must be familiar with their principles and types. There are three major types of maps: planimetric, which present scaled information in two dimensions; topographic maps, which add elevation information; and thematic maps discussed in the next section. All maps must have a scale, which is the ratio between the map's distances and the real (earth) distances. Thus most scales are a unit-less ratio or fraction of two distances. For example, if 1 cm on the map represents 1 km in reality, then the map scale is 1/100,000.

The major types of locational maps include the latitude and longitude system developed in England which divides the earth into parallel (north-south) and meridian (east-west) sections in degrees.

In the United States, the Public Land survey System is used exclusively to locate property lines in legal documents. This system is based on the 34 points that defined the origins of principal meridians and baselines coordinates from which townships (6 square miles) and sections (1 square mile) originate (Fig. 10.3).

Example, the filled square (point A) shown in Fig. 10.3—as representing the NW1/4 corner of section 17 of Township 1S and Range 2W (abbreviated: NW1/4, S17, T1S, R2W).

Topographic maps add relief to land maps with the use of contour lines that defined equal elevations drawn at fixed elevation intervals. The US Geologic Survey generates topographic maps which are used in many disciplines including environmental science. Contour lines are very useful in identifying unique land features such as watersheds, rivers, depressions, and mountains.

Soil survey maps combine land features (aerial photographs) with thematic soil data. These maps are very useful in determining the suitability (use) of land for certain activities, such as agriculture, land development, and septic field location. These maps provide a wealth of data on soil characteristics related to pollution (such as infiltration, soil texture, general composition), which makes them very valuable to soil and environmental scientists. An example of a Soil Survey Map is given in Fig. 10.4

Geographic Information Systems (GISs) are very useful in environmental applications because they provide a way to arrange and present layers of data or themes as maps. Thus GIS maps are decision-making tools that provide a visual representation of large amounts and types of data (see Fig. 10.5), which can be easily sorted because all the data and coordinates are in a digital form. For example, a city manager with a GIS map with data overlay of soil properties could ask the question "show which areas in district X are suitable for septic field systems."

Because GIS uses digital data that can be manipulated using Boolean operations (binary 0 and 1 values), data can be reclassified, sorted, and superimposed (data overlay). GIS data is now an accessible community resource available on the Internet for example, to search and locate parcels of land. GIS methods can now visualize environments using digital photography and data, and virtual reality 3-D modeling.

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MAGNETIC PROPERTIES

E. Sinn , in Encyclopedia of Analytical Science (Second Edition), 2005

Introduction

Mankind has been fascinated by magnetism since before recorded history. Magnetic iron-containing rocks like lodestone show the attraction and repulsion of a natural magnet. Navigating via the magnetic compass was the first widespread application of magnetism, but magnetic navigation predates us by eons: ferromagnetic crystals based on an iron-oxide hydrate complex in the brains of migratory birds allow them to fly halfway around the world without getting lost even when landmarks and stars are obscured.

Everything has magnetic properties, so it is not surprising that magnetism can be used for both quantitative and qualitative analysis. Magnetic studies and data have been widely reviewed and tabulated. Because of the wide range of magnetic properties that quite similar materials can possess, it is important to know about magnetism in general before it can be used analytically. Magnetism can be used for bulk analysis, distinguishing particular elements especially different metals in compounds, and for indicating structure types. Magnetic properties are best used in conjunction with other properties, and the more is known about a sample, the more use can be made of the magnetic properties.

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Is a Compass Needle a Permanent Magnet

Source: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magnetic-compass

Kingston Greste1991

Is a Compass Needle a Permanent Magnet

Magnetic Compass

Paleomagnetism

1.1.2 Main Features of the Geomagnetic Field

Geomagnetism

5.01.1.1.3 Magnetic compass in European documents

Technology, Instrumentation

Directional Measurement

Tectonic and Structural Framework of the Zagros Fold-Thrust Belt

Sampling Strategy

Magnetic stratigraphy

2.1 Introduction

Paleomagnetism

2.1.1 Magnetic Fields, Remanent and Induced Magnetism

Layered (Stratified) Rocks and Topography

How Does the Field Geologist Measure the Orientation of a Plane?

The Role of Environmental Monitoring in Pollution Science

10.4.1 Maps

MAGNETIC PROPERTIES

Introduction

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