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Local attraction
Local attraction
Local attraction
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While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called as local attraction.[1] The external forces are produced by sources of local attraction which may be current carrying wire (magnetic materials) or metal objects.[2] The term is also used to denote amount of deviation of the needle from its normal position. It mostly causes errors in observations while surveying and thus suitable methods are employed to negate these errors.[3]

Sources

[edit]

The sources of local attraction may be natural or artificial. Natural sources include iron ores or magnetic rocks while as artificial sources consist of steel structures, iron pipes, current carrying conductors. The iron made surveying instruments such as metric chains, ranging rods and arrows should also be kept at a safe distance apart from compass.[3]

Detection

[edit]

Local attraction at a place can be detected by observing bearings from both ends of the line in the area. If fore bearing and back bearing of a line differ exactly by 180°, there is no local attraction at either station. But if this difference is not equal to 180°, then local attraction exists there either at one or both ends of the line.[3]

Remedies

[edit]

There are two common methods of correcting observed bearings of the lines taken in the area affected by Local Attraction. The first method involves correcting the bearing with the help of corrected included angles and the second method involves correcting the bearing of traverse from one correct bearing ( in which difference between fore bearing and back bearing is exactly equal to 180°) by the process of distribution of error to other bearings.[4]

References

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Local attraction

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Local attraction in surveying refers to the deviation of a compass needle from its correct alignment with the magnetic north, caused by nearby magnetic influences that distort the Earth's magnetic field. This phenomenon primarily affects compass-based measurements in traversing and orientation tasks, leading to inaccurate bearings that can compromise the precision of surveys for mapping, construction, and navigation. Common sources of local attraction include ferrous materials such as iron or steel structures, underground cables carrying electric current, railroad tracks, and even tools like steel tapes used in the field. These local magnetic fields superimpose on the Earth's geomagnetic field, causing the needle to settle at an incorrect position rather than pointing true magnetic north. Detection of local attraction typically occurs during fieldwork by comparing the fore bearing (direction observed from one end of a line) and back bearing (direction from the opposite end), which should differ by exactly 180° in the absence of errors; any deviation indicates the presence of attraction at one or both stations. Effects are station-specific, meaning all bearings taken at an affected station are equally influenced, but unaffected stations remain accurate. To mitigate local attraction, surveyors employ correction methods such as identifying error-free lines (those with exact 180° fore-back differences) and applying calculated adjustments to affected bearings, or balancing errors across a closed traverse using the known sum of interior angles ((2n - 4) × 90° for n sides). In practice, avoiding magnetic materials near the or using non-magnetic alternatives like theodolites for precise work helps prevent such issues, ensuring reliable geospatial data in and land applications.

Overview and Fundamentals

Definition

Local attraction refers to the deviation of a magnetic needle from its normal position due to the influence of nearby , resulting in erroneous directional readings during or activities. This disturbance arises when external magnetic materials interfere with the needle's alignment, preventing it from accurately pointing toward the magnetic north. The phenomenon specifically impacts the horizontal component of the , to which the needle is responsive in its free within the horizontal plane. As a result, local magnetic influences distort this horizontal component, introducing angular errors in the observed bearings of survey lines or navigational paths. Local attraction is distinct from , which represents the global angular difference between and magnetic north at a given , and from deviation, which stems from inherent magnetic properties within the instrument itself. Unlike declination's broader geographic variation or deviation's consistent instrumental bias, local attraction is inherently site-specific, temporary, and caused by proximate external sources. The relationship between observed and true bearings incorporates local attraction as an error component, expressed in the basic equation:
Observed bearing = True bearing + + Deviation + Local attraction
(with appropriate signs based on direction: east positive, west negative). This formulation accounts for the cumulative effects on readings in magnetic .

Historical Context

The concept of local attraction, referring to site-specific deviations in a magnetic caused by nearby ferromagnetic materials, was first systematically observed during maritime explorations in the . Portuguese navigator D. João de Castro documented compass needle disturbances near iron-rich rocks during his voyages to in 1538–1541, attributing them to local magnetic influences rather than broader geomagnetic variation. These early notations highlighted how environmental factors could alter readings, prompting initial distinctions between global magnetic and localized effects. English physician and natural philosopher William Gilbert expanded on such phenomena in his seminal 1600 work , where he described experiments demonstrating how nearby iron objects deflected magnetic needles, laying foundational insights into terrestrial and its perturbations. By the , as industrialization accelerated with the expansion of railways and operations, local attraction gained prominence in land surveying practices. Surveyors encountered frequent compass errors due to iron tools, deposits, and , necessitating more precise accounting for these deviations. Sir , serving as Surveyor General of from 1830 to 1843, oversaw the (GTS), initiating triangulation efforts across diverse terrains where local magnetic influences later proved significant. observations in the survey revealed inconsistencies traceable to regional iron deposits, underscoring the need for corrections in large-scale geodetic work. A pivotal event in recognizing local attraction's scale occurred during the GTS, where discrepancies in plumb-line deflections and measurements were linked to massive formations in . In 1862, Archdeacon John Henry Pratt analyzed these errors, demonstrating how local attraction from the Satpura and Vindhya ranges caused up to several minutes of arc deviation in survey instruments, affecting the overall accuracy of the meridional arc computations. Pratt's calculations, presented to of , quantified the attraction's influence on the plumb-line, revealing systematic biases in the GTS data and prompting methodological refinements. The terminology evolved from earlier references to "compass variation" or "deviation" to the standardized term "local attraction" by the mid-19th century, particularly in naval and surveying literature. British navigator , in his 1814 A Voyage to , formalized the concept for shipboard contexts by distinguishing ship-induced deviations from true magnetic variation. This naval usage transitioned into land texts, where by the late 19th and early 20th centuries, it was distinctly defined as localized magnetic interference. For instance, in Charles B. Breed and George L. Hosmer's Elementary Surveying (first edition circa 1908, building on 1890s precedents), local attraction is delineated as a correctable error from nearby metals, separate from .

Importance in Surveying and Navigation

Local attraction plays a pivotal role in land by introducing systematic errors in bearings during traverse and surveys, which can propagate through calculations and result in inaccurate positional data for boundaries and layouts. In traverse work, where successive angular measurements define a closed , uncorrected local attraction disrupts the balance of interior angles, leading to misclosures that may exceed acceptable tolerances and necessitate adjustments or re-surveys. Such inaccuracies have historically contributed to boundary disputes, as seen in cases where original surveys in areas with magnetic disturbances resulted in offsets exceeding standard mapping accuracy limits, potentially costing thousands in legal resolutions. Similarly, in networks used for large-scale mapping, angular errors from local attraction can amplify discrepancies across baselines, misaligning structural elements like roads or buildings and posing risks to and project budgets. In navigation, particularly marine and aeronautical applications, local attraction—often termed deviation—compromises compass reliability by deflecting the needle due to nearby ferrous materials on vessels or , such as engines or hull plating, making it essential for maintaining true headings during voyages. Before the widespread adoption of GPS in the late , magnetic es served as primary directional tools, and unaddressed local attraction could lead to significant course deviations, endangering ships in congested waters or during instrument failures. For instance, in marine contexts, deviation errors required ongoing adjustments using celestial observations to ensure safe passage, while in , pilots relied on deviation cards to correct headings, preventing drift from intended flight paths. These errors were especially critical in remote oceanic or polar regions where alternative aids were unavailable. The quantifiable risks of local attraction underscore its practical significance, with deviations reaching 5-10 degrees in severe cases near strong magnetic sources, far exceeding typical correction limits of ±4-6 degrees. Over extended distances, a 1-degree error in bearing can accumulate to approximately 175 meters of lateral displacement after 10 kilometers, as the tangent of the angle approximates the offset in small-angle approximations for traverse legs or navigation tracks. In navigation scenarios, such as a vessel maintaining a steady course, this translates to potential ground track errors of several nautical miles over transoceanic distances, amplifying hazards like collisions or grounding. Even today, despite GPS dominance, local attraction remains relevant in low-tech or backup systems, such as in remote expeditions, emergency aviation procedures, or areas with GPS jamming, where compasses provide redundancy but demand vigilant error monitoring to avoid compounded inaccuracies.

Causes

Natural Causes

Local attraction in compass surveying arises from natural geological influences, primarily the presence of magnetic minerals such as embedded in rocks, which generate localized that deflect the compass needle from true magnetic north. These minerals, common in igneous and metamorphic formations, create anomalies by inducing additional magnetic moments that interact with the Earth's geomagnetic field, leading to deviations of several degrees in affected areas. Such effects are particularly pronounced in regions or hilly terrains where exposed rock outcrops amplify the interference. Soil and terrain variations further contribute to local attraction through ferrous-rich soils and volcanic features like lava fields, which alter the local magnetic field gradient. Ferrous oxides in iron-rich soils containing magnetic minerals, often derived from weathered , produce subtle but measurable distortions in the . Lava fields, composed of basaltic rocks with inherent remanent magnetism from cooled molten material, can cause significant needle deflections due to their strong ferromagnetic properties and irregular -induced variations. These natural features result in non-uniform magnetic gradients that challenge accurate bearing measurements during surveys. Notable examples of these natural causes include deviations observed near deposits in the region, where early surveyors in 1844 noted erratic behavior due to magnetite-rich formations, leading to the discovery of high-grade ores. Similarly, in Australia's region, the vast and deposits in banded iron formations have been associated with magnetic anomalies that affect accuracy in exploration surveys, highlighting the challenges in iron-prosperous terrains.

Artificial Causes

Artificial causes of local attraction arise from man-made objects and structures that introduce magnetic or electromagnetic fields interfering with the needle's alignment to magnetic north. These disturbances are primarily due to ferromagnetic materials or current-carrying conductors in close proximity to the surveying instrument. Unlike natural geological influences, artificial sources are often localized to specific human-engineered environments, such as construction sites or urban areas, and can systematically bias all bearings measured at an affected station. Structural materials in buildings, bridges, and vehicles represent a common source of interference, as reinforcements and frames act as induced magnets when exposed to the . For instance, the framework in modern bridges or the of nearby vehicles can attract the north-seeking end of the needle, causing it to deviate from true magnetic north. In near such structures, this effect is particularly pronounced if the is positioned within 10-20 meters, leading to consistent errors across multiple readings. Similarly, iron or elements in buildings, like in , generate localized fields that pull the needle eastward or westward depending on their . Equipment interference further exacerbates local attraction through everyday tools and devices that produce . Items such as chains, watches, or handheld metal tools carried by surveyors can subtly influence the if placed too close, with the needle experiencing attraction proportional to the object's ferromagnetic properties. , especially in active circuits, generates alternating electromagnetic fields that induce temporary deviations in the needle's . These effects are typically smaller but cumulative, often requiring surveyors to maintain a clear zone around the instrument to minimize . Larger-scale infrastructure like railways, power lines, and buried pipelines can cause more significant deviations, often up to several degrees, due to their extensive use of conductive materials. Railways, with their tracks, create persistent magnetic anomalies that deflect the needle when surveys are conducted parallel or nearby, as the tracks become magnetized by geomagnetic induction. High-voltage power lines produce varying fields from alternating currents, leading to deviations that fluctuate with load but commonly reach 2-5 degrees within 50 meters. Buried pipelines, particularly those made of for or transport, similarly induce fields through currents or proximity to telluric effects, affecting compass accuracy in pipeline-adjacent terrains. Historically, artificial causes gained prominence in the with the rise of iron-hulled ships, where hull led to errors quantified in naval logs from the , sometimes exceeding 30 degrees and prompting early correction techniques. These cases, documented in British Admiralty records, highlighted how uncompensated iron structures could render unreliable, influencing subsequent practices on land and .

Detection

Traditional Detection Techniques

Traditional detection techniques for local attraction in compass surveying rely on manual observations and comparisons of magnetic bearings to identify discrepancies caused by nearby magnetic influences, such as structures. One primary method involves comparing fore bearings and back bearings of a survey line. The fore bearing is the angle measured from the forward direction of the line to magnetic north, while the back bearing is taken from the opposite end in the reverse direction; ideally, the back bearing should equal the fore bearing plus or minus 180°, depending on the quadrant. Any deviation from this 180° difference indicates local attraction at one or both stations, as the magnetic needle is deflected equally in all bearings taken at the affected point. For example, if the fore bearing is recorded as θ, the back bearing should be θ + 180°; a consistent offset signals the error's magnitude and direction. In multi-station surveys, such as closed traverses, consistency checks across adjacent points help detect local attraction. Bearings are observed from multiple stations forming a polygonal traverse, and the angular closure is verified; for a closed traverse with n sides, the sum of interior angles should equal (n - 2) × 180°. Discrepancies in this closure, after accounting for observational errors, point to local attraction at specific stations, as it affects all bearings equally from that point. Surveyors identify unaffected stations by selecting lines where fore and back bearings differ by exactly 180° and use them as references to adjust others. Reciprocal observations extend this approach by directly comparing readings at both ends of a line for angular closure. At each endpoint, bearings to the opposite station are recorded; if local attraction is present, the observed angles will not close properly, revealing inconsistencies beyond typical instrument errors. This method ensures that deflections are isolated to specific endpoints, allowing surveyors to flag and quantify the attraction without advanced equipment.

Modern Detection Methods

Modern detection methods for local attraction leverage advanced instrumentation and digital technologies to quantify magnetic field deviations with high precision, surpassing the limitations of manual observations. Portable magnetometers, such as fluxgate sensors, enable direct measurement of local magnetic anomalies by detecting variations in the caused by nearby materials or geological features. These devices, often integrated with tilt compensation and GPS for positioning, achieve sensitivities down to 0.1 nT, allowing surveyors to map gradients and isolate attraction sources in real-time during field operations. Proton precession magnetometers further enhance accuracy for gradient surveys, utilizing the of hydrogen protons in a to measure total field intensity with minimal orientation dependence, ideal for quantifying subtle deviations in surveying environments. These instruments, with noise levels below 0.05 nT/√Hz, are particularly effective in areas with rapid field changes, providing data that can be compared against regional magnetic models to confirm local attraction. GPS integration complements data by cross-verifying bearings with satellite-derived positions, enabling the detection of magnetic errors through discrepancies in heading calculations. In systems, low-cost GPS receivers paired with digital compasses undergo autocalibration to correct for vehicle-induced distortions, achieving relative positioning accuracies of approximately 1 m and reducing local attraction impacts in dynamic surveys. Software analysis via Geographic Information Systems (GIS) tools models local magnetic anomalies by processing survey data through spatial and filtering algorithms, overlaying magnetic readings with topographic and geological layers for anomaly extraction. Tools like the Anomaly Extractor facilitate automated segmentation and vectorization of geomagnetic datasets, supporting large-scale site assessments with resolutions up to 1 m. Recent advancements include drone-mounted magnetometers, which conduct aerial surveys over expansive areas to detect environmental magnetic anomalies efficiently, as demonstrated in 2020s applications for mineral exploration and archaeological prospection. These systems, equipped with fluxgate or Overhauser sensors, achieve line spacings of 10-50 m and sensitivities of 0.01 nT, minimizing ground access challenges while mapping attraction sources in remote terrains.

Remedies and Corrections

Preventive Strategies

Preventive strategies for local attraction in compass surveying emphasize proactive measures during the planning and setup phases to minimize magnetic interferences that could deflect the compass needle from true magnetic north. Site selection plays a critical role, involving the identification and avoidance of areas with potential magnetic disturbances such as ferrous materials, iron ore deposits, steel structures, power lines, or underground magnetic anomalies. Surveyors should conduct preliminary inspections to map out interference zones before finalizing station locations, ensuring the chosen sites are free from such influences to maintain bearing accuracy. Instrument handling protocols further reduce risks by incorporating non-magnetic equipment and proper positioning. Non-ferrous materials, such as aluminum and , are preferred for tripods and instrument components to eliminate induced ; for instance, specialized non-magnetic tripods with caps ensure zero interference during setup. Additionally, the should be positioned at a safe distance—typically 15 to 30 meters depending on the source strength—from any nearby ferrous objects, with larger separations recommended near significant attractors like power lines or vehicles to prevent deflection. Regular and storage of the in protective cases away from magnetic fields also preserve its integrity. Team protocols focus on human-related factors to safeguard observations. Survey personnel must remove or distance personal metallic items, such as keys, belts, watches, or mobile phones, from the vicinity during readings, as these can introduce subtle attractions. Avoiding times of high electrical fluctuations near artificial sources like power lines helps mitigate transient interferences. Best practices incorporate standardized pre-survey checklists to systematize these efforts. These checklists typically include verifying site clearance for magnetic hazards, confirming non-magnetic equipment readiness, ensuring team compliance with item removal, and logging environmental conditions like values. Adherence to such protocols promotes consistent accuracy and reduces the likelihood of local attraction impacting traverse closures.

Corrective Procedures

Once local attraction has been detected, typically through discrepancies in forward and back bearings, corrective procedures involve adjusting the affected magnetic bearings to restore accuracy in the survey traverse. These adjustments ensure that the directional measurements align with the true magnetic north, minimizing errors in subsequent computations. In bearing adjustment for a traverse, the error due to local attraction at affected stations is calculated and applied to all lines originating from those stations. Identify unaffected lines (where fore and back bearings differ by exactly 180°) and take their bearings as correct. For affected lines, compute included angles at stations and propagate corrections sequentially by adding/subtracting the angles to obtain correct bearings. If all lines are affected, calculate the interior angles of the closed traverse, which should sum to (2n - 4) × 90° for n sides, and distribute the total angular error proportionally among the affected stations to balance the traverse. For instance, if two stations are equally affected with total error α, apply α/2 to each. Mathematical correction in a closed traverse incorporates the closing , which may partly stem from local attraction, and uses to apportion and minimize residuals across observations. The approach estimates corrections by solving for parameters that minimize the sum of squared differences between observed and computed values, effectively attributing a portion of the angular misclosure to local magnetic influences while weighting observations by their precision. This technique is particularly useful in modern software for integrating data with other measurements. Field corrections may require shifting the observation point to a location free from magnetic interference or employing offset readings from a baseline to recalibrate the against a known . Such on-site adjustments allow immediate remediation without restarting the entire , ensuring continued with reduced error. All must be meticulously documented in field notes, including the magnitude and direction of adjustments, affected stations, and rationale, in accordance with standards like ISO 17123-1 for evaluating in geodetic instruments. This documentation facilitates traceability, verification, and integration into final survey reports.

Applications and Limitations

Practical Applications

In land surveying, knowledge of local attraction is essential for accurately correcting compass readings during boundary determinations in urban development projects near steel-framed buildings, where ferrous materials can cause deviations of up to several degrees in magnetic bearings. Surveyors typically detect and adjust for these influences by comparing observed and known true bearings in traverse networks, ensuring precise delineation of property lines and infrastructure alignments. For instance, in areas with nearby iron or steel structures, such as high-rise constructions, local attraction computations are integrated into fieldwork to mitigate errors that could otherwise lead to boundary disputes. In , local attraction principles underpin the use of magnetometry surveys to identify magnetic anomalies produced by buried iron artifacts, aiding non-invasive site assessments. These anomalies arise from the ferromagnetic properties of iron objects, generating localized magnetic fields that deviate needles or register on fluxgate magnetometers, often with signal strengths exceeding 100 nT for metallic remains. Such techniques have been applied in field surveys to map potential artifact distributions, enabling targeted excavations while preserving site integrity. Military navigation training emphasizes compensating for local attraction when using magnetic compasses in ferrous-rich environments, such as armored vehicles or naval vessels, to maintain accurate bearings during operations. On ships, deviation tables derived from onboard steel components correct compass errors, which can reach 10-15 degrees without adjustment, while vehicle-based training includes checks for magnetic interference from engines and weaponry. This preparation ensures reliable dead reckoning and orienteering in scenarios where GPS may be unavailable. In environmental monitoring during the 2020s, magnetic methods informed by local attraction concepts have been employed to detect pollution from industrial sites through soil and atmospheric magnetic susceptibility measurements, identifying heavy metal contaminants like iron oxides emitted from factories. These assessments, often using portable magnetometers, quantify anthropogenic magnetic enhancements in ecological surveys, with susceptibility values elevated by factors of 2-5 times background levels near emission sources, supporting regulatory compliance and remediation planning.

Limitations in Contemporary Practice

In contemporary navigation and surveying, the relevance of addressing local attraction has diminished significantly due to the widespread adoption of Global Navigation Satellite Systems (GNSS) integrated with Inertial Navigation Systems (INS). Magnetic compasses, once primary tools susceptible to local magnetic deviations, now serve primarily as low-cost backups in scenarios where GNSS signals are unavailable, such as underwater or jammed environments. In GNSS/INS setups, heading accuracies of 0.3° root mean square (RMS) are routinely achieved using dual-antenna configurations, rendering local attraction errors—typically on the order of several degrees—negligible for most applications. Increasing artificial magnetic sources in urban environments further complicate the detection and mitigation of local attraction, exacerbating challenges for residual compass-based workflows. Electric vehicles (EVs) generate static magnetic fields up to 0.2 millitesla near batteries and motors, which is approximately four times the intensity of Earth's ambient field (~50 microtesla), potentially causing substantial compass deviations during proximity surveys. These factors demand more sophisticated anomaly isolation techniques beyond traditional compass adjustments. The high cost of advanced magnetometers required for precise local attraction detection also limits their deployment, particularly in developing regions where surveying budgets are constrained. Specialized proton or fluxgate magnetometers, essential for high-resolution anomaly mapping, can exceed $10,000 per unit, posing barriers for small enterprises and resource-limited national agencies that rely on basic equipment. This economic hurdle, coupled with the need for specialized in interpretation and , restricts widespread adoption and perpetuates reliance on less accurate, error-prone methods in such areas. Contemporary resources on local attraction often emphasize outdated traditional methods, with insufficient integration of digital tools like for enhanced anomaly mapping in complex terrains. While excels at generating high-resolution 3D topographic models to contextualize magnetic variations, its combined use with magnetometry remains underexplored, leading to gaps in holistic protocols that could better address modern environmental complexities.

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