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A simple alidade for use with a ceiling projector

An alidade (/ˈæ.lɪ.deɪd/) (archaic forms include alhidade, alhidad, alidad)[citation needed] or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site using a plane table drawing of intersecting lines in the direction of the object from two or more points or to measure the angle and horizontal distance to the object from some reference point's polar measurement. Angles measured can be horizontal, vertical or in any chosen plane.

The alidade sighting ruler was originally a part of many types of scientific and astronomical instrument. At one time, some alidades, particularly using circular graduations as on astrolabes, were also called diopters.[1] With modern technology, the name is applied to complete instruments such as the 'plane table alidade'.

Origins

[edit]
An example of an alidade on a circumferentor. Taken from the Table of Surveying, Cyclopaedia, Volume 2, 1728

The word in Arabic (الحلقة العضدية, al-ḥilqa al-ʿaḍudiyya, lit. 'the ruler'), signifies the same device. In Greek and Latin, it is respectively called δίοπτρα, "dioptra", and linea fiduciae, "fiducial line".

The earliest alidades consisted of a bar, rod or similar component with a vane on each end. Each vane (also called a pinnule or pinule) has a hole, slot or other indicator through which one can view a distant object. There may also be a pointer or pointers on the alidade to indicate a position on a scale. Alidades have been made of wood, ivory, brass and other materials.

Examples of old alidade types

[edit]
Several examples of alidade types.
Traverse plane-table alidade, c. 1898

The figure on the left displays drawings that attempt to show the general forms of various alidades that can be found on many antique instruments. Real alidades of these types could be much more decorative, revealing the maker's artistic talents as well as his technical skills. In the terminology of the time, the edge of an alidade at which one reads a scale or draws a line is called a fiducial edge.

Alidade B in the diagram shows a straight, flat bar with a vane at either end. No pointers are used. The vanes are not centred on the bar but offset so that the sight line coincides with the edge of the bar.

The vanes have a rectangular hole in each with a fine wire held vertically in the opening. To use the alidade, the user sights an object and lines it up with the wires in each vane. This type of alidade could be found on a plane table, graphometer or similar instrument.

Alidades A and C are similar to B but have a slit or circular hole without a wire. In the diagram, the openings are exaggerated in size to show the shape; they would be smaller in a real alidade, perhaps 2 mm or so in width. One can look through the openings and line the openings up with the object of interest in the distance. With a small opening, the error in sighting the object is small. However, if a dim object such as a star is observed through a small hole, the image is difficult to see.

This form is shown in the diagram as having pointers. These can be used to read off an angle on a scale that is engraved around the outer edge (or limb) of the instrument. Alidades of this form are found on astrolabes, mariner's astrolabes and similar instruments.

Alidade D has vanes without any openings. In this case, the object is viewed and the alidade is rotated until the two opposite vanes simultaneously eclipse the object. With skill, this sort of alidade can yield very precise measures. In this example, pointers are shown.

Alidade E is a representation of a very interesting design by Johannes Hevelius. Hevelius was following in Tycho Brahe's footsteps and cataloging star positions with high accuracy. He did have access to the telescopic sights that were being used by astronomers in other countries, however, he chose to use naked-eye observations for his positional instruments. Due to the design of his instruments and the alidades, as well as his diligent practices, he was able to yield very precise measures.

Hevelius' design[2] featured a pivot point with a vertical cylinder and a vane at the observer's end. The vane had two narrow slits that were spaced precisely the same distance apart as the diameter of the cylinder (in the diagram, the portion of the vane between the slits is removed for clarity; the left and right edges of the opening represent the slits). If the observer could sight a star on only one side of the cylinder, as seen in F, the alignment was off. By carefully moving the vane so that the star could just barely be seen on either side of the cylinder (G), the alidade was aligned with the position of the star. This could not be used with a closely located object. A star, being so far away as to exhibit no parallax to the naked-eye, would be observable as a point source simultaneously on both sides.

Modern alidade types

[edit]
A U.S. Navy sailor using a telescopic alidade.

The alidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down.

In a sextant or octant the alidade is the turnable arm carrying a mirror and an index to a graduated circle in a vertical plane. Today it is more commonly called an 'index arm'.

Alidade tables have also long been used in fire towers for sighting the bearing to a forest fire. A topographic map of the local area, with a suitable scale, is oriented, centered and permanently mounted on a leveled circular table surrounded by an arc calibrated to true north of the map and graduated in degrees (and fractions) of arc. Two vertical sight apertures are arranged opposite each other and can be rotated along the graduated arc of the horizontal table. To determine a bearing to a suspected fire, the user looks through the two sights and adjusts them until they are aligned with the source of the smoke (or an observed lightning strike to be monitored for smoke). See Osborne Fire Finder.

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Alidade

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An alidade is a sighting instrument consisting of a straightedge equipped with aligned sights or a telescope, used primarily in surveying, astronomy, and navigation to measure angles and directions by aligning a line of sight with a target and reading a corresponding scale.[1][2] The term derives from the Arabic al-ʿiḍādah, meaning "the revolving radius" or "the rule," reflecting its origins as a rotating arm or pointer on early goniometric devices.[2][1] Originating from ancient Greek instruments like the dioptra and gaining prominence in medieval Islamic astronomy through integration into astrolabes and quadrants, alidades spread to Europe in the late medieval period and evolved into precise tools, including telescopic versions by the early 19th century for enhanced accuracy in land surveying.[1][3][4] Alidades are primarily employed in plane table surveying to create topographic maps by sighting distant points and measuring angles from known stations, a method standard for U.S. Geological Survey mapping until the late 20th century.[5][6][3][7] Though largely supplanted by modern electronic tools like total stations and GPS since the 1980s, alidades remain valuable for educational purposes and in low-technology environments due to their mechanical reliability and portability.[7]

Definition and Principles

Basic Concept

An alidade is a sighting device consisting of a straightedge equipped with a fiducial edge or optical alignment mechanism, designed to measure angles relative to a fixed reference such as a graduated circle or plane table.[5][6] This instrument facilitates the precise observation of distant objects by aligning the line of sight along the fiducial edge, allowing users to determine directions, bearings, or elevations directly.[8] Its primary purpose lies in enabling angular measurements without requiring elaborate computations, making it a foundational tool for fieldwork in disciplines like surveying and astronomy.[9] At its core, an alidade comprises essential components such as a sighting vane or telescope for visual alignment, an index arm to indicate position on a scale, and a fiducial line or edge that ensures accurate referencing against the measurement base.[6][10] These elements work in tandem to project a straight line of sight onto a drawing surface or scale, capturing the orientation of remote points relative to the observer's position.[5] In modern contexts, it plays a key role in plane table surveying by allowing direct plotting of sighted lines onto maps.[5]

Optical and Mechanical Principles

The optical principles of an alidade rely on establishing a precise line of sight to a distant target, enabling accurate angular measurement without distortion from parallax. In simple designs, this is achieved through sighting vanes at each end of a straightedge, featuring pinholes or narrow slits that allow light from the target to form a collimated beam, approximating parallel rays for alignment.[11] More advanced telescopic alidades incorporate lenses in an erect-image telescope to magnify the view (typically 25x) and focus the line of sight via cross-hairs or fiducial marks, ensuring the observer's eye position does not shift the apparent target location, thus providing parallax-free sighting.[12] These fiducial elements, such as etched wires or projected curves, serve as reference points aligned with the optical axis, transferring the target's direction directly onto a base plane.[13] Mechanically, the alidade features a pivoting index arm mounted on a trunnion axis, allowing smooth rotation over a graduated protractor scale on the base, which facilitates azimuthal and elevational adjustments.[6] This arm, often constructed from brass or aluminum for stability, integrates a straightedge parallel to the line of sight, ensuring that the sighted direction corresponds exactly to the drawn line. Vernier scales attached to the vertical or horizontal circles provide fine angular resolution, often reading to single minutes (60 arcseconds) by interpolating between main graduations, enhancing precision in angle transfer.[14] Levels, such as transverse and circular bubbles, maintain the instrument's orientation during pivoting, preventing tilt-induced errors.[12] The alignment process begins with positioning the alidade over the measurement point on a leveled base, such as a plane table, and sighting the target through the optical vanes or telescope until the fiducial line coincides with the object. This action rotates the index arm, directly projecting the angle onto the graduated scale via the parallel straightedge, where the reading is noted for mapping or computation. In telescopic variants, internal focusing adjusts for distance, maintaining collimation without altering the mechanical pivot.[3] Instrumental errors, such as collimation offset—where the line of sight deviates from perpendicularity to the trunnion axis—can introduce angular inaccuracies, conceptually arising from misalignment in the optical train or mechanical wear. Corrections involve adjusting capstan screws on the telescope mount to realign the line of sight with fiducial references, verified by sighting known horizontal lines and conceptually applying trigonometric principles to quantify and nullify the offset angle during transfer to the scale.[13][12]

Historical Development

Ancient Origins

The term "alidade" derives from the Arabic "al-'iḍāda," meaning "the indicator" or "revolving radius," and entered European languages through medieval translations of scientific texts.[15] Although the modern name is of later origin, the underlying concept of a sighting device for measuring angles emerged in ancient civilizations as simple rulers or bars equipped with sights for precise observation.[16] The earliest known uses of such instruments date to around the 3rd century BCE in Greek contexts, where they were integrated into surveying and astronomical tools influenced by prior Babylonian observations of celestial movements.[17] By the 1st century CE, Hero of Alexandria detailed the dioptra in his treatise Dioptra, describing it as a portable device with a rotating sighting bar (an early alidade form) mounted on a stand, used for leveling, angular measurements, and sighting distant points in geodesy and astronomy.[18] This instrument allowed for accurate determination of horizontal and vertical angles, building on simpler Greek prototypes from circa 200 BCE that combined sighting rods with protractors for goniometric purposes. In Egyptian and Mesopotamian contexts, analogous simple wooden or bronze rulers with open sights facilitated solar and stellar observations, predating more complex Greek designs. For instance, the Egyptian merkhet, an L-shaped palm-rib bar paired with a plumb line dating to at least 600 BCE, served as a sighting tool for tracking stars across the horizon during nighttime astronomy and diurnal leveling.[19] Mesopotamian astronomers employed clay tablets to record planetary positions, enabling systematic angular estimates in their sexagesimal system. Such instruments held profound cultural significance in enabling early architectural feats and rudimentary cartography. In Egypt, merkhet sightings aligned major structures like the pyramids to cardinal directions and stellar positions, as seen in the near-perfect north orientation of the Great Pyramid at Giza (deviating by only about 3 arcminutes), achieved through stellar transits over a level horizon.[20] In Mesopotamia, similar sighting methods supported ziggurat orientations and initial mapping of irrigation networks, laying foundations for later Hellenistic advancements in the astrolabe.[17]

Medieval and Early Modern Evolution

During the medieval Islamic Golden Age, scholars advanced the alidade's design within astrolabes, enhancing its precision for astronomical and navigational purposes. In the 11th century, the Andalusian astronomer al-Zarqali (Azarquiel) invented the azafea, a universal astrolabe that incorporated an alidade with refined sighting mechanisms, allowing measurements without latitude-specific plates through stereographic projections centered on the equinoctial point.[21] This innovation included universal diagrams, or rete configurations, and precise engraved scales that facilitated calculations of altitudes, azimuths, and time, proving invaluable for navigation across diverse latitudes.[21] These refinements built on earlier sighting devices, transforming the alidade from a basic ruler into a versatile tool for Islamic astronomers and mariners. European scholars adopted and adapted the alidade in the late medieval period, integrating it into their own instrumental traditions. By the early 14th century, as seen in the 1326 "Chaucer" astrolabe—one of the earliest dated European examples—the alidade appeared as a pivoting ruler with sights for measuring celestial altitudes.[22] Geoffrey Chaucer's 1391 Treatise on the Astrolabe further popularized it, describing the alidade (termed the "label") as essential for aligning sights and reading scales on the astrolabe's back for practical astronomy. In the 15th century, Johannes Regiomontanus advanced its application by incorporating alidades into quadrants and sectors, instruments that enabled more accurate angular measurements for both astronomical observations and land surveying, as detailed in his Scripta on instrument construction.[23] Key advancements in the 16th and 17th centuries standardized the alidade in cartographic practices, supporting the era's expansive mapping efforts. Cartographers like Gerardus Mercator employed alidades in conjunction with plane tables and quadrants for precise triangulation and bearing measurements in regional mapping, as in his surveys for the 1564 Lotharingen map and subsequent charts that required reliable field surveys.[24] By the 18th century, alidades evolved for marine use with the addition of adjustable sight vanes—slotted or pierced plates for alignment—allowing sailors to measure horizontal angles over the horizon despite ship motion, as in designs fitted to compasses. Technological shifts during this period marked a transition from wooden or simple metal rulers to durable engraved brass alidades, which offered finer graduations and greater stability. By the late 18th century, the incorporation of telescopic sights into alidades further enhanced precision for distant observations in surveying and navigation.

Types of Alidades

Traditional Alidades

Traditional alidades, prevalent before the 19th century, were fundamental sighting instruments consisting of a simple straightedge ruler crafted from wood or metal, equipped with open sights or pinhole vanes positioned parallel to the fiducial edge for aligning lines of sight.[3][25] These devices were typically placed atop protractors, astrolabes, or early plane tables to measure angles by sighting distant objects and marking directions directly on a drawing surface.[26] The basic design relied on manual alignment, drawing from principles of sighting where the observer's eye, the vane apertures, and the target formed a straight line for accurate bearing determination.[25] Notable examples include 17th-century alidades adapted for plane table surveying, which facilitated topographic mapping by allowing surveyors to plot features in real-time during fieldwork.[27] Another application appeared in marine navigation, where alidades mounted on a ship's binnacle enabled officers to take relative bearings of landmarks or other vessels against the compass rose for course adjustments.[28] These historical variants, often brass or wooden, emphasized portability and simplicity for use in both land and sea environments.[29] In construction, traditional alidades featured two vertical vanes at opposite ends of the ruler: a fixed eye vane with a narrow slit or pinhole for peering through, and an object vane with a central hair or wire to align against the target, both hinged for folding when not in use.[25] This setup supported forward and reverse sightings by rotating the alidade 180 degrees on its base, ensuring bidirectional alignment without repositioning the instrument.[3] Integration with semicircular scales occurred via the fiducial edge, which rested along the scale's radius to read angles directly after sighting, often incorporating a plumb line or spirit level for orientation.[26] Despite their utility, traditional alidades were susceptible to parallax errors, where misalignment of the observer's eye with the sight line caused apparent shifts in the target's position, degrading precision.[30] Without optical magnification, their accuracy was not very high, sufficient for rudimentary mapping but inadequate for finer measurements.[25]

Modern Alidades

The telescopic alidade emerged in the early 19th century as a significant advancement over open-sight designs, incorporating a telescope to enhance sighting precision in plane table surveying.[3] This innovation allowed for more accurate alignment over longer distances, with typical models featuring achromatic lenses and erecting prisms to produce upright images and minimize chromatic aberration.[31] Magnification levels generally ranged from 15x to 25x, enabling angular measurements with errors reduced to about 1 minute of arc.[32][4] Specialized variants further adapted the alidade for demanding environments, such as early 20th-century marine alidades mounted on gyrocompasses for naval navigation. These devices used the gyroscope's stabilization to maintain orientation amid ship motion, and included auxiliary optics for simultaneous compass card viewing and bubble leveling.[33][34] In recent decades, some digital alidades have incorporated encoders to automate angle readouts, replacing manual verniers with electronic displays for faster data capture and reduced human error in field measurements.[11] Manufacturing shifts emphasized lightweight durability, with aluminum alloys replacing heavier brass in constructions, combined with precision machining to achieve tighter tolerances in optical alignments.[35] This material evolution facilitated integration as detachable sighting units on theodolites, allowing modular use in both standalone and combined surveying setups.[36] As of 2025, alidades persist in niche applications like archaeological site mapping and educational training, where their mechanical simplicity aids hands-on learning of angular sighting principles despite being largely supplanted by electronic total stations in professional surveying.[11] Their value lies in low-cost, battery-free operation for remote field work, though adoption remains limited outside specialized contexts.[37]

Applications

In Surveying and Mapping

In plane table surveying, the alidade is mounted on a drawing board known as the plane table, enabling surveyors to directly plot sight lines from observed points onto the map sheet in real time. This setup facilitates precise angular measurements by aligning the alidade's sighting device—typically a telescope or vane—with distant targets, allowing rays to be drawn along the straight edge to represent lines of sight.[38] Two primary methods employed are intersection and resection: in intersection, rays from two or more known stations are drawn to intersect at the unknown point's location, ideal for locating details without occupying the point; resection, conversely, determines the plane table's position by sighting back to two or three known points and solving for the station through graphical trial or mechanical aids.[39][38] For height and distance determination, the alidade often incorporates a clinometer function through a vertical circle attachment, which measures vertical angles to compute elevations and baselines. The basic formula for elevation derives from trigonometry in a right triangle formed by the horizontal distance dd to the object, the vertical rise hh, and the line of sight as the hypotenuse: the angle θ\theta sighted on the vertical circle is the angle of elevation, where tan⁡θ=hd\tan \theta = \frac{h}{d}, so h=dtan⁡θh = d \tan \theta.[40] To derive this, consider the observer at ground level sighting the top of an object; the horizontal distance dd is measured separately (e.g., by tape or pacing), and θ\theta is read directly from the alidade's scale—the tangent function relates the opposite side (height) to the adjacent side (distance) in the triangle, yielding hh when solved, with adjustments for eye height if needed to refine accuracy.[41] Historically, alidades were integral to 19th-century topographic mapping workflows, such as those conducted by the United States Geological Survey (USGS), where plane table methods with alidades enabled rapid field plotting of terrain features across vast western landscapes, producing foundational quadrangle maps.[42][43] In modern applications, portable alidades continue in archaeological site planning, where teams use plane table setups to map surface features and earthworks on small scales, combining traditional sighting with digital overlays for detailed plans.[27][44] The advantages of alidades in terrestrial mapping include real-time visualization of plotted points against the actual terrain, which minimizes transcription errors and supports on-site adjustments, particularly beneficial in rugged or mountainous areas where direct distance measurement is challenging.[45] This graphical approach excels in contexts requiring quick, low-cost surveys of irregular topography, as the immediate correlation between map and landscape reduces cumulative inaccuracies from separate data collection phases.[46]

In Navigation and Astronomy

In marine navigation, alidades are mounted on azimuth circles fitted to a ship's compass to measure the bearings of stars, landmarks, or other vessels, enabling precise determination of relative positions at sea.[47] These instruments facilitate relative motion plotting, where successive bearings are used to construct lines of position (LOPs) on charts, allowing navigators to compute fixes by intersecting these lines and accounting for the vessel's speed and course.[48] Alidades, used in earlier instruments such as the mariner's astrolabe, contributed to the development of more advanced tools like the sextant, which became standard in the 18th and 19th centuries for angular sightings in basic position estimation.[49] In astronomical applications, alidades are integral to sighting stars on traditional instruments such as astrolabes or quadrants, where they align the observer's line of sight to measure the altitude of celestial bodies above the horizon for latitude determination.[10] For instance, on a mariner's astrolabe, the alidade's vanes are adjusted to sight Polaris or the sun, with the resulting angle read from the instrument's scale and compared to ephemerides to calculate the observer's parallel.[50] Amplitude observations, involving the alidade to note a body's bearing as it rises or sets on the horizon, further aid in verifying compass deviations by comparing observed arcs against tabulated values.[51] Key techniques employing alidades in these fields center on azimuth measurement, defined as the angle α representing the bearing of a celestial or terrestrial object clockwise from true north, typically ranging from 0° to 360°.[52] Alidades provide azimuth bearings for plotting lines of position. Separately, altitude observations with a chronometer enable longitude determination: the time difference between local apparent noon and UTC yields the east-west offset in degrees (15° per hour). Such methods parallel angle sighting in surveying but emphasize dynamic horizon references for positional accuracy in motion.[48] Alidades retain niche roles in traditional sailing vessels and maritime training, where they provide reliable, non-electronic backups for bearing tasks amid potential GPS failures.[53] In planetariums and amateur astronomy, replica astrolabes with alidades continue to demonstrate celestial sighting principles, fostering hands-on understanding of historical navigation without modern electronics.[10]

References

  1. In depth - Alidade - Museo Galileo
  2. ALIDADE Definition & Meaning - Merriam-Webster
  3. Open-sight Alidades | U.S. Geological Survey - USGS.gov
  4. Telescopic Alidade | Smithsonian Institution
  5. Alidade Alliance | U.S. Geological Survey - USGS.gov
  6. [PDF] Chapter 1 Surveying - USDA
  7. [PDF] THE DEVELOPMENT OF SURVEY INSTRUMENTS
  8. Telescopic Alidade
  9. Epact: Scientific Instruments of Medieval and Renaissance Europe
  10. Alidade Definitions for Land Surveyors - Learn CST
  11. The Parts of an Astrolabe | Whipple Museum
  12. What are the Different Types of Alidade? - Geomaster Group
  13. [PDF] Optical and Mathematical Instruments - Telescopic Alidade
  14. [PDF] Lesson 1. Surveying – Introduction - Vcet civil
  15. Telescopic Aildade | Smithsonian Institution
  16. Alidade Definition & Meaning - YourDictionary
  17. In depth - Diopter - Museo Galileo
  18. Diopter | instrument - Britannica
  19. Dioptra | book by Heron of Alexandria - Britannica
  20. Egyptian Merkhet | Science Museum Group Collection
  21. Yale Assyriologist decodes 'writing of the heavens' by ancient ...
  22. Pyramids Aligned with Stars: Ancient Egyptian Sky Secrets
  23. al-Zarqali (1029 - 1100) - Biography - MacTutor History of Mathematics
  24. The 'Chaucer' astrolabe (1326) - IHR Web Archives
  25. Regiomontanus (1436 - 1476) - Biography - MacTutor
  26. [PDF] 19 • Land Surveys, Instruments, and Practitioners in the Renaissance
  27. Alidade | Royal Museums Greenwich
  28. A Few Technical Items: Questions About 18th Century Surveying ...
  29. Plain Alidade - NPTEL Archive
  30. Plane Tables - Compleat Surveyor
  31. [PDF] Graphical and Plane Table Survey of Archaeological Earthworks
  32. A History of the Ship's Compass - U.S. Naval Institute
  33. The Marine Alidade: A Vital Tool for Oceanic Expeditions
  34. Explaining Parallax Error and How to Correct it - Primary Arms Blog
  35. The Alidade: 4 Interesting Facts About This Device - Warren Knight
  36. Telescopic Marine Alidade - Warren Knight
  37. Transit Aluminum Telescopic Alidade Survey & Level Theodolite ...
  38. The Many Uses For Alidades & Telescopic Alidades - Warren Knight
  39. Tools For Tuesday: The Alidade - Land Surveyor Blogs - Land ...
  40. Methods of Plane Table Surveying - Civinnovate
  41. Resection Method - Mapping Around
  42. Vertical Distances and Angles (clinometer) - Field Techniques
  43. Measuring Height or Altitude with an Inclinometer | Science Project
  44. 125 Years of Topographic Mapping - ArcNews Fall 2009 Issue - Esri
  45. The Last Plane Table Surveyor?
  46. Survey and Map - The Hidden Heritage of a Landscape
  47. [PDF] unit-5-plane-table-survey-orientation-radiation-intersection ...
  48. Plane Table Surveying: Advantages and Disadvantages
  49. MD60A2K Telescopic Alidade Sight for Bearing Compass Repeaters
  50. Instill the Fundamentals of Seamanship and Navigation | Proceedings
  51. The History of the Sextant
  52. Mariner's Astrolabe - Ages of Exploration
  53. Amplitude - Celestial Navigation - The Nautical Site
  54. Azimuth method for ship position in celestial navigation
  55. The Evolution of the Sextant - November 1936 Vol. 62/11/405
  56. Atlas Instrument Co.
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