An alidade () (archaic forms include alhidade, alhidad, alidad) 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 In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle m ...

a scale map on site using a

plane table A plane table (plain table prior to 1830) is a device used in surveying, site mapping, exploration mapping, coastal navigation mapping, and related disciplines to provide a solid and level surface on which to make field drawings, charts and maps. ...

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

astrolabe An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...

s, were also called ''diopters''. With modern technology, the name is applied to complete instruments such as the '

alidade'.

Origins

The word in

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

(, , ), signifies the same device. In

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

and

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

, it is respectively called , "''dioptra''", and , "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

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

graphometer The graphometer, semicircle or semicircumferentor is a surveying instrument used for angle measurements. It consists of a semicircular wiktionary:Limb#Etymology 2, limb divided into 180 degrees and sometimes subdivided into minutes. The limb is s ...

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 Limb may refer to: Science and technology *Limb (anatomy), an appendage of a human or animal *Limb, a large or main branch of a tree *Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb *Limb, in botany, t ...

) of the instrument. Alidades of this form are found on

mariner's astrolabe The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude (declination) or the meridian altitude of a star of known declination. Not an astrolabe ...

s 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 Johannes Hevelius Some sources refer to Hevelius as Polish: * * * * * * * Some sources refer to Hevelius as German: * * * * *of the Royal Society * (in German also known as ''Hevel''; ; – 28 January 1687) was a councillor and mayor of Danz ...

. Hevelius was following in

Tycho Brahe Tycho Brahe ( ; ; born Tyge Ottesen Brahe, ; 14 December 154624 October 1601), generally called Tycho for short, was a Danish astronomer of the Renaissance, known for his comprehensive and unprecedentedly accurate astronomical observations. He ...

'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' designJoseph Ashbrook, ''The Astronomical Scrapbook'', Sky Publishing Corporation, Cambridge, MA, 1984 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 Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

to the naked-eye, would be observable as a point source simultaneously on both sides.

Modern alidade types

The alidade is the part of a

theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...

that rotates around the vertical axis, and that bears the horizontal axis around which the

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

(or visor, in early telescope-less instruments) turns up or down. In a

sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of cel ...

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 wildfire, forest fire, or a bushfire is an unplanned and uncontrolled fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identified as a bushfire ( in Australia), dese ...

. A

topographic map In modern mapping, a topographic map or topographic sheet is a type of map characterized by large- scale detail and quantitative representation of relief features, usually using contour lines (connecting points of equal elevation), but histori ...

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 Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...

strike to be monitored for smoke). ''See'' Osborne Fire Finder.

References

* Gerard L'E. Turner, ''Nineteenth Century Scientific Instruments'', Sotheby Publications, 1983, * Gerard L'E. Turner, ''Antique Scientific Instruments'', Blandford Press Ltd. 1980,

External links

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Origins

Examples of old alidade types

Modern alidade types

See also

References

External links