Geodetic Instruments

Geodetic Instruments

 

mechanical, optical-mechanical, electrooptical, and radio electronic apparatus for the measurement of the lengths of lines, angles, and superelevations in the construction of astronomical-geodetic grids and leveling nets; for mapping, construction, and installation; and for the operation of large engineering structures, radiotelescope antennas, and so on. Geodetic instruments also include instruments for astronomical determinations associated with geodetic projects, as well as for surveying.

Instruments and devices for measuring the length of lines. Ordinary measurements of the lengths of lines are performed with steel measuring tapes 20 or 50 m long, which are placed on the ground, while marking their ends with pins. The relative error of tape measurements depends on topographic conditions; it averages 1:2,000. More accurate measurements are performed with invar tapes, which are stretched by dynamometers. This makes it possible to reduce the error to 1:20,000-1:50,000. Even more accurate measurements, usually those of the bases in triangulation, require the use of base instruments with suspended invar measuring wires 24 m long. The relative error of such measurements is of the order of 1:1,000,000, which corresponds to 1 mm per kilometer of measured distance.

Geodetic projects also use rangefinders, which are either combined with a telescope or are attachments for the telescope of a geodetic instrument. Rangefinders make possible the determination of the length of the unknown line by determining the triangle whose apex coincides with the principal front focus of the lens of the instrument’s telescope; the height of the triangle is the line being measured. In this case the base and the opposite angle of the triangle are known.

There are also electrooptical rangefinders and radio rangefinders, which make possible the determination of distance according to the time required by the light or radio waves of known rate of propagation to traverse the distance being measured.

Instruments for determining directions and measuring angles. A compass may be used for the simplest determination of the directions of lines with respect to the meridian. The compass may be either a separate geodetic instrument or an accessory for other instruments. The error of a compass is 10-15’.

More accurate determinations of directions and angles in geodesy require the use of various other instruments. A prototype of these instruments was the astrolabe, which was invented even before the Common Era and consisted of a circle with divisions, from which angles were measured with the aid of a rotating rule with diopters for sighting at an object. Other angle-measuring instruments, such as the pan-tometer (an astrolabe with a vertical circle, permitting the measurement of horizontal as well as vertical angles), appeared during the second half of the 16th century. In the 17th century telescopes (1608), microscopes (1609), verniers (1631), levels (1660), and crosshairs (1670) began to be used in goniometric instruments. The basic angle-measuring instrument, which was called the theodolite, was formed in this way.

The theodolite is placed on a tripod, the vertical axis is brought into the normal position using elevating screws and a level and, by turning the telescope about its vertical and horizontal axes, is aimed at the target point and readings are taken on the circles. This gives the direction, and the angle is obtained as the difference between two converging directions. In modern theodolites the circles are fabricated from optical glass, with a scale diameter of 6-18 cm. The most useful interval between divisions is 20’ or 10’, and either scale microscopes, with a readout accuracy of 1-6”., or so-called optical micrometers, with an accuracy of up to 0.2”-03”, are used as readout devices.

During the 1960’s, so-called gyrotheodolites and various gyroscopic attachments began to be used for determining the direction of the true (geographic) meridian. The error of determinations using a gyrotheodolite is 5”-10”.

The axial, mounting, and aiming devices of goniometric instruments are subject to strict requirements. For example, in high-precision theodolites the angular oscillations of the vertical axes do not exceed 2”, and in transit instruments, the permissible irregularity of their journals (on which the telescope rotates) is fractions of a micron. The mounting devices should not generate elastic deformations in the axial systems and displacements of the attached parts of the instrument at the instant of attachment. The aiming devices should be capable of achieving very fine displacement of the instrument parts—for example, turns with a precision of up to a fraction of a second of arc.

The telescopes of goniometric and other geodetic instruments have 15- to 65-power magnification. The most widespread are the so-called telescopes with inner focusing, which are equipped with telephoto lenses whose rear component, called the focusing lens, may be moved to produce a clear image of objects at various distances. The accuracy of sighting through the telescope depends on its magnification, the diameter of the lens aperture, and the image quality provided by the lens, as well as on the form, size, illumination, and contrast properties of the target being sighted. With increasing target distance, great importance is acquired by the atmospheric interference, which lowers the contrast and causes oscillations of the target image. Under ideal conditions, high-quality telescopes with a 30- to 40-power magnification give a sighting error of about 0.3”.

The so-called automatic and semiautomatic tachymeters, which make it possible without calculations to determine by direct reading from a rod the distances and super elevations of the points of placement of the rod, reduced to the horizontal plane, or to determine only the distances without calculations, while computing the super elevations from the measured distance and angle of inclination, are similar to theodolites.

Instruments for determining superelevations. Mechanical-optical levels with horizontal collimating rays are usually used for leveling. These instruments are used for taking readings from rods that are mounted on points whose height difference is to be determined. Levels with inclined collimating rays are also known. These instruments make it possible to determine considerable superelevations from a single position, but they have not become widespread because of their lesser accuracy. In some cases—for example, for determining the positions of islands relative to a continent—so-called hydrostatic levels, which are based on the property of interconnected vessels of preserving the level of the liquid filling them, are used.

The first mention of levels is associated with the names of Hero of Alexandria and the Roman architect Vitruvius (first century B.C.). Levels began to take on their present form simultaneously with the appearance of clinometers and telescopes (17th century).

Levels with a horizontal collimating ray are distinguished by the type of interconnection of the three basic parts: the telescope with crosshairs, which fixes the collimating ray; the clinometer, which brings this ray into a horizontal position; and the tripod, which supports the telescope and is connected to the vertical axis of rotation. Levels with a fixed telescope, level, and tripod, which are called dumpy levels, began to be used in the mid-20th century.

In the 1950’s, levels with automatic adjustment of the collimating ray came into widespread use. These devices use a compensator, which is an optical part of the telescope that is suspended by a pendulum mount, for the horizontal adjustment of the collimating ray. Such a level was first constructed in the USSR in 1946.

Surveying rods 1.5 to 4 m high are used in leveling. The scales for precise leveling, in which the sighting distance does not exceed 50 m, consist of lines 1 mm wide spaced at 5-mm intervals on an invar tape stretched over a wooden support by springs, which guarantees the constancy of the scale length in case of temperature fluctuations. Lower-class leveling, in which the sighting distance may be as high as 100 m, uses wooden rods with scales consisting of checker squares 1 cm wide with intervening spaces of the same width.

Instruments for graphic surveying. In spite of the large-scale development of stereophotogrammetric surveying methods, graphic or plane-table surveying is still being used. The basic instruments for this technique are the plane table and the telescopic alidade.

During the 19th century, telescopic alidades of the so-called Main Headquarters type were produced and widely used in Russia. In the 1930’s, the KShV (Shiriaev-Vilem) portable telescopic alidade, which was advanced for its time, was produced in the USSR; it was combined with a simplified plane table.

History. The history of geodetic instrument production in Russia began during the reign of Peter I. The most important Russian scientists and inventors were involved in the construction of geodetic instruments, starting with M. V. Lomonosov and I. P. Kulibin. Subsequently (at the turn of the 19th century), geodetic instruments were produced in the shops of the Academy of Sciences, the Main Headquarters, and the Pulkovo Observatory. In this connection the contributions of V. K. Dellen, V. Ia. Struve, and A. S. Vasil’ev were of great importance. However, there was almost no industrial production of geodetic instruments in Russia, and the demand for these instruments was primarily satisfied by imports.

Soviet geodetic instrument science began in the 1920’s in Moscow with the foundation of the Geodeziia and Geofizika factories, where the design and lot production of geodetic instruments of technical precision grades was established. In the late 1920’s, F. N. Krasovskii headed work on the production of high-precision geodetic instruments for the generation of state reference grids. Geodetic instruments were produced at the Aerogeopribor factory (now the Experimental Opticomechanical Factory in Moscow). The opticomechanical industry in the USSR produces tens of thousands of geodetic instruments per year. The design and production technology of these instruments are at the highest level of world technology.

REFERENCES

Krasovskii, F. N., and V. V. Danilov. Rukovodstvo po vysshei geodezii, 2nd ed., part 1, fasc. 1-2. Moscow, 1938-39.
Chebotarev, A. S. Geodeziia, 2nd ed., parts 1-2. Moscow, 1955-62.
Litvinov, B. A. Geodezicheskoe instrumentovedenie. Moscow, 1956.
Eliseev, S. V. Geodezicheskie instrumenty i pribory [2nd ed.]. Moscow, 1959.
Araev, I. P. Opticheskie teodolity srednei tochnosti. Moscow, 1955.
Zakharov, A. I., and I. I. Zuikov. Teodolity srednei tochnosti i opticheskie dal’nomery. Moscow, 1965.
Gusev, N. A. Marksheidersko-geodezieheskie instrumenty i pribory, 2nd ed. Moscow, 1968.
Zakharov, A. I. Novye teodolity i opticheskie dal’nomery. Moscow, 1970.

G. G. GORDON

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