gl Nautical Instruments Italian Terms - Topographical Glossary
Riding the wave

Topographical Glossary

edited by Franca Acerenza, historian of science

ARCHIPENZOLO or Archipendolo: Also called leveling pendulum, it consists of a wooden isosceles or equilateral triangle, with a plumb line fixed at the apex and a reference marking in the center of the base. The instrument is placed on a slope, and if the plumb line passes through the reference, the plane is horizontal. To verify the accuracy of the instrument, after having done a reading, the instrument is reversed and, if the plumb line gives the same reading, then the base is perpendicular to the plumb line as required.

BALAUSTRINO: a compass to draw circles or arcs of small radius.

BAROMETER : device to measure the pressure exerted by the atmosphere at a certain place and at any one time. Since atmospheric pressure varies with altitude at the place of observation, the barometer can be used to determine altitude. Barometers are of three types: mercury b., spring or metal b., boiling point b. The proper barometer is the first one, the second one is called aneroid and the last one hypsometer.

COMPASS : the current and historical meaning is "tool that uses the action of magnetic field on a magnetic needle to give the direction of the magnetic meridian in the place where the instrument is".
Topographic compasses can be of various types: with targets, with prisms, with telescope. They can be handheld or on a tripod.

TELESCOPE ANALLATICO: special telescope used in all topographical tools. The objective is composed of two lenses (objective lens and lens anallatica) arranged so that the prime focus of the system falls on a given point of the optical axis, inside the telescope: if this coincides with the center of the instrument, at the intersection of the vertical axis of rotation and the axis of collimation, the telescope is said centrally anallatico. This innovation, pioneered by Ignazio Porro, was applied since 1823 in surveying instruments and in particular to theodolites . With the telescope centrally anallatico the distance is between the ruler and the axis of the instrument, rather than between the ruler and the front lens.

CELERIMENSURA : planimetric and elevation surveying method invented by Ignazio Porro in 1822 and by him named celerimensura. Essentially consists in determining from the station, and for each survey point, the five quantities or generating numbers: horizontal angle, vertical angle, distance, instrument height and reading at the middle of the ruler.

CLEPS: this was a theodolite, with accessories for astronomical use, built by Ignazio Porro.

CLISIGONIOMETRO: disused term to indicate a telescope clinometer equipped with a goniometer or compass.

CLINOMETER : level to measure the slope of a given view, or the trigonometric tangent of the angle that the line of view forms with the horizon expressed as a percentage. The clinometers may be simple, with viewfinder or with telescope.

COMPASS : an instrument consisting of two arms with a spike at the end, connected by a hinge, which can be adjusted. It is used to draw circles and measure distances between two points on a chart.

MAGNETIC DECLINER : a compass whose pointing is restricted to an arc. Used to orient the surveying instrument with the direction of the magnetic meridian and therefore to know its orientation, or also to set up the Praetorian tablet.

DIOPTER : a tool to determine a line of view. The diopter, also called a viewfinder alidade, is normally formed by a rule with a fin or pinnule at each end, called the ocular and the objective. To determine a line of view, the ocular, that has a hole, is aligned with the objective, that carries two cross hairs. In planimetrics it is often more convenient to use a diopter with the ocular fitted with a vertical slit and the objective having a stretched hair, parallel to the slit. In this way a plane is determined, instead of a line.

ECLIMETER or ECCLIMETER: level supplying the inclination of the axis of collimation or of the position plane over a horizontal plane. Very practical to determine the inclination of lengthmeters in order to correct distances measured on slopes. It is used by placing the flat surface of the instrument on the plane to be measured, and turn the brass knob with a vernier till the bubble is centered. The mark on the scale indicates the angle of inclination.

GONIOMETER : instrument to measure angles in horizontal or vertical planes. The goniometers used in topography may be of different types: azimuthal, eclimeters, theodolites, universal theodolites, tachometers.
The latter, which can considered to belong to the class of theodolites , are correctly called theodolites, if their sensitivity is very high (order of magnitude of resolution in seconds); tachometers , if their sensitivity is not so high, and if they have a rangefinder telescope. There are also theodolites-tachometers, that are tachometers with the highest sensitivity, rivaling that of theodolites. Finally, the name of universal, or universal theodolite, designates the great theodolites for geodetic surveys.

SEXAGESIMAL DEGREE: unit of measurement equivalent to one 360th of a full circle. The degree is divided into 60 minutes, and every minute, in another 60 minutes seconds. This unit was known to the Chaldeans, the Egyptians, the Persians and the Indians.

GRADS: unit of measurement equivalent to one 400th of a full circle. The grads was proposed at the time of the First French Republic, but was adopted around 1850, when Ignazio Porro built the first tools with divisions in hundredths.

HYPSOMETER: Boiling barometer by which we can determine the difference in altitude between two points. It consists of a vessel (in communication with the ambient) in which distilled water is brought to the boiling point, and a mercury thermometer, very sensitive and precise, with a scale resolution in hundredths of a degree Celsius. Thermometers for hypsometers cover of a small temperature range (ten degrees) and often have a pressure scale (saturated vapor pressure) in parallel to the temperature reading. Quote differences and altitudes are so determined by using a hypsometric table.
Contrary to what occurs for mercury barometers, the pressure supplied by hypsometers requires no correction. As the temperature of boiling water varies uniquely with the atmospheric pressure, the altitude can be inferred from it.

TORIC LEVEL: instrument used to check the horizontality of a line. The level is based on a curved cylindrical tube (torus) filled with a liquid, the latter forming a vapor bubble whose center, when the tool is in equilibrium, goes to the highest point. The tangent to the toroidal surface at the midpoint of the bubble (level axis) is horizontal. The glass tube is filled almost entirely with a low viscosity liquid such as alcohol, ether, gasoline, and is closed at both ends. The vial is inserted in a metal frame with open top and a flat base.
To check a horizontal plane or a line, it is necessary that level axis is parallel to the flat support base. The verification is done by laying the instrument on a plane and moving the level around to find the direction in which the bubble is centered. If the level is adjusted, the line on which it rests is horizontal. To check if the level is adjusted, this one is turned in the opposite direction: if the bubble is again centered, the level is adjusted. Otherwise, the adjustment screw is set.

SPHERICAL LEVEL: it is based on the same principle as the toric level, but consists of a flat cylinder, topped by a glass with the inner surface of spherical shape. On the glass are engraved one or more concentric circles, centered on the top of the dome, which act as reference lines. Three leveling screws are used for adjustment. These levels, although not very sensitive, are very convenient because they require less space and may give an initial adjustment to certain instruments, to set a plane horizontal or to make vertical a rod.

LEVEL: surveying instrument used to define a line of view lying in a horizontal plane. It is used by an operator to identify a horizontal line against which to measure the vertical distance from the ground. This category includes many types of instruments: in topography the levels have always a telescope, as only this one allows to point a ruler precisely enough. In the past, many types of levels have been built, the most common may be identified as follows:
a) level with mobile telescope and fixed level
b) level with mobile telescope and mobile level
c) level with fixed telescope and fixed level
d) level with reversible telescope and fixed or mobile level on the telescope
All of them include a base provided with leveling screws, an horizontal support beam, an alidade, a level, a telescope and a tripod. Can be supplied with azimuthal ring, and modern ones and are fitted with elevation screw (type a above) with fixed level on the telescope. The conditions that must meet a level to be considered as correctly adjusted are: mechanical axis coincident with the optical axis, optical axis parallel to the axis of the level.

LENGTHMETER : a tool for direct measurement of distances, normally built with a material insensitive to temperature changes. Examples of lengthmeters are a ruler, an engineer's scale, a measuring tape.

VERNIER : device to reliably read the fractional part of a division of a graduated ring. The divisions in the rings of surveying instruments are marked on the periphery of a silver plated disc of brass or bronze, or engraved on a silver tape, fastened on the edge of the ring. The divisions may be centesimal or sexagesimal, the numbering clockwise or anticlockwise. An interval of division may correspond to one degree or half degree, or one third, or in general to the nth fraction, though the smallness of divisions is limited by mechanical and practical reasons.
The term nonius comes from the mathematician Peter Nunes, who, in 1542, first invented one of these instruments, while the term vernier comes from Peter Vernier, who, in 1631, introduced the device that is still in use, much simpler than the former one.

ARTIFICIAL HORIZON : flat, horizontal specular surface, which is used in surveying for the verification of telescope instruments.

PANTOGRAPH : a tool to amplify or reduce drawings, it is essentially based on the use of an articulated parallelogram.

PLANIMETER : a tool for measuring the area of a plane figure. It can be orthogonal or polar. The polar planimeter is the most widely used, with many types manufactured: single unit, multiple units, with pantograph , Becker type. To use a polar planimeter, the tool is placed on a drawing, normally horizontal, and the a pointed end (not sharp, may be called a rammer) is guided by the operator on the perimeter of the area. The rammer moves a mechanism whose displacement is proportional to the area to be calculated.
The various types of planimeters differ by how the contour tracing, using the rammer, determines the movement of the mechanism and how the law of proportionality mentioned above is obtained. They are widely used in drawing, topographic and cadastral operations.

PROTRACTOR : also said goniograph, allows to duplicate a given angle. It consists of a circle or semicircle, graduated in degrees at its periphery with a small indentation in the center. Its use is simple: just put the instrument on the sheet, with its center on the vertex of the angle to be measured, and with the zero graduation coincident with a side. There are several types of protractors: simple (semicircle), full circle, with one or three arms, direct reading or indirect.

GRID or LATTICE : used for the measurement of areas on a drawing with a known scale. The transparent plate is laid over the drawing and the number of elementary squares that are located inside the contour of the figure is counted, adding an estimate of the fractions of squares crossed by the boundary. Finally, the extent of the area of interest is obtained by taking into account the scale of the drawing.

HORIZON TRANSFORMATION SCALE : it is used to translate distances to the horizon, ie to transform the actual distances with various slopes to horizontal or topographical.

DUMMY SQUARE : this tool is used tigether with the praetorian tablet. It projects a point on the mirror or on the drawing board to the station. It consists of two rulers, forming the catheter and the hypotenuse of a right triangle. When the notch at the end of the catheter is put in coincidence with a convenient point of the mirror, a plumb line suspended at the free end of the hypotenuse vertically projects this point on the ground. That way it is possible to determine the projection of a mirror point on the ground, and the position, on the mirror, of the point corresponding to the station.

SURVEYING SQUARE : it allows to determine alignments perpendicular to a given alignment. It consists of a box, octagonal, cylindrical or spherical, that can be supported on a perch or a tripod. The box has narrow slits which identify two mutually orthogonal planes. Each slit can serve as the eyepiece or the target, and the collimation is done by putting the eye on a slit and targeting with the opposite one: if the image is in the middle, then the image is on target. Sometimes it is useful to identify alignments to 45°, so the box carries four more shorter slots, equidistant from the main ones. If the land is very uneven, the visuals directions can be very inclined; a spherical box is therefore used, where the slits run along the meridians of the surface, or also a cylindrical one, whose top cover is provided with slits exactly matching those on the side.

STADIA : a ruler used in distance measurements made with distance meters. Both when leveling or measuring altitudes, the stadia must be kept vertical. It is therefore provided with a spirit level of low sensitivity or a plumb line, suspended where it can be seen by the operator.

TACHEOMETER : type of telescope distance meter equipped with a goniometer, it allows, in addition to angular measurements, to determine the indirect measure of the distance between the station and the point observed. It differs from the theodolite because of the presence, before the eyepiece, of a grid distance meter, consisting of at least three horizontal wires, and because the accuracy of reading the angles is usually 1' or less. Often, the telescope is equipped with a fixed level and can also be used as a level. The tachometers are the tools of choice of celerimensura : tacheometer etymologically means "fast meter". The instrument consists of five parts: base, azimuth circle, zenith circle, telescope and accessories. The base has the shape of a triangle, with a leveling screw at each vertex.
The azimuth circle measures horizontal angles and is divided into degrees numbered clockwise. It has two verniers diametrically opposite on the alidade, this feature being typical of nineteenth-century tacheometers. The zenith circle measures vertical angles, with the same technical features of the azimuthal one. The telescope is anallatico and can be reversed, that is it can rotate around a horizontal axis connected to the alidade by two uprights.
The devices that are part of the instrument are: -a toric spirit level on the alidade; -a magnetic declinator that can be fixed to the supports of the azimuthal circle to determine the magnetic orientation and then to measure the relative azimuth; -four microscopes, which are used for the reading the verniers of the azimuth and zenith circles; -the plumb line, used to set up the instrument as a station; -the set screws and micrometer screws, in the number of six: set and micrometer screw to set the alidade on the base, set and micrometer screw to set the alidade on the azimuth circle, and set and micrometer screw to set the telescope on the uprights; -tripod consisting of three articulated legs and a top platform upon which rests the instrument. In the center of the platform there is a hole through which passes the screw to lock the instrument onto it. Under the platform, the head of this bolt is a hook to which is attached the plumb line. To be operational, the tacheometer must meet the following conditions: main axis of the instrument vertical; optical axis perpendicular to the axis of rotation of the telescope; axis of rotation of the telescope horizontal. In the nineteenth century the tacheometers were called: the big model, the normal one, and medium and small model.

PRETORIAN TABLET : the name derives from Pretorius, professor in Altdorf, who invented it in 1590. It was used to draw maps of medium and small scale, because it allows to trace the drawing in detail and contours in sight of the land. It consists of a wooden tablet, called mirror, combined with a base supported by a tripod: the base allows leveling, being equipped with three leveling screws and, thanks to a ball joint on top of the tripod, it can be rotated. On the table is placed in a diopter with telescope. The pedestal supporting the diopter is integral with a rule, whose outer edge (line of faith) is at an appropriate distance to the vertical collimation axis of the diopter.
The station point is obtained using a dummy square while the distances are obtained from the rangefinder built into the telescope. Directions are identified with the diopter and drawn with the rule, so it is possible to do surveys by radii and intersection, that are solved graphically on the drawing sheet placed on the mirror. The telescope diopters are all rectifiable and, to be operational, the conditions are the same as for any telescope goniometer: collimation axis perpendicular to the rotation axis of the telescope and rotation axis of the telescope horizontal.

THEODOLITE : telescope goniometer for precision measurement of azimuth and zenith distances. Essentially consists of a sighting telescope, with orthogonal wire frame. The station tool is moving along two axes: a vertical axis or rotation of the alidade or main axis of the instrument, and a horizontal axis or rotation of the telescope, or secondary axis. The telescope is fixed to a shaft perpendicular to the main axis. The shaft is fastened to a graduated ring normal to it, which can rotate in a vertical plane. The vertical ring, equipped with a level, measures zenith distances, performing the readings on indices fixed to the uprights supporting the axis of rotation of the telescope. This one, being able to fully turn around its axis, can take measurements in two opposite conjugate positions. The operating devices are identical to those described for tacheometers. In the instruments of the past century, usually the bridge level was available to make horizontal the axis of rotation of the telescope with the best precision. The conditions for the set up as a station are the same as described for the tacheometer.