Bihar Special Survey Amin key terms

A fixed, permanently marked point whose reduced level above mean sea level is known and recorded, used as the starting reference for any levelling work in the area.

The height of a point expressed relative to a chosen datum, usually mean sea level, obtained by adding or subtracting staff readings from a benchmark level.

The reduced level of the line of sight of a levelling instrument, found by adding the backsight reading to the reduced level of the point on which the staff stood.

The first staff reading taken on a point of known reduced level after setting up the level, used to establish the height of the instrument for subsequent readings.

The last staff reading taken on a point before the instrument is moved to a new position, giving the reduced level of that change point.

A stable point on which both a foresight and the next backsight are taken, allowing the levelling instrument to be shifted while keeping the level run continuous.

A rapid levelling run made only to connect a starting benchmark with a distant point, without taking intermediate readings, usually to check or transfer a level.

The constant vertical distance between two successive contour lines on a map; a small interval shows gentle slopes in detail while a large one suits steep or rough terrain.

An imaginary line on the ground joining points of equal elevation, drawn on a map so that the shape and steepness of the terrain can be read directly.

The horizontal distance on the ground between two adjacent contour lines; it is small on steep ground and large on flat ground for the same contour interval.

A method of surveying in which lengths are measured with a chain or tape and offsets are taken at right angles to a framework of triangles, without measuring angles with an instrument.

The longest and most carefully measured line of a chain survey, from which the main triangles of the framework are built and to which all other measurements are related.

An additional line measured in a chain survey purely to verify the accuracy of the plotted framework; its measured value is compared with the value scaled from the plan.

The lateral distance measured from a survey line to a detail such as a building corner or boundary, taken either perpendicular or at a fixed oblique angle.

The shortest distance from a survey line to a detail point, measured at right angles; it is preferred because it is short, simple to set out and easy to check.

A straight painted pole of standard length used to mark survey stations and to keep intermediate points visually in line when chaining over long distances.

A small hand instrument with two mirrors set at forty-five degrees that projects a line of sight at exactly ninety degrees, used to set out perpendicular offsets quickly.

A simple instrument with vertical slits or vanes mounted on a staff, used to sight a right angle to a chain line when locating perpendicular offsets on the ground.

A drawing board mounted on a tripod with an alidade, allowing field observations to be plotted directly on the sheet at the moment they are taken.

The sighting straightedge used with a plane table; its fiducial edge passes through the plotted station so that the sighted line can be drawn on the sheet immediately.

The operation of turning the plane table so that all lines already plotted stay parallel to the corresponding ground lines, commonly done by magnetic needle or by backsight.

A plane table method in which the position of the instrument station itself is found by sighting two or three known points and drawing back rays to intersect on the sheet.

A plane table method in which rays are drawn from a single set-up to several surrounding detail points and distances are scaled along them to fix those points.

A precision instrument for measuring horizontal and vertical angles, whose telescope can be completely reversed in its standards, so angles can be doubled and prolonged.

The observation position in which the vertical circle of the theodolite lies to the left of the observer, that is, with the telescope in its normal upright position.

The observation position obtained by transiting the telescope and swinging it about the vertical axis, placing the vertical circle on the observer's right side.

A series of connected survey lines whose lengths and directions are measured, forming a framework used to locate detail and to compute coordinates of the stations.

The component of a traverse line measured along the reference meridian, equal to the length multiplied by the cosine of the bearing; north is positive and south negative.

The component of a traverse line measured at right angles to the reference meridian, equal to the length multiplied by the sine of the bearing; east is positive and west negative.

The gap between the computed end of a closed traverse and its starting station, expressed as a distance and a bearing, showing the accumulated error of the fieldwork.

A method of adjusting a closed traverse in which the total closing error is distributed among the sides in proportion to their lengths, correcting latitudes and departures.

The horizontal direction of a survey line, stated as an angle measured clockwise from a chosen reference meridian such as true north, magnetic north or an assumed line.

The horizontal angle between true north and magnetic north at a place; it changes with location and time and must be applied when converting magnetic bearings to true bearings.

The disturbance of a magnetic needle by nearby iron objects, rails or electric currents, which makes observed magnetic bearings unreliable and must be detected and corrected.

A survey carried out to define, mark and record the boundaries of individual land parcels, together with their areas and ownership, for revenue and land records.

The numbered plot in the village land register that identifies a particular parcel of agricultural land, its area, soil class and the names of the persons holding it.

The record of rights listing the tenants of a village or circle with the khasra numbers they hold and the nature of their interest in each plot.

The fixed ratio between a distance on the map or plan and the corresponding horizontal distance on the ground, chosen according to the detail the drawing must show.

A scale expressed as a ratio such as 1:2500, independent of any unit of measurement, so the same map can be read in metres, feet or any other unit.

The ruled notebook in which chain lines, offsets, station sketches and remarks are recorded on site in a standard order, forming the primary evidence for later plotting.