Describe the following in brief:
Local attraction
The North end of a freely suspended magnetic needle always points to the magnetic north, if it is not influenced by any other external forces except the earth’s magnetic field. It is a common experience that the magnetic needle gets deflected from its normal position if placed near magnetic rocks, iron ores, cables carrying current or iron electric poles. Such a disturbing force is known as ‘Local attraction’. Magnetic bearings are, therefore, not reliable unless these are checked against the presence of local attractions at each station and their elimination.
Sensitivity of the bubble
Sensitiveness of Level Tube Sensitivity of a level tube depends upon the following:
- The radius of curvature of the internal surface: Larger the radius, the greater is the sensitiveness.
- Diameter of the level tube: Larger the diameter, the greater is the sensitiveness.
- Length of the vapour bubble: Larger the length, the greater is the sensitiveness.
- Viscosity and surface tension of the liquid: Lesser the less viscosity, the more is the more sensitiveness.
- The smoothness of the finish of the internal surface of the tube: Greater the smoothness, more is the more sensitivity.
α' = l / R = (S / nD) 206 265 (in seconds)
where α' = sensitivity of bubble tube.; S = staff intercept; l = length of one division of bubble tube (usually 2 mm); D = distance of the instrument from the staff; n = number of divisions through which the bubble is moved; R = radius of curvature of the tube.
Subtense bar
It is also known as a horizontal stave. It is used for measuring both the horizontal well as the vertical distances in places where chaining is impossible because of undulations and rough country. It is used to determine short distances of up to 200 m. A subtense bar is shown below.
Condition equation
An equation that expresses exactly certain relationships that must exist among related quantities. For example: In measuring the angles of a triangle, no condition exists until all three angles are measured. The condition equation will then express the condition that the three measured angles plus certain corrections must equal 180° plus the spherical excess of the triangle. The various condition equations set up in surveying are—angle equation, side equation, length equation, latitude equation, longitude equation, and azimuth equation.
Well-conditioned triangle
The triangles with the angle A in the range 30⁰ < A < 120⁰ is called well-conditioned triangles. An equilateral triangle is the best well-conditioned triangle (Ideal Triangle) possible.
The well-conditioned triangle has perfect apex points that are sharp and are located by a single ‘dot’. This will bring no possibility of relative displacement of the plotted point.
An ill-conditioned triangle is a triangle with an internal angle (A) in the range 120⁰ < A < 30⁰.
Bilby tower
A tower (see figure) is erected when it is desired to elevate the instrument or signal, above the instrument station, to obtain a line of sight clear of obstructions. These are built-in duplicate, one to support the instrument and the other to support the observer and signal. Usually, these are made of steel sections that can be bolted together. These are known as ‘Bilby towers’.
Tilted photograph
In case the vertical axis of the camera is unintentionally inclined to the vertical by not more than 3°, the photograph is called a tilted photograph. However, sometimes the vertical axis of the camera is intentionally inclined to the vertical and then the resultant photograph is known as oblique.
Stereoscopy
The principle of stereoscopy is similar to that of a binocular vision enabling an observer to view an object or to be more precise, two different perspectives of an object, so as to obtain therefrom a mental impression of the three-dimensional image. Stereoscopic depth perception is of fundamental importance in photogrammetry, for it enables the formation of a three-dimensional stereo model by viewing a pair of overlapping photographs. For the stereoscopic vision of photographs, the third dimension, i.e.. depth or height is obtained by using the principle of parallax.
Parallax bar
It is an instrument used to determine the parallax difference between any two points. The instrument, as shown in the given figure. consists of a pair of glass graticules held in frames attached to a rigid bar such that the lower surface of each graticule is in contact with one of the pair of stereo photographs. A small opaque circular dot is marked on each graticule.
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