How does refraction affect surveying measurements?
For long sights and accurate levelling work, the effects of curvature of the earth and refraction of the line of sight shall have to be taken into consideration. Due to curvature, the points appear to be lower than they actually are; while due to refraction, they appear to be higher than they actually are.
What is angle of refraction in surveying?
refraction, terrestrial (angle of)— The angle of refraction, or the refraction, of a survey line is the angle at the point of observation between the true direction of the observed object and the direction as shown by light coming from it (direction of object as seen).
What is meant by atmospheric refraction of light?
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. Astronomical or celestial refraction causes astronomical objects to appear higher above the horizon than they actually are.
What is atmospheric refraction explain with example?
The light coming from a star undergoes atmospheric refraction due to varying optical densities of air at various altitudes. When an object transmits light rays in the atmosphere, these light rays pass through the atmosphere having different air layers of different densities and get refracted by the atmosphere.
What is trigonometric levelling in surveying?
Trigonometric Leveling is the branch of Surveying in which we find out the vertical distance between two points with the help of some measurements of the vertical angles and the known distances. The known distances are either assumed to be horizontal or the geodetic lengths at the mean sea level(MSL).
What is the rise and fall method?
Introduction. The rise and fall method is the method of calculating the difference in elevation between consecutive points in levelling work. Some of the points you have to know before starting numerical are: Back sights: The first reading after seeing the instrument is called back sights.
How does atmospheric refraction effect the apparent time of sunset?
At a result, when sunlight passes through the atmosphere, successive refraction occurs and bends the sunlight towards the ground (see Figure 1). The remaining -0o34′ is the effect of atmospheric refraction under average atmospheric condition, leading to the apparent effect of advanced sunrise and delayed sunset.
Why is atmospheric refraction important?
The atmosphere is continuously changing (due to which the optical densities of air at different levels in the atmosphere keep on changing). The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next.
How does atmospheric refraction occur?
Atmospheric refraction actually occurs because the velocity of light becomes different when it passes through an optical medium denser than air. The velocity decreases due to the higher optical density, which results in deviation from the straight line path.
What is atmospheric refraction give two example?
1) The apparent random wavering or flickering of object seen through a turbulent stream of hot air rising above a fire or a radiator. 2) Twinkling of stars. 3) Advanced sunrise and delayed sunset. 4) Apparent flattening of the sun’s disc at sunrise and sunset.
What is the main cause of atmospheric refraction?
Atmospheric refraction is caused by the bending of light rays when they pass through the layers of the earth’s atmosphere by varying optical densities.
What is trigonometric leveling used for?
Trigonometric leveling is often used where accurate elevations are not available or when elevation of inaccessible points must be determined.
What kind of refraction do land surveyors use?
A careful reconnaissance is required, especially when surveying urban areas, to prevent the inclusion of lines so affected in a survey net. In some publications terrestrial refraction has been referred to as “atmospheric refraction.” See also refraction, horizontal.
How is the angle of incidence of light related to refraction?
Light entering or leaving a new medium perpendicular to its surface passes straight though without any deviation (Figure 1 – ray A). At other angles the ray will be refracted and the greater the angle of incidence the greater the amount of refraction (Figure 1 – rays B & C).
How is atmospheric refraction related to astronomical refraction?
The same principle can be applied to derive astronomical refraction, although this is not the topic of this page. Atmospheric Refraction is the bending of light due to a gradient of the refractive index caused by a density gradient of the air.
How does refraction affect the line of sight?
Atmospheric Refraction: This effect increases the closer to the ground the light ray gets and errors in the region of 5 mm/km have resulted. The atmosphere refracts the horizontal line of sight downward, making the level rod reading smaller. The typical effect of refraction is equal to about 14% of the effect of earth curvature.