Propagation of Refraction Error on Different Textures in Geometric Levelling

Geometric levelling is a process in which the difference of elevation between points on the earth surface is determined. In establishing higher accurate Bench Marks in 1storder for surveying purposes, refraction error would act as a significant factor in maintaining the accuracy of the task. Generally, refraction causes to small height differences and it is usually considered as the largest systematic error in levelling with in mm/cm levels. Refraction is largely a function of atmospheric pressure and temperature gradients and it would change the light path of the line of sight. Generally, temperature gradients vary with the material of the surface of running such as grass, gravel, cement and asphalt. Present study models the refraction error occurred in determining the height difference for different textures such as asphalt, grass and gravel in order to identify the optimal surface material and time period for high accurate levelling tasks in geometric levelling. Level measurements were taken for a 1.5 km distance with an Auto Level at pre scheduled different time periods on three different days. This task was carried out for different textures and the gradients of temperature were determined. The temperature values were measured 02 minutes after setting up the instrument. The refraction correction was calculated by using relevant formula. The minimum refraction error, ranged from 0.000, 04 to 0.000, 06 mm, was recorded for grass surfaces and the study identified the time slot from 0800h to 1000h as the best time duration for a levelling task. Further, the study concludes that the levelling task should be done on grass surfaces in establishing 01st order bench marks.