![]() 2-Height relation between entrance point and exit point is problematic and time consuming For example, in a 3 km long tunnel (in Arak-Khoram Abad freeway), to relate entrance point to exit point, it is necessary to perform levelling about 90 km. Furthermore, the calibration methods that will be mention in this article, have significance effects on underground transverse. In this article, the role of astronomy is defined in two subjects: 1-Azimuth determination of directions from entrance and exit nets of tunnel and also calibration of gyro-theodolite to use them in Underground transvers: By astronomical methods, azimuth of directions can be determine with an accuracy of 0.5 arcsecond, whereas, nowadays, no gyroscope can measure the azimuth in this accuracy For instance, accuracy of the most precise gyroscope (Gyromat 5000) is 1.2 cm over a distance of one kilometre (2.4 arcsecond). To solve this problems, astro-geodetic that is part of geodesy science, can help surveying engineers. This kind of underground surveying, because of particular condition, has some different points in relation to the ground surveying, including Improper geometry in underground transverse, low precise measurement in direction and length due to condition such as refraction, distinct gravity between underground point and corresponding point on the ground (both value and direction of gravity) and etc. One of prime aspects of surveying projects is guidance of paths of a long tunnel from different directions and finally ending all paths in a specific place.
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