![]() The assumed arbitrary bench mark values may be 50 m, 100 m, 200 m etc.Datum plays an important role in GD & T. So, some fixed points in that area are arbitrarily taken as bench marks and values are assumed. When this happens, we cannot predict the exact reduced level of that area with respect to mean sea level. In Small survey works or engineering projects it is important to know the difference in elevation of two points. The temporary bench mark point should be decided in a way that the object or point should be permanently fixed in that position. Then, it is easier to continue the survey from that point after large gap or on the next day of work. Temporary benchmarks are created by the surveyors in the field to mark the point in the field up to which the survey is completed. Permanent benchmarks are useful for future references also. They are established by local state government agencies or railways at railway stations, public buildings, at bridges etc. Permanent benchmarks are established with reference to GTS benchmarks.
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