Datum line definition
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. The datum feature symbol identifies physical features and shall NOT BE APPLIED TO CENTER LINES, CENTER PLANES, OR AXES except as defined in paras. 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. Engineers use the datum line as a horizontal base line to. In general, a datum line is a tool used by engineers to ensure the accuracy of measurements. The term is primarily used in reference to surveying and engineering, where it is used in the measurement of depths and heights. Permanent benchmarks are established with reference to GTS benchmarks. A datum line is a point of reference or standard for comparison. The United States Army Corps of Engineers.Here is some of the list of agencies for different countries, who established the GTS benchmarks in their respective countries. These pedestals are generally protected by masonry structure build around it. The bronze plate consists the information or value of the benchmark on its top. These are generally established by higher survey authorities of particular country in all points of the country.Ī concrete pedestal with bronze plate at its top is provided at a point where GTS benchmark is located. GTS benchmarks are decided by taking mean sea level as datum. Great trigonometrical bench mark or shortly GTS benchmarks are very accurate, and they are established by conducting high precise surveys. Great Trigonometrical Survey benchmarks.
There are different types of bench marks are available in the surveying as follows: Bench mark is considered as check for the other level points. Reduced level is the vertical difference between the level point and datum line or the mean sea level.īenchmark is the point laid above or below the datum line with a known elevation. Mean sea level or MSL is used as datum level for all important surveys of that zone. By this system a new date is born every hour when the time at the central meridian of a time zone is midnight. To simplify timekeeping, modern nations divide the Earth into 24 north-south zones of standard time. The need for help arises from the way new dates are born at midnight in any locality. Mean sea level is established by taking the average height of tides in the sea over a very long period (generally 19years). This date line also helps to keep dates straight in all parts of the world. Even though it is a curved surface, it is considered as plane surface for smaller area works.ĭatum line is the reference line with respect to which the levels of other station points are fixed. It means the any point on level line is equidistance from the center of earth. The level line makes right angles to the vertical line or plumb line at any point. The line representing the level surface is termed as level line. Level surface is the continuous surface parallel to the mean spheroid of the earth. The surface along horizontal line of sight is called as horizontal surface. Horizontal line is the line of sight of instrument which is tangential to the level surface and It is perpendicular the plumb line. A plumb is released from the instrument height with the help of thread and instrument is set up at that point as its center. Vertical line connects the station point to the center of the earth. It’s just decided based on the consideration of earth’s gravity. The underside of the movable jaw is 10mm from the datum surface but it is in the opposite direction and hence is labelled '-10'.It is the line which is indicated by plumb at required station. The 15mm dimension to the centre line of the bolt and the 30mm dimension to the top of the movable jaw are dimensioned from the datum surface. In the example shown in Figure 4.2, the datum faces are the flat faces where the underside of the hardened insert contacts the step in the movable jaw. The definition of a datum feature will be dependent upon the functional performance of a particular component.
Thus, the various features on a component are dimensioned with respect to datum features. Reality datum features since it is with respect to features that other features relate. The datum point or points on a component are in A datum, is a point, line or surface of a component to which dimensions are referred and from which measurements are taken during inspection.