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A typical datum reference frame is made up of three planes. For example, the three planes could be one "face side" and two "datum edges". These three planes are marked A, B and C, where A is the face side, B is the first datum edge, and C is the second datum edge. In this case, the datum reference frame is A/B/C.
A vertical datum is a reference surface for vertical positions, such as the elevations of Earth features including terrain, bathymetry, water level, and human-made structures. An approximate definition of sea level is the datum WGS 84 , an ellipsoid , whereas a more accurate definition is Earth Gravitational Model 2008 (EGM2008), using at least ...
Furthermore, the features studied in these approaches are usually over simplified. The bulk of the feature recognition literature normally deals with 2.5D features (those made by sweeping a 2D profile along a linear axis). Graph representations, hint definitions or volume decompositions are much more difficult to define for 3D and free form ...
Tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level. for the London Underground, a tunnel datum of ODN −100 m is used; [7] thus a depth of −60 m AOD is 40 m ATD (above tunnel datum) for the Channel Tunnel, a tunnel datum of ODN −200 m is used; [8] thus a depth of −60 m is 140 m ATD
Example of true position geometric control defined by basic dimensions and datum features. Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances via a symbolic language on engineering drawings and computer-generated 3D models that describes a physical object's nominal geometry and the permissible variation thereof.
Vertical datums in Europe. In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built structures) and altitudes of satellite orbits and in aviation.
In a technical drawing, a basic dimension is a theoretically exact dimension, given from a datum to a feature of interest. In Geometric dimensioning and tolerancing, basic dimensions are defined as a numerical value used to describe the theoretically exact size, profile, orientation or location of a feature or datum target.
This updated version of the standard has been restructured to be more aligned to the workflow of designers and engineers to assist throughout the design process. The standard now references 3D geometry, not only as drawings but also allowing a 3D surface to be used as a datum feature. [2]