#True position one datum series

However, as the benefits of a global system outweigh the greater accuracy, the global WGS 84 datum has become widely adopted. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid. This phenomenon is called datum shift.īecause Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS 84 can. Using local datums, the disparity on the ground between a point having the same horizontal coordinates in two different datums could reach kilometers if the point is far from the origin of one or both datums. Because the ellipsoid or geoid differs between datums, along with their origins and orientation in space, the relationship between coordinates referred to one datum and coordinates referred to another datum is undefined and can only be approximated. Then the coordinates of other places are measured from the nearest control point through surveying.

Even GPS requires a predefined framework on which to base its measurements, so WGS 84 essentially functions as a datum, even though it is different in some particulars from a traditional standard horizontal or vertical datum.Ī standard datum specification (whether horizontal or vertical) consists of several parts: a model for Earth's shape and dimensions, such as a reference ellipsoid or a geoid an origin at which the ellipsoid/geoid is tied to a known (often monumented) location on or inside Earth (not necessarily at 0 latitude 0 longitude) and multiple control points that have been precisely measured from the origin and monumented. Astronomical and chronological methods have limited precision and accuracy, especially over long distances. The WGS 84 is intended for global use, unlike most earlier datums.īefore GPS, there was no precise way to measure the position of a location that was far from universal reference points, such as from the Prime Meridian at the Greenwich Observatory for longitude, from the Equator for latitude, or from the nearest coast for sea level. Since the rise of the global positioning system (GPS), the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications. A horizontal datum is used to measure a location across the Earth's surface, in latitude and longitude or another coordinate system a vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL). Datums are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. Whether or not that 'suffices' is up to the designer and what the design intent is.A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other planetary bodies by means of geodetic coordinates. It can even have no datum features (datumless). Your position tolerance can have one datum feature. In this case your datum feature B is likely the best candidate for the primary datum feature as it will constrain your rotation and translation in practice, and then A can be specified secondary to constrain translation/location.

#True position one datum series

You mentioned the example of a flange, which could also be your textbook example - if A is a clearance fit and is secured with a series of bolts the clamping force will tightly pull your surface B in contact with the mating part surface. This may all be intuitive, but I just wanted to clarify that when you say you don't care about orientation, you are likely only talking about one form of orientation (rotation about the z-axis).Īs mentioned above it is important that our datum features are robust for simulation ie: the low length to diameter ratio of A could potentially make it unstable/unreliable as a primary datum feature but we also want to make sure our datum features reflect part function/design intent. Position tolerance also controls both orientation AND location of each hole axis to within the stated tolerance. You said you don't care about orientation, but you do likely care about ie: rotation about the x and y axes respectively (assuming x and y axes are in plane with B and z is perpendicular to B), just not necessarily orientation/clocking about the z-axis aka rotation.