This requires the calculation of moment for each variable mass in the aircraft (fuel, passengers, cargo, etc.), by multiplying its weight by its distance from a datum reference. In aviation, an aircraft is designed to operate within a specified range of weight and (chiefly longitudinal) balance an airman is responsible for determining these factors for each flight under his or her command.

In subsequent marking out, all measurements are then taken from either of the two datum references. If necessary they may be cut, planed or filed to make them so. For most work, the datum references of the workpiece need to be square. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). The artisan nominates two straight edges on a workpiece as the 'datum edges', and they are marked accordingly. In carpentry, an alternative, more common name is 'face side' and 'face edge'. The concept of datums is used in many fields, including carpentry, metalworking, needlework, geometric dimensioning and tolerancing (GD&T), aviation, surveying, geodesy ( geodetic datums), and others. This matters because the hub and rim need to be concentric to within close limits (or else the wheel will not roll smoothly). For example, on a car's wheel, the lug nut holes define a bolt circle that is a datum from which the location of the rim can be defined and measured.

JSTOR ( November 2015) ( Learn how and when to remove this template message)Ī datum reference or just datum (plural: datums ) is some important part of an object-such as a point, line, plane, hole, set of holes, or pair of surfaces-that serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value.

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Datum engineering