Datum reference

Datum reference is a concept used in carpentry, metalworking, needlework, Geometric dimensioning and tolerancing (GD&T), aviation and similar activities.

In carpentry, an alternative, more common name is "face side" and "face edge". The artisan nominates two straight edges on a workpiece as the "datum edges", and they are marked accordingly. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). For most work, the datum references of the workpiece need to be square. If necessary they may be cut, planed or filed to make them so. In subsequent marking out, all measurements are then taken from either of the two datum references.

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. 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. The datum for light airplanes is usually the engine firewall or the tip of the spinner, but in all cases it is a fixed plane perpendicular to the aircraft's longitudinal axis, and specified in its operating handbook.