# Datum reference > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Datum_reference > Markdown URL: https://mediated.wiki/source/Datum_reference.md > Source: https://en.wikipedia.org/wiki/Datum_reference > Source revision: 1318739741 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Point of reference against which some others are calculated This article needs more citations. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Datum reference" – news · newspapers · books · scholar · JSTOR (November 2015) (Learn how and when to remove this message) Technical drawing with references and geometric specifications A **datum reference** or just **datum** (plural: **datums**[Note 1]) is some [geometrically](/source/Geometry) important part of an object, such as a [point](/source/Point_(geometry)), [line](/source/Line_(geometry)), [plane](/source/Plane_(geometry)), hole, set of holes, or pair of [surfaces](/source/Surface_(mathematics)). It serves as a reference in defining the [geometry](/source/Geometry) of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the [nominal](/source/Real_versus_nominal_value) value, which may be an ideal, standard, average, or desired value. For example, on a car's wheel, the [lug nut](/source/Lug_nut) holes define a [bolt circle](https://en.wiktionary.org/wiki/bolt_circle#Noun) that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be [concentric](/source/Concentric_objects) to within close limits (or else the wheel will not roll smoothly). The concept of datums is used in many fields, including [carpentry](/source/Carpentry), [metalworking](/source/Metalworking), [needlework](/source/Needlework), [geometric dimensioning and tolerancing](/source/Geometric_dimensioning_and_tolerancing) (GD&T), [aviation](/source/Aviation), [surveying](/source/Surveying), [geodesy](/source/Geodesy) ([geodetic datums](/source/Geodetic_datum)), and others. ## Uses In carpentry, an alternative, more common name is "face side" and "face edge". The artisan nominates two straight edges on a [workpiece](/source/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](/source/Perpendicular). 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](/source/Flight_envelope) of mass and (chiefly longitudinal) [balance](/source/Center_of_gravity_of_an_aircraft); an airman is responsible for determining these factors for each flight under their command. This requires the calculation of [moment](/source/Moment_(physics)) 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](/source/Aircraft_principal_axes), and specified in its operating handbook. ## Engineering Screenshot from [AutoCAD](/source/AutoCAD) with various [geometric dimensioning and tolerancing](/source/Geometric_dimensioning_and_tolerancing) datum reference symbols ([total indicator reading](/source/Total_indicator_reading), [perpendicularity](/source/Perpendicular), and [parallelism](/source/Parallel_(geometry)) An engineering datum used in [geometric dimensioning and tolerancing](/source/Geometric_dimensioning_and_tolerancing) is a feature on an object used to create a reference system for [measurement](/source/Measurement).[1] In [engineering](/source/Engineering) and [drafting](/source/Engineering_drawing), a *datum* is a reference point, surface, or axis on an object against which measurements are made. These are then referred to by one or more 'datum references' which indicate measurements that should be made with respect to the corresponding datum feature . In [geometric dimensioning and tolerancing](/source/Geometric_dimensioning_and_tolerancing), datum reference frames are typically [3D](/source/Three-dimensional_space). Datum reference frames are used as part of the [feature control frame](/source/Feature_control_frame) to show where the measurement is taken from. 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/B/C is shown at the end of feature control frame to show from where the measurement is taken. (See the ASME standard Y14.5M-2009 for more examples and material modifiers.) The engineer selects A/B/C based on the dimensional function of the part. The datums should be functional per the ASME standard. Typically, a part is required to fit with other parts. So, the functional datums are chosen based on how the part attaches. However, typically, the functional datums are not used to manufacture the part. The manufacturing datums are typically different from the functional datums to save cost, improve process speed, and repeatability. A [tolerance analysis](/source/Tolerance_analysis) may be needed in many cases to convert between the functional datums and the manufacturing datums. Computer software can be purchased for [dimensional analysis](/source/Dimensional_analysis). There are typically 6 [degrees of freedom](/source/Degrees_of_freedom_(mechanics)) that need to be considered before choosing which feature is A, B, or C. For this example, A is the primary datum, B is the secondary, and C is the tertiary datum. The primary datum controls the most degrees of freedom. The tertiary datum controls the least degrees of freedom. For this example, of a block of wood, datum A (the face) controls 3 degrees of freedom, B (first edge) controls 2 degrees of freedom, and C (second edge) controls 1 degree of freedom. 3+2+1 = 6, all 6 degrees of freedom are considered. The 6 degrees of freedom in this example are 3 translation and 3 rotation about the 3D [coordinate system](/source/Coordinate_system). The face datum A controls 3: translation along the z-axis, rotation about the x-axis, and rotation about the y-axis. The edge datum B controls 2: translation along the y-axis and rotation about the z-axis. Finally, The edge datum C controls 1 degree of freedom, namely the translation along the x-axis.[2] ## See also - [Datum (geodesy)](/source/Datum_(geodesy)) - [Exact constraint](/source/Exact_constraint) - [Reference frame](/source/Reference_frame) - [Surface plate](/source/Surface_plate) - [Spatial reference system](/source/Spatial_reference_system) ## Notes 1. **[^](#cite_ref-plural_1-0)** The plural of this [sense](/source/Word_sense) of the word *[datum](https://en.wiktionary.org/wiki/datum#Noun)* is *datums* by convention, in contrast with the other senses of the word in which *[data](/source/Data)* usually serves as both the plural form and the [mass noun](/source/Mass_noun) counterpart.) ## References 1. **[^](#cite_ref-2)** ANSI Y14.5M ([ISBN](/source/ISBN_(identifier)) [0-7918-2223-0](https://en.wikipedia.org/wiki/Special:BookSources/0-7918-2223-0)) for engineering datums. 1. **[^](#cite_ref-3)** *Dimensioning and tolerancing : engineering drawings and related documentation practices : an international standard*. New York, NY: American Society of Mechanical Engineers. 2009. [ISBN](/source/ISBN_(identifier)) [978-0791831922](https://en.wikipedia.org/wiki/Special:BookSources/978-0791831922). v t e Woodworking Overviews History Glossary Wood (lumber) Styles Boat building Bow and arrow Bush carpentry Cabinetry Caning Carpentry Certosina Chainsaw carving Chip carving Ébéniste Fretwork Intarsia Japanese carpentry Khatam Kohlrosing Log building Luthier Marquetry Millwork Pallet crafting Parquetry Pyrography Relief carving Root carving Segmented turning Shingle weaving Shipbuilding Spindle turning Timber framing Treen Whittling Wood carving Woodturning Woods Soft Cedar (Calocedrus, Cedrus) Cypress Douglas fir Fir Juniper Larch Kauri Pine Rimu Spruce Yew Hard Afromosia Alder Andiroba Anigre Ash Apple Aspen Avodire Balsa Beech Bilinga Birch African Blackwood Australian Blackwood Boxwood Bubinga Camphor Cedrela Cherry Chestnut Cocobolo Cumaru Ebony Elm Eucalyptus Hazel Hickory Hornbeam Idigbo Imbuia Ipê Iroko Jarrah Jelutong Lignum vitae Linden (lime, basswood) Lovoa Merbau Mahogany (American, African) Maple Meranti Oak Padauk Pear Plum Poplar Purpleheart Ovankol Ramin Red Quebracho Rosewood Rubberwood Sapele Teak Totara Utile Walnut Wenge Willow Zebrano Engineered Cross-laminated Glue laminated Hardboard MDF OSB Particle board Plywood Wood–plastic composite Tools Abrasives Axe Adze Burnisher Chisel Drawknife Drill Fence Float Gimlet Gauge Hinge bender Impact driver Janka hardness test Jointer Mallet Milling machine Miter box Rasp Router Sandpaper Shaper Shaving horse Square (Carpenters, Combination, Miter, Speed, Try) Thickness planer Timber-framing Veneer hammer Vise Warrington hammer Winding sticks Wood scribe Workbench Clamps Band clamp C-clamp F-clamp Flooring clamp Gripe Holdfast Miter clamp Pipe clamp Sawbuck Saws Backsaw Bandsaw Bow Bucksaw Chainsaw Circular Compass Coping Crosscut Frame Fretsaw Hand saw Jigsaw Keyhole Miter Ripsaw Scroll Table Veneer Whipsaw Planes Bedrock plane Block plane Chamfer plane Compass plane Finger plane Fore plane Grooving plane Jack plane Japanese plane Jointer plane Moulding plane Razee plane Rebate plane Router plane Scrub plane Shoulder plane Smoothing plane Spokeshave Surform Geometry Joints Birdsmouth Biscuit Box Bridle Butt Butterfly Coping Crown of thorns Dado Dovetail Finger Groove Halved Hammer-headed tenon Knee Lap Mason's miter Miter Mortise and tenon Rabbet/Rebate Scarf Splice Tongue and groove Profiles Bead Bevel Chamfer Ogee Ogive Ovolo Surface piecing Binding Edge banding Intarsia Marquetry Oystering Parquetry Purfling Treatments Adhesive French polish Heat bending Lacquer Oil Paint Paint stripper Steam bending Thermal Varnish Wax Wood drying Wood preservation Wood stain Wood finishing Organizations American Association of Woodturners Architectural Woodwork Institute British Woodworking Federation Building and Wood Workers' International Caricature carving International Federation of Building and Wood Workers National Wood Carvers Association Society of Wood Engravers Timber Framers Guild Conversion Chainsaw mill Hewing Sawmill Whipsaw Wood splitting Flat sawing Quarter sawing Rift sawing Techniques Frame and panel Frameless construction Green woodworking Category WikiProject Commons --- Adapted from the Wikipedia article [Datum reference](https://en.wikipedia.org/wiki/Datum_reference) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Datum_reference?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.