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Datum (GIS)

A set of parameters and control points used to accurately define the three-dimensional shape of the Earth (e.g., as a spheroid). The corresponding datum is the basis for a planar coordinate system.

datum - Defines the origin, orientation and scale of the coordinate system and ties it to the earth, ensuring that the abstract mathematical concept 'coordinate system' ...

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GIS Dictionary > datum
GIS Dictionary

Map Datums
A datum describes the model that was used to match the location of features on the ground to coordinates and locations on the map. Maps all start with some form of survey.


Reference Datum
A reference datum is a known and constant surface which can be used to describe the location of unknown points. On Earth, the normal reference datum is sea level.

You are publishing a tiled map service using the ArcGIS Online/Google Maps/Bing Maps tiling scheme and there is no datum transformation from your data frame's datum to WGS 1984 Major Auxiliary Sphere.

A reference location or elevation which is used as a starting point for subsequent measurements. Sea level is a datum for elevation measurements.

4-1. The earth is an ellipsoid, not a sphere, flattened slightly at the poles and bulging somewhat at the equator.

Datum, Spheroids and Ellipsoids
Unlike local surveys, which treat the Earth as a plane, the precise determination of the latitude and longitude of points over a broad area must take into account the actual shape of the Earth.

Datums 1 - the Basics
A datum is a system which allows the location of latitudes, longitudes and heights to be identified onto the surface of the Earth. As datums is a jargon rich subject it is recommended that you read this section first.

A datum is a mathematical surface on which a mapping and coordinate system is based.

Datum: In mapping, a numeric or geometric quantity which serves as a reference or base to accurately define other quantities.

Datum - A reference surface for a particular coordinate system. Defines the mathematical relationship by which a feature relates to its original surface in a specific coordinate system.

Datums are sets of parameters and ground control points defining local coordinate systems.

A datum is a starting point for locating features on the Earth's surface; it is the origin point of a coordinate system. It defines the position of the ellipsoid (or spheroid) relative to the Earth's center.

The Datum
For highly precise maps of smaller regions, the basic ellipsoidal shape can not be ignored. A geodetic datum is a set of parameters (including axis lengths and offset from true center of the Earth) defining a reference ellipsoid.

The Datum Selection Option
The selection of the DRG Clipper Button will prompt the user to identify the current datum of the 24K DRG files as shown below.

Datum A point, line, or surface reference from which coordinate systems are derived for surveying and mapping.
DNR Louisiana Department of Natural Resources.

Datum transformations using Molodensky equations (implemented in most geo-software) are becoming increasingly important, because of the growing use of GPS data.

Datum conversions are automatically handled by the PROJ.4 library if "+datum" setings are specified on both the input and output projections on the command line.

Datum Notes:
Spatial data are referenced to a horizontal datum and a vertical datum, both of which are based on a mathematically calculated shape of the earth.

Datums can be vertical, horizontal, or complete. Vertical datums deal primarily with elevation and are usually developed to supplement existing datums.

Point, line or surface used as a reference for a measurement of another quantity. Point, line, or surface used as a reference (i.e., surveying, mapping, or geology).

Datum - A point, line or surface selected as the origin or reference for measurements. A datum can be mathematically defined (e.g. the Earth's ellipsoid) or established by field observations (e.g. the mean sea level).

The datum established in 1929 by the U.S. Coast and Geodetic Survey as the surface against which elevation data in the United States is referenced. Referenced by: North American Vertical Datum of 1988; Elevation; Sea Level Datum of 1929.

Geodetic Datum: A set of parameters defining co-ordinate systems for all or parts of the earth. These datums have been refined and revised over time. NAD 27 is the North American datum for 1927, for example.

Geodetic Datum

Multiple datum LRS and color coding of complex infrastructure objects often result in misleading or incorrect computation, identified by Sutton (1996) as network pathologies.

Geodetic Datums
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Changing datums (for example, from NAD 27 to NAD 83) can be difficult or cumbersome.
Projections are usually implemented using power series approximations that might be inadequate for high precision work.

Geocentric datum relate coordinates to the earth's center of mass. Such datum have been improved by modern satellite data.

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See also: See also: Map, Geographic, Coordinate, Information, Surface