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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' can be applied to the practical problem of describing positions of features on or near the earth' ...

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GIS Dictionary > datum
GIS Dictionary
Definitions for GIS terms related to operations such as analysis, GIS modeling and web-based GIS, cartography, and Esri software.

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.

Geodetic ~ - WGS 84 - NAD 83 - GPS
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A reference ~ is a known and constant surface which can be used to describe the location of unknown points. On Earth, the normal reference ~ is sea level. On other planets, such as the Moon or Mars, the ~ is the average radius of the planet.

The Geocentric ~ of Australia (GDA) is the new Australian coordinate system, replacing the Australian Geodetic ~ (AGD). GDA is part of a global coordinate reference frame and is directly compatible with the Global Positioning System (GPS).

You are publishing a tiled map service using the ArcGIS Online/Google Maps/Bing Maps tiling scheme and there is no ~ transformation from your data frame's ~ 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 ~ for elevation measurements.

4-1. The earth is an ellipsoid, not a sphere, flattened slightly at the poles and bulging somewhat at the equator. ~s are reference surfaces that consider the curvature of the earth for the mathematical reduction of geodetic and cartographic data.

~, 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.

A ~ is a mathematical surface on which a mapping and coordinate system is based.
Digital elevation model (DEM)
A topographic surface arranged in a data file as a set of regularly spaced x, y, z coordinates where z represents elevation.

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

~s are sets of parameters and ground control points defining local coordinate systems. Because the earth is not a perfect sphere, but is somewhat "egg-shaped," geodesists use spheroids and ellipsoids to model the 3-dimensional shape of the earth.

A ~ 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. There are many different ~s and hence many different starting positions.

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.
Digital Elevation Model (DEM) ...

A ~ is a system which allows the location of latitudes and longitudes (and heights) to be identified onto the surface of the Earth - ie onto the surface of a 'round' object.
The basic mathematical/geometric principle which is used is that: ...

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

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

~s (sets of data) are the basis for all geodetic survey work. They act as reference points in the same way that starting points do when you give someone directions.

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

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

~ conversions are automatically handled by the PROJ.4 library if "+~" setings are specified on both the input and output projections on the command line. The "+towgs84" parameter can be used to define either 3 or 7 term ~ transform coefficients, satisfying this requirement.

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).
Combination of parameters and control points used to accurately define the three dimensional shape of the Earth (spheroid).

A reference point or surface against which position measurements are made, and an associated model of the shape of the earth for computing positions.

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


The ~ 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 ~ of 1988; Elevation; Sea Level ~ of 1929.
National Spatial Data Infrastructure ...

Geodetic ~
These materials were developed by Peter H. Dana, Department of Geography, University of Texas at Austin, 1995. These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the author, Peter H.

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

Geodetic ~s
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Changing ~s (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 ~ relate coordinates to the earth's center of mass. Such ~ have been improved by modern satellite data. The North American ~ of 1983 is based on the modern GRS 1980 ellipsoid - almost identical to the most widely used WGS 1984 ellipsoid which is also geocentric.

Geodetic ~
A math model representing the size and shape of the earth (or a portion of it).

(singular: ~) information collected during a scientific study.
data capture
Noun ...

Land Surface ~; Lands and Survey Div. (Trinidad and Tobago)
Landsat Signature Development Program (software) ...

North American ~ 1983
North American Presentation Level Protocol Syntax ...

data update cycle Source: GETIS glossary Data update interval dataset series Source: ISO 19115; ISO 19113; ISO 19114 Collection of datasets sharing the same product specification ~ Defines the origin, orientation and scale of the coordinate system and ties it to the earth, ...

The global WGS84 ~ (i.e., World Geodetic System of 1984) uses the center of the earth as the origin of the GCS and is used for defining locations across the globe.

Map Projections and ~ and Coordinate Systems Overview, by Peter H. Dana - part of the The Geographer's Craft project.
Return to The Map Projection Home Page ...

Typically consists of a projection or a system of projection zones plus a geodetic ~ and ellipsoid. NDVI p. 220 difference between two bands (near infrared minus visible red) divided by the sum of the two bands; high values indicate active vegetation growth; often applied to NOAA AVHRR data.

NDVI = (IR - Red)/(IR + Red) North American ~ (NAD) ~s used as a basis for large scale mapping, particularly NAD27 and NAD83 Northings The y-coordinates in a plane-coordinate system; see eastings.
O ...

Then I went through the source code to try to find out where the CH1903 ~ came into the picture. The source code is just about unreadable, being made up mostly of #defines for all the functions.

bench mark - Relatively permanent material object, natural or artificial, bearing a marked point whose elevation above or below an adopted ~ is known.

map hypsographic Map showing relief with elevations referred to the national geodetic vertical ~ of 1929. map, hypsometric Map showing relief by any convention, such as contours, hachures, shading, or tinting.

BenchmarkA brass or bronze disk set in a concrete base or other permanent structure, inscribed with a mark showing its elevation above or below an adopted vertical ~.~In geomatics, a mathematical model used to approximate the size and shape of the earth.

digital elevation model The digital cartographic representative of the surface of the Earth or a subsurface feature through a series of three-dimensional coordinate values: a continuous variable over a two-dimensional surface by a regular array of z values referenced to a common ~.

An estimate of the earth's surface based on an ellipsoid provides a determination of the elevation of every point on the earth's surface, including sea level, and is often called a ~. Over time, and in different countries, many ~s have been developed and used.

A digital representation of a continuous variable over a two-dimensional surface by a regular array of z values referenced to a common ~. Digital elevation models are typically used to represent terrain relief. This is a common GIS product created at a variety of scales.

There is another critical bit of information that is required before you are totally confused: the ~. The need for a ~ is the consequence of man's inability to describe mathematically the exact shape of the earth.

This function supports most geodetic ~ used around the world. This function can convert the control points or be applied to a single point.
For 3D VRML export, the output file now includes color and image draping information.

Geodetic ~s (Dana/Geographer's Craft) -- Introduction to geodetic ~s; Figure of the earth: geometric models, reference ellipsoids, global coordinate systems, earth surfaces; Geodetic ~s.

This conversion requires both a transformation between ellipsoids and a ~ shift. Numerous ~s have been used for mapping around the world. For details, see Snyder (1987; 1989).

The software performs transformation on raster and vector data sometimes of differing ~s, grid system, or reference system, into one coherent image. It can also analyse changes over time within a region. This software is central to the professional analysis and presentation of GIS data.

All parametric equipment types include at least one nozzle manager ~.
Vertical vessel with legs (TANK_V4LEG) correctly orients legs straddling the axis of the vessel.
Label tool selects individual equipment nozzles.

Z-score The (signed) number of standard deviations an observation or ~ is above or below the mean. A positive standard score represents a ~ above the mean, while a negative standard score represents a ~ below the mean.

Both 32661 and 4326 use the WGS 84 ~ and ellipsoid but one is long lat (4326) and one is a meter based one. The WGS 84 simply defines the ellipse and ~.

But the earth's mountains bump-up and valleys bump-down from the ellipsoid so a ~ is designed to fit the earth's surface that accounts for the actual wrinkling of the globe as established by orbiting satellites.

Take a look at the projection definition you copied (Imported) for the la_to_ny shapefile you created with ArcToolbox, what Spheroid was used with the ~?
3b. Briefly state what the difference between Spheroid and ~ is.

Geodetic ~ Overview, Department of Geography, University of Colorado at Boulder
Global Positioning System Overview, Department of Geography, University of Colorado at Boulder
Coordinate Systems Overview, Department of Geography, University of Colorado at Boulder ...

Geodetic ~s Overview
The Mercator Conformal Projection Norris Wiemer, University of Alberta.
John Snyder An obituary of the man who achieved immortality by computerizing the mathematical algorithms for transforming map projections.

altitude -- elevation above or below a reference ~, as defined in Federal Information Processing Standard 70-1. See also elevation.
area -- a generic term for a bounded, continuous, two-dimensional object that may or may not include its boundary.

Wide range of Map Projections and ~s supported
Non-destructive, parameter based image chains
Native file access
Precision Terrain correction and ortho-rectification
Advanced Mosaicing, compositing and fusions
Elevation support
Vector and shapelib support ...

Attributes (see description above);
Geographic information describing the position of the ~ in space relative to other data; and
Temporal information describing the instant or period of time for which the ~ is valid.

HOW WE KNOW WHERE SOMETHING IS First, a disclaimer: This text does not pretend to cover issues such as geodetic ~s, projections, coordinate systems, and other terms from the fields of geodesy and surveying.

This should at least include dataset names with descriptions of what they contain, coordinate system & ~, theoretical accuracy, source document names with dates & scales, data conversion methods, update history, lists of field names & what they contain, ...

The data related to a particular part of the Earth surface are often in different format, have different georeferencing (~ heterogeneity, projection heterogeneity, etc.), end different resolution.

Manual, semi-automated, and automated digitizing
Data layering
Data editing
Data manipulation
~ conversions, transformations, and projections
Topology building
Data structuring
Attaching data attributes to graphics
Data format conversions ...

With names like State Plane, UTM, Albers and many others, projections can be quite thorny for novice GIS users (compounded by a related factor, called a '~' which defines the surface of the earth as of a certain date).

A reference system that uses latitude and longitude to define the locations of points on the surface of a sphere or spheroid. A geographic coordinate system definition includes a ~, prime meridian, and angular unit.
Layer1 ...

The 'srid' column is one of the most important because it stores the 'Spatial Reference Indentifier' value which dictates the projection and ~ to be used to correctly display the geographic data on the surface of the earth.

These include data layer classifications, source media and digitizing standards, ~s, projections, coordinate systems, cartographic elements, and output scales. Standards and guidelines for these aspects of GIS are scheduled for development within WGIAC's GIS Standards Plan document.

NAD - North American ~: The official reference ellipsoid used for the primary geodetic network in North America.
Node: The beginning and ending locations of a line on a digital map. A node is topologically linked to all lines that meet at the node.

ArcInfo supports these transformations: similarity, affine, piecewise linear, projective, NADCON ~ adjustment using minimum-derived curvature transformation, and a polynomial transformation to warp grids and images.

See also: See also: What is the meaning of Map, Information, Coordinate, Geographic, Location?

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