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Terms and Acronyms

A   B    C    D   E    F  G    H    I    J    K    L    M    N    O    P    Q    R   S    T    U    V    W    X    Y    Z


A

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Accuracy

Precision Accuracy

The ability to hit the target or goal

Altitude

The Altitude Elevation height of an independent object, such as an aircraft, above ground level (AGL)

Angular Unit of Measure

Any Unit Circle division of circles accepted as a standard of measurement for angles

Arc Degree

The full name for degrees when referencing the angular unit of measurement of plane angle, representing 1⁄360 of a full rotation (circle)

Arc Minute

The full name for minutes when referencing the angular unit of measurement of plane angle, representing 1⁄60 of a degree

  • Usually denoted by "

Arc Second

The full name for seconds when referencing the angular unit of measurement of plane angle, representing 1⁄60 of a minute

  • Usually denoted by '

Azimuthal (Planar) Projections

Azimuthal Planar Projection projections, also known as planar projections, are projections where a rectangular developable surface is tangent at a single point or secant along a path (the developable surface 'slices' through part of the globe) and map elements are projected from a single light source.

  • Azimuthal projections can be normal - which in an azimuthal projection, the developable surface is tangent at either the North or South pole; transverse - the point of tangency is somewhere along the equator; or oblique - the point of tangency is anywhere else.
  • Examples of azimuthal projections include: Azimuthal Equidistant, Lambert Azimuthal Equal-Area, Gnomonic, Stereographic, and Orthographic projections.


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Benchmark

Real-world Benchmark locations which have been carefully surveyed with locations to match a specific geoid.


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Cartesian coordinate system

An Cartesian equally spaced grid that exists in a single geometric plane where intersections of perpendicular lines are labeled with the count of units from a specified (0,0) point

Compromise Map

A Robinson Projection type of projection attempting to balance all of the distortions in one map. This means that none of the six are "perfect", but each one is is balance with the others, the idea being that no one place is grossly distorted in comparison to any other place on the map

Conformal Maps

Serve Mercator Projection the purpose of preserving shape, distance, and bearing, at the expense of area and scale

Conic Projections

Conical Conic Projection projections use a developable surface which start in the shape of a cone. The cone is slipped over the Earth and is either tangent along a single line or secant along two parallel lines around the entire planet. After the projection is completed, the cone is removed and slit up one side. Conical projections reduce distortion closest to the tangent or secant lines, with the distortion increasing as one moves away from these areas.

  • Conical projections can be equatorial (normal) - meaning the cone is tangent at the equator; transverse - the cone is tangent along a meridian; or oblique - the cone is tangent along another path.
  • Examples of conic projections include Lambert Conformal Conic, Albers Equal Area Conic, and Equidistant Conic projections

Control Points

Mathematically derived points that connect a reference ellipsoid to a geoid

Cylindrical Projections

Cylindrical projections use a cylinder slipped over the Earth with either a single line of tangency or two secant lines. After the map elements are projected onto the cylinder, it is slit and rolled flat.

Cylindrical projections are probably the most common, for they result in a rectangular map that does not have distortion like an azimuthal projection. Cylindrical projections, like conical have the least amount of distortion near the tangent or secant lines, then the distortion increases one moves away.

  • Equatorial Cylindrical Projection (normal) projections are tangent along the Equator, with the cylinder parallel to the poles. Mercator is one of the most popular ways to see this projection.
  • Transverse Transverse Mercator Oblique projections are tangent along a meridian (most often along the Prime Meridian, but it's not mandatory). Transverse Mercator projections are popular; Universal Transverse Mercator (UTM) is a transverse Mercator projection superimposed with a grid for navigation purposes. UTM is just as popular as latitude and longitude when it comes to the settings on your GPS unit.
  • Oblique projections are a cylindrical projection along any line that is not the equator or a meridian. Oblique cylindrical projections are used to reduce distortion locally, not just at the Equator or Prime Meridian.
  • Examples of cylindrical projections include Mercator, Transverse Mercator, Oblique Mercator, Plate Carré, Miller Cylindrical, Cylindrical equal-area, Gall–Peters, Hobo–Dyer, Behrmann, and Lambert Cylindrical Equal-Area projections.

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Datum

Datum Shift

When control points are adjusted via better mathematical calculations or real-world surveying.

  • Benchmarks cannot move, but control points can change via datum shifts.
  • Major: Large effort; many points change; expensive and time-consuming. Noted with a two-digit year (ie NAD*#)
  • Minor: Just a few points change. Less expensive; less involved. Noted with a four-digit year (ie. NAD83(1985))

Degree of Arc

The full name for degrees when referencing the angular unit of measurement of plane angle, representing 1⁄360 of a full rotation (circle)

Degrees

angular unit of measurement of plane angle, representing 1⁄360 of a full rotation (circle)

Developable Surface

A Projsurf geometric shape which will not be distorted when flattened. Used as the base shape to transfer features during projections. Most often a cone, cylinder, or plane (azimuthal)

Distortion Ellipses (Tissot's indicatrix)

Start Mercator Projection as circles placed on the globe. As the projection is created, the distortion ellipses distort in a manner equal to the map's distortion at the place upon which they are centered. This method allows for a user to visualize the map's distortion without any measuring equipment.

For example, if size being distorted, the circles closest to the line of tangency will remain the original size while those further away will increase in size.

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Elevation

Altitude Elevation The measured distance between local mean sea level and the topographic surface

Ellipsoid

A Triaxial Ellipsoid three-dimensional, sphere-like shape which is longer then it is wide

Ellipsoid Height

The Ellipsoid Geoid Orthometric Height measured distance between the reference ellipsoid and the topographic surface

Error

In Precision Accuracy GIS and Cartography, the linear distance between the represented data and reality.

ESRI

(Pronounced "ESS - REE") Formally Environmental Sciences Research Institute. Now actually "ESRI". Creators of the ArcGIS Suite of software.

Equidistant Maps

The Equirectangular Projection product of a projection designed to preserve distance, but only from the line or lines of tangency

Equal Area Map

A Gall Peters Projection map where each of the land masses represented is given an equal amount of area

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Geodetic Datum

The result of attaching a "free-floating" reference ellipsoid to a specifically measured geoid via control points and benchmarks.

Geodesy

The science of measuring and monitoring the size and shape of the Earth and the location of points on its surface

  • Geodetic - an action relating to geodesy
  • Geodesist - the person performing that action

Geographic Coordinate Systems

A system of labeling locations on the Earth's surface that must include a datum, an angular unit of measure, and a principal meridian

Geoid

A
A model of a geoid: continents are gold, areas with a greater gravitation pull are red, areas with a lower gravitational pull are green.
A model of a geoid: continents are gold, areas with a greater gravitation pull are red, areas with a lower gravitational pull are green.
word used to describe the shape of the Earth, expressing the changes in gravitational pull due to the density of the crust. Areas of higher gravitational pull, most often associated with mountainous regions, rise above mean sea level, while areas of lower gravitational pull, most often associated with ocean floors, fall below mean sea level

Geoid Height

Geoid Separation

AKA Ellipsoid Geoid Orthometric Height geoid height

The measured difference between the reference ellipsoid and the geoid

GIS

(Pronouced "Gee - Eye - Ess") Geographic Information Systems

The software used to create, store, and manage spatial data, analyze spatial problems, and display the data in cartographic layouts.

GISci

Geographic Information Sciences

The branch of geospatial sciences concerned with the underlying structure of how to collect and analyze data

GNSS

Global Navigation Satellite System

A general term for the technology of using satellites and a signal receiver to pin-point a location anywhere on the surface of the Earth

GPS

Global Positioning System

The GNSS specific to the United States


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Horizontal Datum

Used to reference location on the Earth's surface, regardless of elevation


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J

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K

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Line of Tangency

Latitude

The Latitud east-west portion of a specific geographic coordinate system measured with angles between 0 and 90° degrees used to locate specific locations

Local Mean Sea Level

zero elevation

Determined by averaging several sea depth measurements from one area to assure local accuracy

Longitude

The Longitude1 north-south portion of a specific geographic coordinate system measured with angles between 0 and 180° degrees used to locate specific locations


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Map Distortion

In GIS, the unavoidable inaccuracies which occur when transferring features from a geographic coordinate system to a developable surface. Comes in six flavors:

  • Shape: the shape of the geographic feature vs. the shape drawn on the map
  • Area: the measured area of a world feature
  • Distance: the measured distance between two world features
  • Direction: the cardinal direction between two world features, minus distance information
  • Bearing: the cardinal direction measuring from one world feature to any other
  • Scale: comparing the size of two world features vs. the same two drawn on a map

Map Scale

A mathematical representation expressing distance on a map vs distance on the ground.

Mean Sea Level

zero elevation

Determined by averaging several sea depth measurements from around the globe

Minutes

angular unit of measurement of plane angle, representing 1⁄60 of a degree

MXD

The file extension for saving map documents in ArcMap. This format saves the data loaded into the project, the symbology of that data, table of contents order, and any cartography created in layout view

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Normal Aspect

Normal Flashcard *see also: developable surface, point or line of tangency, transverse aspect and oblique aspect
When a developable surface is oriented with the polar axis (cones and cylinders) or tangent with either of the poles (azimuthal).

Equatorial: Specifically tangent with the Equator

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Oblate Spheroid

A OblateSpheroid spheroid that is wider then it is tall

Oblique Aspect

Oblique Flashcard

When an aspect is neither a normal nor transverse. All other positions of aspects.

Orthometric Datums

Orthometric Height

The Ellipsoid Geoid Orthometric Height measured distance between the geoid and the topographic surface

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Planar Projections

Point or Line of Tangency

The single point (in azimuthal developable surfaces) or line (cones and cylinders) where a developable surface touches the geographic coordinate system. The line of tangency is almost always the area of least distortion.


Precision

the Precision Accuracy ability to repeat your results almost exactly the same every time

Principal Meridian

Any of the true geographical meridians established by authority of the surveyor general of the U.S. that serves as the meridian of reference for subdividing public lands in a given region

Projection

The result of using one of variety of methods to transfer the geographic locations of features from a geographic coordinate system to a developable surface

Prolate Spheroid

A ProlateSpheroid spheroid that is taller then it is wide


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R

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Reference Ellipsoid

An Global Local Ellipsoids ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid

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Secant

In projections, when a developable surface touches or intersects the datum in two places.

Seconds

angular unit of measurement of plane angle, representing 1⁄60 of a minute

Spheriod

A bulging sphere

SQL

(Pronounced "See-QWELL") Structured Query Language - The 'search and find' computer language for tables in ArcGIS


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Tangent

In projections, when a developable surface touches or intersects the datum in only one place.

Tie Points

Mathematically derived points that connect a reference ellipsoid to a geoid

Tidal Datums

Three Dimensional Datums

Topographic Surface

The surfacecomparison.gif topographic surface is the actual surface of the Earth and sea floor, minus the oceans. It takes into account all the variations of mountains and valleys without any averaging or smoothing

Transverse Aspect

When Transverse Flashcard a developable surface is perpendicular to the polar aspect (cones and cylinders) or tangent with the Equator (azimuthal)

  • Polar: Centered on the North Pole or the South Pole

Trilateration

A technique for establishing the distance between any two points, or the relative position of two or more points, by using such points as vertices of a triangle or series of triangles, such that each triangle has a side of known or measurable length (base or base line) that permits the size of the angles of the triangle and the length of its other two sides to be established by observations taken either upon or from the two ends of the base line.


Trilateration differs from triangulation in that trilateration is about finding distance via the legs of a triangle where triangulation uses known angles.

True Direction Map

An equidistant map specifically for azimuthal projections

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Vertical Datum

Used to reference locations and distances above mean sea level; elevation.

  • Orthometric datums: Shows the changes in the Earth's gravitational pull from 0 - any height referenced to the Earth's gravity field can be called as "geopotential heights"
  • Tidal datums: Show the changes in sea level due to tides and are based on local mean sea level
  • Three dimensional datums: Combine horizontal datums with ellipsoidal height


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X

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Y

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Z 

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ZIP File or Data

A computer file whose contents of one or more files are compressed for storage or transmission, often carrying the extension .ZIP.

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