## Terms and Acronyms

**A**

#### Altitude

*see also:elevation*

#### Arc Degree

*see also: arc minutes and arc seconds*

#### Arc Minute

*see also: arc degrees and arc minutes*

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

*see also: arc degrees and arc minutes*

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 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.

**B**

**C**

#### Conic Projections

Conical 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

*see also: tie points*

#### 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 (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 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.

**D**

#### Datum

*see geodetic 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)

- In full, a degree of arc or arc degree
- Usually denoted by °

#### Developable Surface

A 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 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.

**E**

#### Elevation

*see also:altitude*

#### Ellipsoid Height

*see also: orthometric height and geoid separation*

**F**

**G**

#### Geodetic Datum

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

#### 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 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

*see: geoid separation*

#### Geoid Separation

*see also: orthometric height and ellipsoid height*

**H**

#### Horizontal Datum

- see also geodetic datum

**I**

**J**

**K**

**L**

#### Latitude

*see also: longitude*

#### Local Mean Sea Level

*see also: mean sea level*

*zero elevation*

#### Longitude

*see also: latitude*

**M**

#### 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

#### Mean Sea Level

*see also: local mean sea level*

*zero elevation*

#### Minutes

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

- In full, arc minute
- Usually denoted by “

**N**

#### Normal Aspect

**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*

**O**

#### Oblique Aspect

*see also: developable surface amd point or line of tangency*

#### Orthometric Datums

*see: vertical datum*

#### Orthometric Height

*see also: ellipsoid height and point or line of tangency*

**P**

#### 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.

#### 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

**Q**

**R**

**S**

#### Secant

- see also: tangent’’

#### Seconds

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

- In full, arc second
- Usually denoted by ‘

#### Spheriod

A bulging sphere

*Oblate spheroid*- A spheroid that is wider then it is tall*Prolate spheroid*- A spheroid that is taller then it is wide

**T**

#### Tangent

- see also: secant’’

#### Tie Points

*see also: control points*

#### Tidal Datums

*see: vertical datum*

#### Three Dimensional Datums

*see: vertical datum*

#### Transverse Aspect

*see also: normal aspect and oblique aspect*

When 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.

**U**

**V**

#### Vertical Datum

*see also geodetic 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