Glossary

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A

Altitude: the height of an independent object, such as an aircraft, above ground level (AGL) altitude_elevation-display

Angular Unit of Measure: the selected units for measuring angles.  Choices include degree and radians. 

B

Benchmarks: Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid. benchmark

C

Cartesian coordinate system: An 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 cartesian-display

Compromise maps: projections attempt 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 the purpose of preserving shape, distance, and bearing, at the expense of area and scale

Control Points or Tie Points: mathematically derived points that connect a reference ellipsoid to a geoid  

D

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 NAD83)  ''Minor'' Just a few points change. Less expensive; less involved. Noted with a four-digit year (ie. NAD83(1985))

Degrees: 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. 

E

Earth-centered, Earth-fixed System: Earth-centered, Earth-fixed systems use the center of the Earth as a start point for measurements, while local-north systems use a smaller area affixed to the Earth's surface as the start point

Elevation: the vertical distance between local mean sea level and a single point on the Earth's surface altitude_elevation-display

Ellipsoid: a 3D sphere-like shape which is wider than they are long along the same plane instead of perpendicular planes like a spheroid.

Ellipsoid Height the measured distance between the reference ellipsoid and the topographic surface ellipsoid_geoid_orthometric_height-display

Equal area maps: a map where each of the land masses represented is given an equal amount of area

Equidistant maps: aim to preserve distance, but only from the tangential line or lines

F
G

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 Information Systems: the software used to create, store, and manage spatial data, analyze spatial problems, and display the data in cartographic layouts

Geoid: a model of the variation between global mean (average) sea level and local mean sea level, which is used to measure precise elevations on the topographic surface

Geoid Separationthe measured difference between the ellipsoid and the geoid ellipsoid_geoid_orthometric_height-display

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

Global mean sea level: the average of the sea level as affected by the pull of gravity when there is a finite amount of water upon a model of the Earth.

Global Navigation Satellite System or GNSS: 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

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

Geographic Grid: the result of using an established angular unit of measure to label the intersections of north-south and east-west lines on the surface of the Earth starting the labels at a principal meridian.

H

 Horizontal datums: used to reference location on the Earth's surface, regardless of elevation 

I
J
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L

Lambert Conformal Conic Projection: projection developed by Johann Heinrich Lambert in 1772 utilizing a conic developable surface designed to preserve shape and size (to conform) of land masses.

Large Scale Map: a map where the representative fraction is close to one and the objects in the map are relatively large (zoomed in)

Latitude or ''parallels": the east-west portion of the grid measured with angles between 0 and 90°

Local mean sea level: the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface's relief 

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: 1. Shape: the shape of the geographic feature vs. the shape drawn on the map  2. Area: the measured area of a world feature 3. Distance: the measured distance between two world features 4. Direction: the cardinal direction between two world features, minus distance information 5. Bearing: the cardinal direction measuring from one world feature to any other 6. 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 (MSL): global or local, a measurement of the sea level above or below the average 

Minutes: 1/60the of a degree.  In full, arc minute.  Usually denoted by "

Modern geodesy: precise global and regional locations, both horizontal and vertical (along the Earth and above the Earth), mapping the land sea, and ice, and determining the variations in the Earth’s gravitational pull and how this effects measurements

N

Normal 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

Normal Mercator Projection: a projection developed by Gerardus Mercator in 1569 utilizing a cylindrical developable surface tangent at the Equator and designed to produce a map with parallel lines of longitude and latitude.  The main purpose of this projection is navigation in the mid-latitudes for east-west travel.

O

Oblate Spheroid: a sphere-like object which is wider than it is tall OblateSpheroid

Oblique aspect: all other orientations after normal and transverse

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"

Orthometric Height the measured distance between the geoid and the topographic surface. ellipsoid_geoid_orthometric_height-display

P

Planar Coordinate System: the result of converting an angular unit of measure used to locate objects on a geographic coordinate system (a round Earth) to a linear unit of measure via a Cartesian Coordinate System (a flat map). Planar Coordinate Systems utilize linear units such as feet, meters, and international feet.

Prime Meridian: the name of the principal meridian in the latitude/longitude system

Principal Meridian: the north-south line from which the labeling begins.  East-west lines have a very obvious start point: the equator.  North-south lines must start somewhere, so when it is established for a particular geographic grid, it can be considered the principal meridian.

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 sphere-like object which is taller than it is wide ProlateSpheroid

Q
R

Reference Ellipsoid: an 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

Relief: the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features 

Representative Fraction: expression of map scale in ratio form utilizing non-specific linear unit, such as 1 map unit equals 250,000 real world units, or 1:250,000

S

Seconds: 1/60the of an arc minute; 1/3600 of a degree.  In full, arc second.  Usually denoted by ' 

Small Scale Map: a map where the representative fraction is far from one and the objects in the map are relatively small (zoomed out)

Spheroid: a sphere-like 3D object where the radius in one direction is longer than the radius in a direction at a right angle to the first

State Plane Coordinate System (SPCS): a planar coordinate system for the United States which breaks states into zones and uses either a Lambert Conformal Conic projection (east-west trending states) or a Transverse Mercator projection (north-south trending states) to create small pieces with little distortion.  The zones are stitched together to create a US wide map.

Statement of equivalency (also known as ''verbal scale''): the relative scale is expressly defined on a map: 1 cm = 1 kilometer; 1 inch = 10 miles

T

Three dimensional datums: combine horizontal datums with ellipsoidal height

Tidal datums: show the changes in sea level due to tides and are based on local mean sea level

Tie Points or Control Points: Mathematically derived points that connect a reference ellipsoid to a geoid  

Topographic Surface: a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write."

Transverse 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 

Transverse Mercator Projection: a variation of the Normal Mercator Projection where the tangential line is switched from a parallel to a meridian.   Lines of longitude and latitude appear circular and the map is optimized for north-south travel. 

Trilateration: the process of determining absolute or relative locations of points by measurement of distances, using the geometry of circles, spheres or triangles. ... In contrast to triangulation, it does not involve the measurement of angles

True direction maps: equidistant maps specifically for azimuthal projections

U

Universal Transverse Mercator (UTM): a Planar Coordinate System (via a projection) which divides the globe into 60 separate 6° wide zones, each one with the principle meridian 3° from either side and cut half at the Equator.  Each strip, or zone, is then stitched together to create an entire Earth flat map.  To prevent negative numbers, each zone is assigned a origin arbitrarily labeled 500,000 mE, 10,000,000 mN. Using a Cartesian Coordinate System, locations are measured in meters.  Since both the north and the south half of each zone will contain the same coordinates, and there are 60 zones, all possible coordinate pairs will appear 120 times, forcing the need to use the zone number and designate North or South is needed. 

V

 Vertical datums: used to reference locations and distances above mean sea level; elevation.

W
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