Terms and Acronyms
- see also:elevation
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.
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
- see also: tie points
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.
- see geodetic datum
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))
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.
ESRI(Pronounced "ESS - REE") Formally Environmental Sciences Research Institute. Now actually "ESRI". Creators of the ArcGIS Suite of software.
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
GeoidA mean sea level, while areas of lower gravitational pull, most often associated with ocean floors, fall below mean sea level
- see: geoid separation
(Pronouced "Gee - Eye - Ess") Geographic Information SystemsThe software used to create, store, and manage spatial data, analyze spatial problems, and display the data in cartographic layouts.
Geographic Information SciencesThe branch of geospatial sciences concerned with the underlying structure of how to collect and analyze data
Global Navigation Satellite SystemA general term for the technology of using satellites and a signal receiver to pin-point a location anywhere on the surface of the Earth
Global Positioning SystemThe GNSS specific to the United States
- see also geodetic datum
Line of Tangency
Local Mean Sea Level
- see also: mean sea level
zero elevationDetermined by averaging several sea depth measurements from one area to assure local accuracy
- 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 ScaleA mathematical representation expressing distance on a map vs distance on the ground.
Mean Sea Level
- see also: local mean sea level
zero elevationDetermined by averaging several sea depth measurements from around the globe
MXDThe 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
- see: vertical datum
Principal MeridianAny 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
SQL(Pronounced "See-QWELL") Structured Query Language - The 'search and find' computer language for tables in ArcGIS
- see also: control points
- see: vertical datum
Three Dimensional Datums
- see: vertical datum
Trilateration differs from triangulation in that trilateration is about finding distance via the legs of a triangle where triangulation uses known angles.
True Direction MapAn equidistant map specifically for azimuthal projections
- 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