Projections and Coordinate Systems Flashcards
Geographic Coordinate System vs. Projected Coordinate System
GCS- records locations on a sphere.
PCS- based on a GCS and defined on a 2D surface.
Datums
Defines the position of the spheroid relative to the center of the earth.
- provides frame of reference for measuring locations on earth’s surface. It defines origin and orientation of lat and long lines.
- datum is built on top of selected spheroid and can incorporate local variations in elevation.
Map Projections
Defines manner in which 3D info about Earth is transformed to a 2D surface for display and analysis.
- They introduce error and distortion into spatial data.
- Certain projections are designed to minimize a particular type or types of distortions but NO MAP IS COMPLETELY DISTORTION-FREE
Geographic Coordinate System
Earth is divided into lines of latitude and longitude.
- System of measurement
- Units are called degrees
Spheroid/Ellipsoid
Earth is not a perfect sphere (bulges at the equator do to its rotation). The spheroid (aka ellipsoid) is used with GCS because latitude and longitude are more accurate than those on a sphere.
-geoid can only be approximated mathematically by a spheroid because of local anomalies at various scales.
Geoid
The shape of the Earth without the influence of Earth’s gravitation and rotation alone (no wind, current, etc.) It has an equipotential surface (same gravity everywhere)
- actual shape of Earth is best represented by the geoid.
- geoid can only be approximated mathematically by a spheroid because of local anomalies at various scales.
Geographic Coordinate System Problems
GCS records locations on a sphere (3D) whereas GIS coordinates are drawn in 2D. Distortions are introduced by this practice.
Common Datums
- World Geodetic Survey of 1984 (WGS84) is a satellite-measured spheroid
- North American Datum of 1983 (NAD83) based on GRS_1980 spheroid
- North American Datum of 1927 (NAD27) based on Clarke 1866 spheroid
- Coordinates of a particular location can differ up to several 100 meters between NAD83 and NAD27
Map Projection Classification- CONFORMAL
Local shapes are preserved. Conformal maps are well-suited for navigation.
-Examples: Lambert conformal conic, transverse Mercator, Mercator
Map Projection Classification- EQUAL-AREA
Areas are preserved. Equal-area maps are well-suited for general thematic mapping.
-Examples: Albers equal-area conic, Lambert azimuthal equal-area, Mollweide
Map Projection Classification- EQUIDISTANT
Distances from all locations to one or two points are preserved. Equidistant maps are useful for measuring distances from fixed locations.
-Examples: Azimutha equidistant (distances from projection center are true), two-point equidistant (distances from two central points are true).
Map Projection Classification- AZIMUTHAL
Directions from all locations to one or two points are preserved. Azimuthal is well-suited for mapping polar regions, and for achieving certain special characteristics.
-Examples: Gnomonic projection (great-circle paths between any two points appear on the map as straight lines), Stereographic (also conformal), Lambert azimuthal equal-area (also equal area.
Classification based on Geometric Surface Used- CYLINDRICAL
Scale is true along the Equator or along two parallels equidistant from the Equator.
Classification based on Geometric Surface Used- CONIC
Scale is true along one or two standard parallels.
Classification based on Geometric Surface Used- AZIMUTHAL
Typically, scale is true at the projection center or on a circle around the center.
