Term
Map projections and Coordinate Systems |
|
Definition
All spatial data are referenced to a datum and coordinate system... either geographic or projected |
|
|
Term
Spatail referencing system
relevant components...
|
|
Definition
ellipsoid > datum > geographic coordinate system > projected coordinate system |
|
|
Term
|
Definition
Mathematical estimation of earth's general shape, it's shape is smooth |
|
|
Term
Ellipsoid, What does it do? |
|
Definition
establishes the reference system for measuring horizontal location |
|
|
Term
Geometric properties of an Ellipsoid |
|
Definition
semi major axis:
the estimated radius of the earth in the equitorial direction
Semi minor axis:
The estimated radius of the earth in the polar direction |
|
|
Term
Ellipsoid
Flattening Factor |
|
Definition
An expression of the degree of "squashing" in the ellipsoid
( a function of the defference between the semi-major and semi-minor axes) |
|
|
Term
Why do we have so many different ellipsoids?
Explain |
|
Definition
|
|
Term
|
Definition
Ties an estimated ellipsoid to the earth by "fixing" it to the earths surface through a physical network of precisely measured points
( sets the position and oreintation of the ellipsoid, i.e. its center, relative to the earths center (of mass) |
|
|
Term
|
Definition
|
|
Term
|
Definition
Defines the origin of where gratucules of latitude and longitude will lie on the earths surface, i.e. the origin for a geographic coordinate system |
|
|
Term
|
Definition
based on astonomical observations, local surface measurements and older estimated ellipsoids as such, the ellipsoid is aligned to closely fit the earths surface only in one particular region; as a result, the origin, or ellipsoid center, is generally offset relative to the earths true center of mass; only applies to a particular area of the earth and not another; North American Datum (NAD) 1927 is a local datum |
|
|
Term
Earh centered (geocentric) datum |
|
Definition
Based on satellite measurements and newer ellipsoid estimates (best fitting); as such the ellipsoid is aligned to fit the earths surface globally; as a result the origin, or ellipsoid center, is aligned to the earths true center of mass. Applies tot he entire world; North Americna Datum (NAD) 1983 and WGS 1984 are geocentric datums
GIS natively records horizontal location using WGS 1984 datum
|
|
|
Term
Datum Shift
(motherfucker) |
|
Definition
Since different datums are based on different ellipsoids and sets of measurements, the estimated coordinates (location) for benchmark points typically differ between datums
The latitude and longidute estimate of a given location point in one datum will be different from the latitdue and longitude estimate of the same location in a different datum ( as a function of each datum having diff. origins, i.e. ellipsoid centers). The feature has not physically moved, just the estimate of its location has changed. |
|
|
Term
Geographic (datum) transformatino
|
|
Definition
Because coordinates are based off of datums and each datum is based on one particular ellipsoid, a change in a datum changes the underlying ellipsoid, and thus the origin of the spatial reference system based off of it.
Geographic transformation converts latitude, longitude from one datum to another through a series of mathematical calculations (transformations) that account for this defference in origins |
|
|
Term
Geographic Coordinate Systems |
|
Definition
Is is a cartesian coordinate system? NO, it is a spherical (polar) system based on angles from an origin (or baselin)
Latitude and longitude are measured as degrees or angles from the center of the earth, i.e. ellipsoid origin; a geographical coordinate systems origin is always established by a datum
All geographic coordinate systems are based on one particular datum |
|
|
Term
Map projections and projecte coordinate systems |
|
Definition
Transfer the geographical coordinate system established by a datum, i.e. a particular ellipsoid, to a flat surface (3D to 2D)
- mathematical expressions that transform geodetic coordinates to a flat surface
- generally made on to simple geometric shapes called develoable surfaces
- Why? the shapes can be flattened without stretching thier surfaces
- combine developable surfaces and different perspective views (light) to generate a projection
|
|
|
Term
|
Definition
All map projections involve some level of distortion
spatial properties that are inevidably distorted on a map, shape, area, distances and direction
- size of mapped area influences amount of expected distortion; function of how much of the curvature of the earth must be projected (flattened)
- small scale maps: large area, greater degree of distortion
- large scale maps: small area; minimum degree of distortion expected
|
|
|
Term
Projections by surface (families) |
|
Definition
- conical (cones)
- cylindrical (cylindars)
- azimuthal or planar (planes)
|
|
|
Term
Projections by preservation of property |
|
Definition
- equal area (preserve area)
- conformal (preserve local shape)
- equidistant (preserve distance)
- true-directional or azimuthal (preserve direction)
No map can be entirely equal area and conformal at the same time
|
|
|
Term
Lines of tangency (standard parallels) of a projection
How/why are they significant to map projections? |
|
Definition
- points where the developable surface touches the surface of the earth and comprise areas of zero distortion, areas of true scale
- distortion increases away from the lines of tangency
- lines of tnagency can intersect in one place (tangent) or in two (secant)
|
|
|
Term
Linear Unit of a projection |
|
Definition
- the X and Y values are stored, e.g. meters, feet
- also establishes the unit of measurement, e.g. area, lenght etc. for spatatial data with coordinates stored in that projection
|
|
|
Term
|
Definition
- Sea surface as a function of gravity alone; no tidal, atmospheric or surface influence
- resulting surface is a lumpy and irregular ( from gravitational anomalies) surface that approzimates mean sea level
- establishes the reference system for measurein vertical location ( elevation)
- orthometric height ( height above the geoid; more accurate) ellipsoidal height ( height above the ellipsoid; less accurate)
- GPS measures elevation as hight above the ellipsoid, thus relatively inaccurate way to measure elevation in that regard
|
|
|