Glossary

Type of units for measuring rotation

An area of use is a polygon that defines the applicable extent of an object in the datasource, such as a coordinate reference system or transformation.They are defined as both a polygon and a minimum bounding rectangle in Latitude / Longitude coordinates in WGS84.

Combined Factor is simply Grid scale multiplied by the Orthometric Height Scale. This factor is used to calculate ellipsoid distance from a grid distance above or below the ellipsoid. Example: The Grid Scale for two points is 0.99968900, and the Orthometric Height scale for the points is 0.999999123, the combined factor is: 0.99968900 x 0.999999123= 0.999688123 If the measured horizontal distance between the two points is 1000 meters, then calculate grid distance as: 0.999688123272747 x 1000meters = 999.69 meters

Convergence is the angle of difference in direction between Grid North and True North. This will vary across a projected coordinate system and can be used as a measure of accuracy of angular measurements at a given point on the map. Note: The Geographic Calculator uses the GaussBomford convention for calculating convergence. For more information see http://www.iogp.org/pubs/37321.pdf

A Point Style is used to define the orientation of axes used and the type of units used in the system.

A Coordinate System is a complete definition needed to express the context of a set of map data.

Coordinate Transformations (Concatenated, Datum, Parametric or Vertical)  Parameters to transform coordinates from one Coordinate System to another.

The database of geodetic parameters. This includes objects like Coordinate Systems, Datums, Coordinate Transformations, Units of Measure, etc. The Geographic Calculator includes both a builtin default datasource, and a custom datasource, which contains edits and additions made by the user or administrator.

A Datum is the base model that maps are built on. All coordinate systems must have a datum associated with them to be related to any other map. Without a known datum, coordinates are meaningless.

Digital Elevation Models. This is data stored in a pixelbased raster format, where the pixel value represents elevation.

A system that has a dynamic reference frame. This means the position of points may change over time, usually due to crustal deformations such as tectonic motion and glacial isostatic adjustment. A dataset tied to a dynamic coordinate reference system should include epoch information (date).

An Ellipsoid gives a Datum its size and shape. An Ellipsoid does not have an origin and cannot be used as a base model for coordinates on its own.

A datum ensemble is a group of multiple realizations of the same reference system that are not significantly different when using approximation.

A local survey grid or engineering system paired with a polynomial transformation to relate the system to a known geodetic or projected system. The transformation may be calculated by minimizing the error for a set of control points that define coordinates in both the local system and a known system.

A geocentric coordinate system is a 3dimensional coordinate system represented by X,Y,Z coordinates. The origin of the system is the center of the earth.This is also refered to as earthcentered, earthfixed (ECEF).

Geodesic distance is the shortest possible line between two points on the surface of the datum. This is also referred to as Great Circle distance. It is the shortest distance between two points along the surface of the ellipsoid.

The decimal precision of Geodetic (Latitude, Longitude) coordinates. See Unit Display Settings to set the default precision.

Grid distance measures the distance following a straight line between two points in grid space.

Grid Scale Factor, often simply called "Scale Factor" is a measure of distortion at a given point on a projected map. The scale factor is not cartographic scale, but a factor used to calculate actual ellipsoidal distances rather than distances on the projected surface.

Units for measuring straight line, Cartesian distances.

The angle between magnetic north and true north. There are multiple models of magnetic declination which can be selected in preferences.

Also known as Elevation Factor. Orthometric Height scale represents a factor of elevation that can be used to calculate geodetic distances above or below the ellipsoid (also known as reducing to the ellipsoid). This scale is determined using a constant radius for the earth in the area of the calculation, but this is typically considered accurate enough for most applications.

A Point Style is used to define the orientation of axes used and the type of units used in the system.

The Prime Meridian represents the line of longitude that is designated as 0 degrees. All other lines of longitude are expressed by the angle by which they deviate from the Prime Meridian. Together with the equator of an Ellipsoid, the Prime Meridian defines the origin of a Geodetic Coordinate System. A Prime Meridian is defined by the angle that separates it from the Greenwich Meridian.

A Rhumb Line is a line of constant bearing. Rhumb line distance measures the distance between two points maintaining a line of constant bearing relative to true north. Rhumb Lines are frequently used to plot navigation lines.

Unit of Measure for scale factor

Grid Scale Factor, often simply called "Scale Factor" is a measure of distortion at a given point on a projected map. The scale factor is not cartographic scale, but a factor used to calculate actual ellipsoidal distances rather than distances on the projected surface.

Scope is a description of the primary purpose to which a coordinate reference system, datum or coordinate operation is applied.

A grid coordinate system where the coordinates are represented as an alphanumeric string.

A usage defines how and where a coordinate system, datum or transformation is intended to be used. A usage includes both scope and extent. Area of use represents the geographic extent. A coordinate system or transformation can be tied to multiple usages.
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