Working with Data Source Objects
Individual entries in the geodetic data source are known as Data Source Objects. Geographic Imager can support an unlimited number of custom objects. There are different types of objects for different types of definition. Objects contained in the geodetic data source are:
Data object |
Description |
*All* |
Used to define the orientation of axes used and the type of units used in the system. |
Angular Units |
Type of units for measuring rotation. |
Coordinate Systems |
A complete definition needed to express the context of a set of map data. |
Datum Transformations |
Parameters to transform coordinates from one datum to another. |
Ellipsoids |
An ellipsoid gives a horizontal 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. |
Envelopes |
Defines a geographic area of use for a particular object. |
Horizontal Datums |
More commonly referred to as just "datum" is the base model 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. |
Linear Units |
Units for measuring straight line, Cartesian distances. |
Prime Meridians |
Defines longitude values of meridians. |
*All*
For coordinate points, specify the coordinate point dimensions in either 2D or 3D. For Geodetic and Projected Coordinate Points, the Longitude and Latitude style of units must be entered for all 2D Dimensional Points. The Longitude, Latitude and Ellipsoid Height style of units must be entered for 3D Dimensional Points.
Angular Units
For angular units, enter a conversion for the new unit in terms of the scientific standard Degrees. The Abbreviation is used to identify the unit within the application interface (Example: The abbreviation for degrees is "deg").
Coordinate Systems
For Coordinate Systems, select an Envelope defining the appropriate area of use. If you are unsure of the appropriate envelope, you can leave it set to the default World envelope. The Point Style is how you will select the style of linear units for your system. For most projected systems, you will want to select "Projected point in (units)". Then select the Geodetic model that using the appropriate datum for your system. When you select the appropriate projection for your system, the parameters needed to define the system will appear in the table below. Enter the needed parameters and define the units each parameter is specified in.
Datum Transformations
For Datum Shifts, select an Envelope (appropriate geographic area of use), Source, Target and Method. If you are unsure of a more specific envelope, leave it set to the default "World" Envelope. The Source and Target fields are used to specify the two geodetic models the Transformation is valid for. The Method specifies the necessary parameters to define a particular datum transformation. With the proper method selected, enter the appropriate parameters for your datum transformation. Be sure to define the appropriate units for each parameter by clicking the Units button.
Ellipsoids
For Ellipsoids you must enter the parameters for the Semi-Major Axis and Semi-Minor Axis (often noted as "a" and "b" respectively) define the linear units the axes are specified in by selecting a predefined unit in the appropriate fields. The Inverse Flattening (often noted as "1/f") will automatically calculate in the field below. Alternately, you can make the Inverse Flattening definitive by enabling the check box at the bottom. You can then manually enter the Inverse Flattening rather than the Semi-Minor Axis parameter.
Envelopes
For Envelopes, specify the extents of the envelope (it is not required but recommended). If the Specify Extents Points check box is checked, it is required to enter the minimum and maximum points of the envelope and the point style the value represent.
Horizontal Datums
For Horizontal datums you must select the Ellipsoid the datum is based on, as well as the prime meridian used.
Linear Units
For Linear units, enter a conversion for the new unit in terms of the scientific standard Meters. The Abbreviation is used to identify the unit within the application interface (Example: The Abbreviation for Meters is "m")
Prime Meridians
For Prime Meridians you must enter the longitude value of the Prime Meridian and the angular unit that value is in.
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