Custom Datum Definition
Click the New button in the Assign Coordinate System dialog to open the Define Coordinate System dialog. Use the Datum group to define a custom datum and specify the exact datum parameters and values.
Datum
The Datum list contains predefined datums. Select a datum to populate the Parameter and Value columns. Customize the parameters and values as needed.
Conversion Method
The Conversion Method controls the method of datum conversion and the conversion parameters. The conversion methods include Molodensky, Bursa-Wolfe, and None.
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The Molodensky method is the most widely used method of datum conversion. It adjusts latitude and longitude coordinates by taking into account the displacement between two datum's ellipsoids on all three axes. It does not take into account any rotational differences between the two ellipsoids.
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The Bursa-Wolfe method is similar to the Molodensky method, but in some instances it produces more accurate results because it takes into account both displacement and rotational differences between two ellipsoids.
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Choose None if the predefined methods do not suite your purpose. If you select <custom> from the Ellipsoid Name list, you can specify your own ellipsoid model parameters.
Conversion Parameter and Values
You can edit the conversion parameter and values directly by selecting the value you want to edit and typing a new value over it. Parameters vary depending on the selected Conversion Method.
Ellipsoid
The Ellipsoid group contains options for defining the ellipsoid. Use these settings to define the ellipsoid model that best approximates the curvature of the Earth's shape in the map region. If you do not understand ellipsoids and datum definitions, it is recommended you use the defaults.
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The Name list contains a collection of ellipsoid models that cartographers have historically used in attempts to produce more accurate maps of different regions of the Earth. The Name list also contains the option to create a <custom> ellipsoid.
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The Semimajor Axis box can be edited to define the major axis.
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The Semiminor Axis box can be edited to define the minor axis.
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The Inverse Flattening (1/f) can be edited to define the flattening ratio.
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The X Displacement, Y Displacement, and Z Displacement are the axis displacements in meters.
Prime Meridian
The Prime Meridian section contains options for defining the prime meridian Name and Prime Meridian Shift. A meridian is a line of constant longitude running north-south on a map. The zero meridian or prime meridian is used as a reference line from which longitude east and west is measured. The prime meridian passes through Greenwich, England. The Prime Meridian Shift is the shift from the prime meridian, typically 0 degrees, in decimal degrees.
See Also
Introduction to Map Projections
New Projected Coordinates - Worksheet