2011

False Origin
When defining a custom coordinate system, you may want to enter a false
origin, with northing and easting values. For example, if you are using the
Transverse Mercator projection, and the central meridian bisects the mapping
region, half of the X coordinates are negative values. Coordinate system
definitions usually include an offset called the false origin that is added to all
coordinates to make them positive. The X coordinate of this offset is called
the false origin easting. The Y coordinate of this offset is called the false origin
northing.
Scale Reduction Factor
To produce the smallest possible distance between the projection surface and
any point in the region you are mapping, you must specify a scale reduction
factor. This is particularly important when you are mapping large regions. For
the Transverse Mercator projection, you enter this value in the Scale Reduction
field in the Define Global Coordinate System dialog box.
See also:
■
Overview of Coordinate Systems (page 143)
■ Overview of Grid Data Files and Datum Shift Issues (page 103)
To define a coordinate system
1 Click Map Setup tab ➤ Coordinate System panel ➤ Define Global
Coordinate System.
2 In the Global Coordinate System Manager dialog box, select a category.
3 Do one of the following:
■ To create a new coordinate system, click Define.
■ To create a new coordinate system based on an existing, predefined
coordinate system, select the coordinate system from the Coordinate
Systems In Category list. Click Modify.
4 On the General tab of the Define Coordinate System dialog box, enter a
unique code in the Code box.
The code must be unique within a category and across all coordinate
systems and must not contain spaces. Only the following characters are
valid: A-Z (upper or lower case), 0-9, dash, and underscore.
92 | Chapter 2 Setting Up