The Projection panel of the Configure dialog (pictured below) is used to modify the projection and datum that the currently loaded data is displayed in.
Use the Projection pull-down menu to select the projection you wish to view the currently loaded data in. The Geographic projection is really the absence of a projection. It is just a linear mapping of latitude/longitude to a grid. For a good description of most of the other supported projection systems and a discussion of the tradeoffs in using various projections, we suggest visiting http://www.progonos.com/furuti/MapProj/Normal/TOC/cartTOC.html.
The Datum pull-down is used to select the datum to use in association with the projection. The most commonly used datums are WGS84, NAD27, and NAD83. If you find that the datum that you need is not available, you can use the Add Datum button to add your datum. When that button is pressed, the Create New Datum dialog (pictured below) is shown, allowing you to enter the parameters for either a 3-parameter Molodensky datum transformation or a 7-parameter Bursa-Wolfe datum transformation, or even specify a transformation defined by a text file with a list of control points. If you provide parameters for a Bursa-Wolfe transformation, note that the Position Vector format is used for rotation angles and not Coordinate Frame Rotation, so you will need to negate the rotation angles if you are provided the latter.
If you choose the Custom Shift Based on Control Point File option, the text file that you select should have a series of lines defining the mapping from a lat/lon coordinate in the new datum and the corresponding lat/lon value in WGS84. This is a very easy way to define a datum if you don't know any transformation parameters, but do know the WGS84 coordinates for at least 2 coordinates in the new datum. The format of each line would be as follows:
deg_longitude_in_new_datum,deg_latitude_in_new_datum,deg_longitude_in_WGS84,deg_latitude_in_WGS84
The Abbreviation field is what will be displayed for the datum name in the status bar. If your datum uses a prime meridian other than Greenwich, enter the longitude of the meridian used (relative to Greenwich) in the Prime Meridian entry box.
The datum information will be stored in the custom_datums.txt file in the Global Mapper Application Data folder (see the Help->About dialog for the location of this folder), so to share your datums just copy that file to that folder on another user's computer. If a needed ellipsoid is not built in to Global Mapper, use the Add Ellipsoid button to add a new one or Edit Ellipsoid to edit an existing custom ellipsoid definition.
The Planar Units pull-down select the ground units to use with the projection.
The Zone selection allows you to select the zone for use with the UTM, State Plane, Gauss Krueger, and other zoned projections.
For those projections that require additional parameters to fully define them, the Attributes table allows you to enter the values for each applicable parameter.
You can save the currently selected projection to a PRJ file using the Save to File button. Previously saved PRJ files can be loaded using the Load from File button. By default, the PRJ file will be saved in the newer WKT (well-known-text) format used by most products. There is an Advanced Option on the General tab of the Configuration dialog allowing the older ESRI PRJ format to be exported instead if you would like. If for some reason a WKT-representation of the projection cannot be generated, the old ESRI PRJ format will be written out instead.
The Init From EPSG button allows you to initialize a projection definition based on a numeric EPSG projected coordinate system definition. There are numerous built-in EPSG code definitions, but by no means is there an exhaustive set. You can add your own EPSG code definitions (or modify built-in ones) by modifying the epsg_codes.txt file installed in your Global Mapper installation folder. The format is described in the file header. You must re-start Global Mapper after changing this file for the changes to have any effect.