It's recommended to read the VTP website regarding Coordinate Systems and Map Projections.
Technically speaking, what VTBuilder refers to as a "projection" is actually a "coordinate reference system", or CRS for short. However, since a great may CRS are projected, it is a common colloquial usage to refer to a CRS as a projection.
This dialog is displayed on the menu items View: Set or Convert Projection, and also when VTBuilder needs to ask you for a CRS, such as when importing a file which lacks a CRS.
There are several fields which define a CRS, which are displayed in the dialog:
- Projection
- This can be either Geographic (longitude-latitude, not projected) or any of several styles of projection, such as Transverse Mercator or Lambert Conformal Conic.
- Datum
- You can choose from a short list of commonly-used Datums, or click "Show All Datums" to select from a long list of more unusual Datums.
- Horizontal Units
- This is always Degrees for a Geographic CRS. For most projections, it can be Meters, Feet, or US Survey Feet. For the Dymaxion Projection, this value is always Unit Edges (1.0 = length of a projected face edge)
- Zone
- Certain projections, such as UTM, can be specified quickly with a Zone number.
- Parameters
- If your projection is unusual (not common, like UTM), then you may have to manually enter the parameters. To edit them, select any value then right-click it to supply a new value.
There are also some commands to make specifying projections easier:
- Set State Plane Projection
- If you know that your CRS is a U.S. State Plane CS, then you can use this command to specify it, and the parameters will be automatically supplied.
- Load / Save to File
- It is often convenient to define a projection once, then recall it easily later. These commands will save and load the current projection to a standard projection file (.prj) which contains a Well-Known-Text representation of the CRS.