There are a couple of conventions to be aware of when using Terragen. These conventions may be familiar from other 3D applications, though they can vary from application to application.
In the XYZ coordinate system, the Y axis is the vertical axis (i.e. height).
Default Unit of Measurement
The default unit of measurement is meters. This is used for all size and distance values.
Displacement as the Primary Method for Shaping Terrain
In Terragen, terrain is created through “displacement” of a surface. By default, all terrain is acting on a planet-sized sphere and displacing the surface of that sphere to create landscape features ranging from mountain ranges to pebbles. At render time, the surface is subdivided into micropolygons according to where the camera is placed. The micropolygons are displaced to break up larger shapes and give them fine texture and detail.
This may differ from other 3D programs that handle terrain as geometry or as a standalone heightfield object.
Literally, to change the position of something. In graphics terminology to displace a surface is to modify its geometric (3D) structure using reference data of some kind. For example, a grayscale image might be taken as input, with black areas indicating no displacement of the surface, and white indicating maximum displacement. In Terragen 2 displacement is used to create all terrain by taking heightfield or procedural data as input and using it to displace the normally flat sphere of the planet.
A heightmap or heightfield is an array of height values, usually in a grid which describe the height at specific points in a defined area. Heightfields are used to represent real-world and virtual terrain in a specific, easily converted format. Most heightfields can be represented as simple image data in grayscale, with black being minimum height and white being maximum height.