Off-road terrain presents many challenges to real-time simulation. There is a large amount of information necessary to describe high resolution terrain surfaces with sufficient detail to support high-fidelity vehicle dynamics over large geographic regions.

NADS off-road terrain is divided into two parts: Logical Database Creation (offline) and Real-time Database Interrogation.

Logical Database Creation

In order to provide vehicle dynamics with elevation data, a terrain database stores elevation and surface material for an X Y axis constrained grid. Because terrain surfaces are frequently highly variable, it is most efficient to define elevation posts using variable sized datazone regions. This implementation permits the elevation grid resolution to be more detailed where required, and takes advantage of advanced memory management techniques.

The terrain database is created by extracting triangles from the Out-The-Window visual database; therefore there is a correlation between the level of fidelity between the visual database and the virtual terrain database.

The Datazone Layout Tool provides an easy means to define datazones and resolution. The entire database is covered with datazones, and then passed to a scanner which analyzes the datazone layout. The scanner makes a recommendation how to subdivide the terrain database to the user via the Datazone Layout Tool. The scanner also outputs a file for each datazone defined by the user. Finally the datazone layout and terrain surfaces are passed to the terrain compiler, which ensures that all terrain is properly covered with no gaps or holes, even between adjacent datazones with different resolutions. The compiler also generates datazone adjacency lists. Datazone adjacency lists are simple data structures that facilitate finding datazones that share edges. For each datazone there are four lists maintained, one each for zones that share top, bottom, left and right edges.

Real-time Database Interrogation

The components of Real-time Database Interrogation consist of the logical terrain database and software. It is anticipated that most off-road terrain will be too large to load entirely into memory. Efficient real-time operation is made possible by using a paging mechanism, known as the Look-ahead loader, which reads datazones from disk and loads them into shared memory. This method permits reading terrain elevation directly from memory, which is more efficient than reading continually from disk.

In addition, there is a terrain interrogation function which is responsible for obtaining terrain elevation. If the vehicle query points are not on a X Y grid intersection it becomes necessary to interpolate the height of the terrain using the nearest known elevation points. The terrain interrogation function also must calculate the inclination of the terrain (surface normal) at the vehicle position. This is accomplished using the terrain elevation points nearest to the vehicle position.

Terrain interrogation supports overlapping surfaces, such as bridges, overpasses or parking ramps.

Selected references:
Generation of Real-Time Synthetic Environments Using a Mobile Sensor Platform, Papelis Y., Bossard B., HCTSC conference, 2001

Automatic correlated database generation and management for ground vehicle simulators, Papelis Y., Allen S., Wehrle B., AIAA Conference, 1999

Terrain modeling issues for high fidelity ground vehicle simulators, Papelis Y., IEEE conference, 1994