Overview

VR-Forces uses coordinates for various purposes throughout the API, and they can be in various different coordinate systems. This section lays out the naming conventions used in the API for coordinates and how they relate to each other.

The main terms used are:

• World (Geocentric)
• Local (Database)
• Relative (Parent)
• Topographic (Topo, ENU)

These terms are used in various functions and parameter names. For example:

• setLocalPosition()
• worldPosition()
• worldOrientation()
• setRelativeVelocity()
• terrainHeightAboveSeaLevel(const DtPoint& databaseLocation)

World Coordinates (ECEF)

World coordinates use an earth-centered-earth-fixed (ECEF) cartesian system. This system, also called geocentric coordinates, has its origin at Earth's center with axes:

• X-axis: Through the prime meridian (Greenwich) at the equator
• Y-axis: 90 degrees east at the equator
• Z-axis: Through the North Pole

This is the coordinate system used by DIS and RPR FOM network standards. API functions may refer to this system as either "geocentric" or "DIS" coordinates.

Local (Database) Coordinates

Local coordinates match the terrain database's native coordinate system. This system varies depending on the loaded terrain:

• May be geocentric (matching world coordinates)
• May be UTM or another supported coordinate system
• Always uses cartesian coordinates

VR-Forces primarily stores and processes coordinates in this system for efficiency, avoiding frequent coordinate conversions when accessing terrain data. Some API components refer to these as "database" coordinates.

Relative (Parent) Coordinates

Relative coordinates are centered on a specific simulation object, useful for embarked entities or describing points on an object. The axes are:

• X-axis: Forward (+) / Backward (-)
• Y-axis: Right (+) / Left (-)
• Z-axis: Down (+) / Up (-)

This system is essential for:

• Positioning embarked entities
• Locating articulated parts
• Defining attachment points
• Specifying sensor locations

Geodetic Coordinates

Geodetic coordinates use latitude, longitude, and altitude. While VR-Forces can convert to and from this system, it:

• Is used primarily for input/output
• Is not used for internal calculations
• Does not serve as primary storage format

Topographic (ENU) Coordinates

Topographic coordinates (also called "topo" or ENU) provide a localized flat-earth approximation at a specific location with axes:

• X-axis: East
• Y-axis: North
• Z-axis: Up

Key characteristics:

• Commonly used for computations within simulation components
• Origin at specified surface location
• Accuracy decreases with distance from origin
• Commonly called ENU (East-North-Up) coordinates