RelativeToGround

RelativeToGround is a term used to describe a coordinate frame, pose, or measurement that is defined with respect to the ground plane rather than a global or distant reference frame. In practice, this means that the origin, axes, or values are anchored to the local ground, with the vertical direction typically aligned with gravity.

In surveying and geodesy, relativeToGround often refers to measurements taken with respect to the local ground surface, such as height or position above the terrain at a specific location. In robotics and autonomous systems, a ground-based reference frame can be a local tangent plane or a ground plane extracted from a map, used to express a robot’s position and orientation relative to the surrounding terrain. In computer graphics and simulation, objects may be positioned and oriented relative to a ground plane to ensure stable contact with uneven terrain or to simplify collision and physics calculations.

Implementation commonly involves a transformation between the ground frame and other frames (world, map, or body frames). The ground frame may be defined by a detected or modeled ground plane, terrain height data, or a gravity-aligned reference, with the ground normal serving as the up direction. When the ground plane moves or tilts (for example on sloped terrain), the relativeToGround reference must be updated to maintain accurate positioning.

Common considerations include handling uneven terrain, slopes, and dynamic changes in the ground model, as well as distinguishing relativeToGround from other references like relativeToWorld or relativeToParent.