.. _coordinate_systems:

Coordinate Systems
==================

Below are the most common coordinate systems used. In addition to those each sensor usually has its own coordinate system too.

Coordinate positions are always measured in SI units, i.e. meters for distances and rads for angles.
Velocities are measured in m/s and rad/s.

Base Link (base_link)
---------------------

The platform's own coordinate system, usually the center of mass. X points forward, Y points to the left and Z points upwards.
The internal name is ``base_link``.

Odometry (odom)
---------------

This is an arbitrary coordinate system with its origin at the starting position (at the time of power on) of the platform.
X and Y form the plane of movement, while Z is pointing upwards and aligned with gravity. Usually there is no motion in Z direction,
except when a roll and pitch sensor is installed.
The internal name is ``odom``.

Map (map)
---------

This is the coordinate system of the currently used :ref:`GridMap` and :ref:`RoadMap`.
Usually the :ref:`GridMap` is created first and its origin is the starting position of where the map was created initially.
X and Y form the plane of movement, while Z is pointing upwards and aligned with gravity.
The internal name is ``map``.