.. Copyright 2017 The Cartographer Authors

.. Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

..      http://www.apache.org/licenses/LICENSE-2.0

.. Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

===========
Terminology
===========

This documents a few common patterns that exist in the Cartographer codebase.

Frames
======

global (map) frame
  This is the frame in which global SLAM results are expressed. It is the fixed
  map frame including all loop closure and optimization results. The transform
  between this frame and any other frame can jump when new optimization results
  are available. Its z-axis points upwards, i.e. the gravitational acceleration
  vector points in the -z direction, i.e. the gravitational component measured
  by an accelerometer is in the +z direction.

local (map) frame
  This is the frame in which local SLAM results are expressed. It is the fixed
  map frame excluding loop closures and the pose graph optimization. For a given
  point in time, the transform between this and the global map frame may change,
  but the transform between this and all other frames does not change.

tracking frame
  The frame in which sensor data is expressed.



Transforms
==========

local_pose
  Transforms data from the tracking frame to the local frame.

global_pose
  Transforms data from the tracking frame to the global frame.