frame_conventions
Gazebo's simulation is always performed in Cartesian coordinates (good old XYZ). You can visualize the Gazebo origin by selecting View->Origin in the GUI menu.
Additionally, Gazebo supports the use of real world latitude and longitude coordinates in its simulations using the WGS84 geodetic system. The world's Cartesian coordinates correspond to a local tangent plane at a given point on the planetary surface. By default, this plane follows the ENU (East-North-Up) convention, as shown on the image below:
Gazebo follows the right-hand rule. The robot front facing surface is in the positive x direction, left facing being in the positive y direction, top facing being in the positive z direction and so forth.
The next diagram shows the transformation of the model when applying positive roll, pitch and yaw respectively.
