The population element defines how and where a set of models will be automatically populated in Gazebo. A unique name for the population. This name must not match another population in the world. The number of models to place. Specifies the type of object distribution and its optional parameters. Define how the objects will be placed in the specified region. - random: Models placed at random. - uniform: Models approximately placed in a 2D grid pattern with control over the number of objects. - grid: Models evenly placed in a 2D grid pattern. The number of objects is not explicitly specified, it is based on the number of rows and columns of the grid. - linear-x: Models evently placed in a row along the global x-axis. - linear-y: Models evently placed in a row along the global y-axis. - linear-z: Models evently placed in a row along the global z-axis. Number of rows in the grid. Number of columns in the grid. Distance between elements of the grid.