nav_grid_server

nav_grid_server is a drop-in replacement for map_server but with added flexibility. As such, it is a convenient tool for publishing maps (nav_grids) from image files. It can also save image files from ROS topics.

Map Server

Classic Usage

rosrun nav_grid_server server path/to/map.yaml

This reads map meta-data from the yaml file and exposes the map in three different ways.

• Publishes a nav_msgs/OccupancyGrid message on the /map topic (along with nav_msgs/MapMetaData on the /map_metadata topic).
• Provides the nav_msgs/GetMap service with the name /static_map
• Publishes a nav_2d_msgs/NavGridOfChars message on the /static_map topic (which natively contains the metadata)

Direct Image Usage

rosrun nav_grid_server server path/to/map.png

You can also just provide the image file as a command line argument and use the default metadata.

Map Saver

rosrun nav_grid_server saver

This retrieves a nav_msgs/OccupancyGrid message from the /map topic and saves as map.png and map.yaml.

Images

Image data is managed using opencv2 and can use many formats, including (but not limited to)

• bmp
• jpg
• png
• webp
• pgm

Metadata

Classically, the map metadata has been contained in a yaml file stored alongside the image file, e.g.

image: testmap.png
resolution: 0.1
origin: [0.0, 0.0, 0.0]
occupied_thresh: 0.65
free_thresh: 0.196
negate: 0

In addition, nav_grid_server provides the ability to also specify this information as ROS parameters, or not at all. The order of precedence is

• ROS Parameters (highest precedence)
• YAML File
• Default values (lowest precedence)

See the nav_grid documentation for further definitions of the metadata.

Server Params

The following parameters are able to be specified in either the yaml or ROS parameter server (with defaults in parentheses).

• resolution (0.05) - Resolution of the map, meters / pixel
• negate (false) - Whether the image intensity should be negated
• occupied_thresh (0.65) - Pixels with intensity greater than this threshold (scaled 0-1) are considered occupied.
• free_thresh (0.196) - Pixels with intensity less than this threshold (scaled 0-1) are considered free.
• mode ("trinary") - Default cost interpretation. See the documentation here.
• origin_x/origin_y (0.0 / 0.0) Origin offset in meters. The yaw is assumed to be zero. Note:: For backwards compatibility, the origin in the yaml file is specified with the name origin as an array of doubles, the first to values of which are used for origin_x and origin_y.

If using a yaml file, image must also be specified as the path (absolute or relative) to the image file.

Additionally, you can set the following as ROS Parameters:

• frame_id ("map") - TF frame of the map
• occupancy_grid_topic ("map") - Topic on which the nav_msgs/OccupancyGrid is published.
• nav_grid_topic ("static_map") - Topic on which the nav_2d_msgs/NavGridOfChars is published.

Saver Params

• topic ("map")- Topic to subscribe to
• nav_grid (false) - If true, the topic is expected to be of type nav_2d_msgs/NavGridOfChars. Otherwise, nav_msgs/OccupancyGrid
• once (true) - If true, only save the first map. Otherwise, save the map repeatedly (useful for mapping).
• trinary_output (true) - If true, uses the trinary cost interpretation. Otherwise, the values are scaled [0, 100]
• Filename Parameters
  • map_extension ("png")
  • map_prefix ("map")
  • output_directory (“.”)
  • write_unstamped (true) If true, will write to output_directory/map_prefix.map_extension
  • write_stamped (false) If true, will write to output_directory/map_prefix-timestamp.map_extension