The Map Server provides maps to the rest of the Navigation2 system using both topic and service interfaces.

While the nav2 map server provides the same general function as the nav1 map server, the new code has some changes to accomodate ROS2 as well as some architectural improvements.

In addition, there is now two new "load_map" and "save_map" services which can be used to dynamically load and save a map.

In contrast to the ROS1 navigation map server, the nav2 map server will support a variety of map types, and thus some aspects of the original code have been refactored to support this new extensible framework.

Currently map server divides into tree parts:

• map_server
• map_saver
• map_io library

map_server is responsible for loading the map from a file through command-line interface or by using serice requests.

map_saver saves the map into a file. Like map_server, it has an ability to save the map from command-line or by calling a service.

map_io - is a map input-output library. The library is designed to be an object-independent in order to allow easily save/load map from external code just by calling necessary function. This library is also used by map_loader and map_saver to work. Currently it contains OccupancyGrid saving/loading functions moved from the rest part of map server code. It is designed to be replaceble for a new IO library (e.g. for library with new map encoding method or any other library supporting costmaps, multifloor maps, etc...).

The Map Server is a composable ROS2 node. By default, there is a map_server executable that instances one of these nodes, but it is possible to compose multiple map server nodes into a single process, if desired.

The command line for the map server executable is slightly different that it was with ROS1. With ROS1, one invoked the map server and passing the map YAML filename, like this:

$ map_server map.yaml

Where the YAML file specified contained the various map metadata, such as:

image: testmap.png
resolution: 0.1
origin: [2.0, 3.0, 1.0]
negate: 0
occupied_thresh: 0.65
free_thresh: 0.196

The Navigation2 software retains the map YAML file format from Nav1, but uses the ROS2 parameter mechanism to get the name of the YAML file to use. This effectively introduces a level of indirection to get the map yaml filename. For example, for a node named 'map_server', the parameter file would look like this:

# map_server_params.yaml
map_server:
    ros__parameters:
        yaml_filename: "map.yaml"

One can invoke the map service executable directly, passing the params file on the command line, like this:

$ map_server __params:=map_server_params.yaml

There is also possibility of having multiple map server nodes in a single process, where the parameters file would separate the parameters by node name, like this:

# combined_params.yaml
map_server1:
    ros__parameters:
        yaml_filename: "some_map.yaml"

map_server2:
    ros__parameters:
        yaml_filename: "another_map.yaml"

Then, one would invoke this process with the params file that contains the parameters for both nodes:

$ process_with_multiple_map_servers __params:=combined_params.yaml

Like in ROS1 map_saver could be used as CLI-executable. It was renamed to map_saver_cli and could be invoked by following command:

$ ros2 run nav2_map_server map_saver_cli [arguments] [--ros-args ROS remapping args]

• Occupancy grid (nav_msgs/msg/OccupancyGrid)

MapIO library contains following API functions declared in map_io.hpp to work with OccupancyGrid maps:

• loadMapYaml(): Load and parse the given YAML file
• loadMapFromFile(): Load the image from map file and generate an OccupancyGrid
• loadMapFromYaml(): Load the map YAML, image from map file and generate an OccupancyGrid
• saveMapToFile(): Write OccupancyGrid map to file

As in ROS navigation, the map_server node provides a "map" service to get the map. See the nav_msgs/srv/GetMap.srv file for details.

NEW in ROS2 Eloquent, map_server also now provides a "load_map" service and map_saver - a "save_map" service. See nav2_msgs/srv/LoadMap.srv and nav2_msgs/srv/SaveMap.srv for details.

For using these services map_server/map_saver should be launched as a continuously running nav2::LifecycleNode node. In addition to the CLI, Map Saver has a functionality of server handling incoming services. To run Map Saver in a server mode nav2_map_server/launch/map_saver_server.launch.py launch-file could be used.

Service usage examples:

$ ros2 service call /map_server/load_map nav2_msgs/srv/LoadMap "{map_url: /ros/maps/map.yaml}"
$ ros2 service call /map_saver/save_map nav2_msgs/srv/SaveMap "{map_topic: map, map_url: my_map, image_format: pgm, map_mode: trinary, free_thresh: 0.25, occupied_thresh: 0.65}"