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A partially working solution for my particular case is to put default parameters in a yaml file, and include this file from all launch files. I have no more launch file hierarchy: they all launch the OCU directly. However, each launch file overrides the appropriate parameters and adds new ones. I get such a structure:

<!-- This launch file is used for the UAV -->

<launch>

<node name="$(anon ocu)" pkg="ocu" type="OCU" output="screen">

    <!-- Loads the OCU default parameters -->

    <rosparam command="load" file="$(find ocu)/launchers/ocu.yaml"/>

    <!-- Redefines and adds parameters -->

    <param name="showUAVCams" value="true"/>
    <param name="show3DMap" type="bool" value="true"/>
    <param name="robotType" type="string" value="UAV"/>

    <!-- These ones are necessary until they are placed in the yaml file -->
    <param name="ugv3DModelDescription" textfile="$(find ocu)/media/robot_model/NIFTi.urdf" />
    <param name="folderNameForSnapshots" value="$(find ocu)/snapshots" />

</node>

</launch>

As you can see, it does not yet completely work because my parameters with commands cannot be placed in the yaml file directly. I need to investigate more on that.

A partially working solution for my particular case is to put default parameters in a yaml file, and include this file from all launch files. I have no more launch file hierarchy: they all launch the OCU directly. However, each launch file overrides the appropriate parameters and adds new ones. I get such a structure:

<!-- This launch file is used for the UAV -->

<launch>

<node name="$(anon ocu)" pkg="ocu" type="OCU" output="screen">

    <!-- Loads the OCU default parameters -->

    <rosparam command="load" file="$(find ocu)/launchers/ocu.yaml"/>

    <!-- Redefines and adds parameters -->

    <param name="showUAVCams" value="true"/>
    <param name="show3DMap" type="bool" value="true"/>
    <param name="robotType" type="string" value="UAV"/>

    <!-- These ones are necessary until they are placed in the yaml file -->
    <param name="ugv3DModelDescription" textfile="$(find ocu)/media/robot_model/NIFTi.urdf" />
    <param name="folderNameForSnapshots" value="$(find ocu)/snapshots" />

</node>

</launch>

As you can see, it does not yet completely work because my parameters with commands cannot be placed directly interpreted in the yaml file.

Solution #2

Another partially working solution is to create a homologous argument for each parameter.

There is first a base launch file:

<!-- ocu_base.launch -->
<launch>

    <!-- Here all arguments must be declared and their default values given -->
    <arg name="_imageTransportMode" default="theora"/>
    <arg name="_showArmControlPanel" default="false"/>
    <arg name="_robotType" default="UGV"/>
    <arg name="_ugv3DModelDescription" default="" />
    ...

    <node name="$(anon ocu)" pkg="ocu" type="OCU" output="screen">

        <!-- Here I map the parameters to take the values of the arguments -->
        <param name="imageTransportMode" type="string" value="$(arg _imageTransportMode)"/>
        <param name="showArmControlPanel" type="bool" value="$(arg _showArmControlPanel)"/>
        <param name="robotType" type="string" value="$(arg _robotType)"/>   
        <param name="ugv3DModelDescription" textfile="$(arg _ugv3DModelDescription)" />
        ...

    </node>

</launch>

And several robot-specific launch files:

<!-- ocu_uav.launch -->
<launch>

<!-- Launches the OCU with default parameters -->
<include file="$(find ocu)/launchers/ocu_base.launch">

        <arg name="_imageTransportMode" value="raw"/> 
        <arg name="_robotType" value="UAV"/>    

</include>

</launch>

The problem that I still have is that not all parameters are necessary in all robot-specific files. For example, in the case of the UAV, we do not have a URDF robot model. With standard parameters, I would simply not include the parameter ugv3DModelDescription at all. With arguments, I MUST give a value. I tried giving a blank string as the default argument, but then the parameter ugv3DModelDescription tries to load the text file directly. I need to investigate more on that."blank" and roslaunch fails.