xacro property names not recognised by roslaunch

edit

Operating System: Ubuntu 18.04 LTS

ROS version: Melodic

I've been trying to implement xacro into a very simple robot to help tidy up and future-proof it, however when I use roslaunch to load the file into rviz it gives me the following error:

Unable to parse component [${wb_w}] to a double (while parsing a vector value)

Ive tried launching the file with the following commands with the same results:

roslaunch omni_description display.launch model:='$(find omni_description)/urdf/omni.urdf.xacro'

roslaunch omni_description display.launch model:='$(find omni_description)/urdf/omni.urdf'

The code in question is shown below (Only the sections where xacro has been implemented are shown as i have only tried it on one link so far, let me know if you need the rest.)

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="omni">
<xacro:property name="wb_w" value="0.18"/>
<xacro:property name="wb_h" value="0.005"/>
<xacro:property name="wb_d" value="0.18"/>
<xacro:property name="wb_m" value="0.23"/>

<xacro:property name="w_r" value="0.18"/>
<xacro:property name="w_l" value="0.18"/>
<xacro:property name="w_m" value="0.60"/>

<xacro:property name="b_r" value="0.024"/>
<xacro:property name="b_m" value="0.60"/>

<xacro:macro name="wb_inertial" params="mass depth width height">
  <inertial>
    <mass value="${mass}"/>
    <inertia 
              ixx="${(1/12)*mass*(width+height)}" 
              ixy="0.0" 
              ixz="0.0" 
              iyy="${(1/12)*mass*(depth+height)}" 
              iyz="0.0" 
              izz="${(1/12)*mass*(width+depth)}" />
  </inertial>
</xacro:macro>

  <link name = "wheel_base">
    <visual>
      <geometry>
        <box size = "${wb_w} ${wb_d} ${wb_h}"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <box size = "${wb_w} ${wb_d} ${wb_h}"/>
      </geometry>
    </collision>
    <xacro:wb_inertial mass="${wb_m}" depth="${wb_d*wb_d}" width="${wb_w*wb_w}" height="${wb_h*wb_h}"/>
  </link>

EDIT 1:

The typos have been fixed and the same error has presented itself, when running via roslaunch the same error message is returned:

Unable to parse component [${wb_w}] to a double (while parsing a vector value)

EDIT 2

I have included the launch file below:

<launch>

  <arg name="gui" default="true" />

  <param name="robot_description" textfile="$(find omni_description)/urdf/omni.urdf" />
  <param name="use_gui" value="$(arg gui)"/>

  <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" />
  <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
  <node name="rviz" pkg="rviz" type="rviz" args="-d $(find omni_description)/urdf.rviz" required="true" />

</launch>

EDIT 3

I have checked the file using check_urdf and recieved a slightly more detailed error:

Error:   Unable to parse component [${wb_w}] to a double (while parsing a vector value)
         at line 157 in /build/urdfdom-ACOcA8/urdfdom-1.0.0/urdf_parser/src/link.cpp