This mod allows you to toggle the altimeter to show the radar altitude. This tells you the height above the terrain, sea or building if you're above one.
By default the backquote key (` ) will toggle it but you can change this by opening the settings window by pressing '0' - this key can also be changed in the settings
When enabled the altimeter will change to blue
note: this altitude is from the control part (ie command pod or probe core), not the very bottom of the craft