This app shows what altitude you are at (in meters). For most official maps it shows how much higher you are than the lowest point on that map. It may or may not work on non-official maps depending on how the map creator designed it. If they made a conscious effort to make sure that the low point on the map was at a z value of 0, this app should work the same way. Otherwise you can use it as a relative altimeter.
Many thanks to @Neo who helped with hooking up the altitude stream to the display
(altimeter units shown in video are feet, not currently available on the repo)