You'll need to do a few steps
from a PC

• Intro

  You can download maps from any map server and with any app!
  We'll use jTileDownloader. You don't need to deal with depedences with this snap version ;)
  Or you could use downloadosmtiles too in a Terminal.

• Install java

  jTileDownloader is a java aplication

  ~$sudo apt-get install openjdk-8-jdk unzip

• Download jTileDownloader

  Check here you're downloading the last version

  ~$cd ~

  ~$wget http://svn.openstreetmap.org/applications/utils/downloading/JTileDownloader/trunk/release/jTileDownloader-0-6-1.zip

  ~$unzip jTileDownloader-0-6-1.zip

  ~$rm jTileDownloader-0-6-1.zip

  ~$cd jTileDownloader

  ~$chmod +x jTileDownloader-0-6-1.jar

• Run jTileDownloader

  Use your file browser to open ~/jTileDownloader/jTileDownloader-0-6-1.jar with java:

  

• Download the map

  Slippy Map chooser > Choose the area to download.
  Please, avoid to select so big areas like a complete country!

  Output Zoom Levels: You can download different maps in several times.
  We would recommend you a zoom 16,17 for cities and a zoom 12,13 for not cities. You can download them in several times.

  In Mapnik choose your favourite map or set a custom tile map in Alt. TileServer

  Outputfolder: Download folder, recommended maps.

  And just click on Download Tiles button.

• Move maps folder into phone

  Move that maps from PC to ~/Downloads into the phone.

  Then move it to the final folder with the phone Terminal:

  ~$mv ~/Downloads/maps ~/.local/share/navigator.costales

  Then, you will have a three like this:

  ~$~/.local/share/navigator.costales/maps

  ~$~/.local/share/navigator.costales/maps/10/

  ~$~/.local/share/navigator.costales/maps/10/523/

  ~$~/.local/share/navigator.costales/maps/10/523/331.png

• Set offline maps

  Open uNav and go to Settings. Choose Offline maps:

  

• It's done |o/ Enjoy it!

  Remember, this will allow offline maps, but not offline routing.

  Tip: If you don't see the map, you didn't download that zoom. FYI, when you click on map or search a place, the zoom is 17 and you should see that downloaded map.