Google: One of The Secrets of Google Maps

View an area of the map even when you aren't connected to the Internet. For example, before you go on a camping trip, download a map of the area so you can use the map offline during the trip.

1. Make sure your phone or tablet is connected to the Internet, then open the  Google Maps app.
2. Zoom in to an area on the map.
3. Search for "ok maps." To search by voice, touch the  microphone in the search box then say "ok maps."
4. After a few moments, you'll see a message confirming that the on-screen map area has been cached or downloaded.
5. Next time you aren't connected to the Internet, you can go to this area of the map to view it offline.

Troubleshooting
The on-screen map area is too large: If you see this error, zoom in to a smaller area on the map and repeat the steps.


Reference:
View maps offline
https://support.google.com/gmm/answer/3273567?hl=en