The other option would be to set the phone to not go to sleep when using maps/navigation or with an external power source. Since my phone is waterproof I charge via a wireless puck it sits on so I've thought about making a small case that holds it next to the charging puck which is fed off a USB cable hanging from my backpack strap. This way I can simply set it to stay awake while externally powered and not have to turn it on.