While accumulating gear for a paddle trip I couldn't find my Garmin eTrex unit, and realized that my iPhone 6+ might make a reasonable substitute, while providing a color screen 3 or 4 times the size of the Garmin.

I found 'Map Plus' at Apple's app store. I believe the basic app is free, but there are in-app purchases that drove the price to around $18. This may have been the best money I've ever spent on tech. Rather than think about which of the various in-app options I wanted I just bought them all. I soon decided that the author had done such a good job that he deserved every penny.

First, to get an estimate of power consumption, I put the phone in both airplane mode and low power mode (simultaneously) and ran the app around home on my phone for ~24 hours. (Obviously, it wasn't working as a phone or internet device at the time.) The battery waned by roughly 1% per hour in this way, all the while recording my position every 5 minutes or so. Extrapolating, that should be good for a 4-day trip. For insurance, I bought a 6800mAH external battery from Jackery. This would let me recharge in the wild. More testing showed that the Jackery unit should be good for an extra 140% on the phone, beyond the original charge.

I then downloaded the BWCA & Quetico campsites and portages from paddleplanner. 

At that point I had everything I needed, and started playing with the base maps. This was the geekiest part of the exercise, and the part that took the longest to figure out. You have your choice of several base maps (USGS Topo, satellite photo, ThunderForest and several other variants). For a trip off-grid, you should pre-download the base map you like for the area of interest. You can specify that area as a simple rectangle or as a complex polygon. The latter can save lot of memory if you crank up the resolution. The choice of which resolutions to pull down took some head-scratching. They range from level 1 (very coarse) to 20 (extremely-detailed). I found that downloading levels 1 through 15 was plenty, and didn't consume too much memory on the phone. Choice of base map will be a personal preference. If you're serious about fishing, you'll find depth countours for some BWCA lakes on some base maps but not on most. Of course if you're using your phone this way you'll probably consume more battery because the screen will be on a lot.

In the end, I left the GPS (and my digital camera) at home and just brought the phone (and real maps of course). It worked beautifully. I started with 100%, and had 2% left when I returned to the put-in after 103 hours (4 nights), all without using the external battery. I'll admit I turned the phone off for a few hours on two nights. On the other hand, I discovered that there was cell reception on Cummings lake, and 'tuned in' to get internet weather forecasts a couple of times.

If I owned stock in Garmin I think I'd sell it right away. The only advantage I see in a dedicated GPS unit is that they're better waterproofed and ruggedized. I kept my phone in a ziplock in a zipped pocket of my life jacket most of the time, but still worried about cracking or drowning it.