Larry, it's a fair question. First, if cell service is required for iPhone map apps to work, that would explain a lot. For instance, my wife will use a Garmin Colorado GPS and I will fire up Avenza, a map app for which a local hiking group has painstakingly created custom maps of the main trail system for southern Illinois. After a day of riding, we compare our tracks. Mine will consist of several straight lines where we absolutely did NOT travel in straight lines! Hers will accurately show our traveled trail. Mine will underreport miles traveled. My conclusion is the app was losing connection and therefore averaging, creating a straight line between two points where it did have reception. The same thing happens when I use AllTrails, another commonly used app. The averaging appears not to be as prevalent when hiking as when horseback riding as most hikers in this area use Avenza. Avenza, when checked against a GPS unit, is usually within 50-100 feet, but no more accurate. That's been my experience. For those reasons, I know of no horse riders that successfully use phone apps. Hikers, yes.
A large group that uses GPS on horseback in the Forest are the squirrel hunters that bring along dogs. They seem to love the Garmin Alpha 100 because it is not only a GPS, but a dog tracker, when used with corresponding collars for the dogs. It has an external antenna and a touchscreen and has proven to have a good track record of keeping reception in the Forest. That will be my default device, I guess. But, I hoped to find something with a larger screen.
There's even a group of endurance riders that use Garmin Fenix watches! They provide just a bread crumb trail, but will get you out of the Forest if you've become disoriented. At the speeds they travel, the possibility for that exists!
Brendan, to be honest, I've not heard of external GPS receivers for my phone! I will investigate that! Thanks for the tip.
It has been awhile since I tried the Gaia app. If I remember correctly, it might have been a map issue that turned me away. Not all the apps have detailed topo maps available for the Shawnee Forest in southern Illinois. And, as I mentioned, the local folks have created very detailed custom maps of local trails for the Avenza app. Back to more research! Thank you all for your thoughts.