I agree with not using GPS on trail. I had one never took it out of my pocket on trail and just got annoyed with the loss of pocket space , lost it never cared to replace it.

However what I was always tought, growing up and in military land-nav courses is to use a pencil so you can erase and then to make lines for grids if you need or coordinates I guess. Use a yardstick/ ruler and your pencil as well. We marked the crap outsome maps before just erase after trip and they look pretty ok