A few years ago I tried using an app called "Save Location" to map out tree locations at my orchard. Even with trees at 15 or 30 foot apart, the resolution of standard cell phone GPS wasn't good enough to make a worthwhile map.
RTK GPS, such as used by John Deere's systems, would have enough resolution, but it's big $$$.
My next thought was to just get a drone to create an ultra high map, but instead I just laid out a grid system and that has worked well. Permaculture designs seem to avoid setting things out on straight lines, but by golly are they useful in keeping track of things.
Although the locations aren't very accurate, Google Maps is a rather good way of saving things, as it lets you save photos and text along with each tree. Here's mine, but it hasn't been updated in a while.
https://www.google.com/maps/d/edit?mid=1C0E1iqIPWtbITbd3SAROkyzuLL_1G-4&usp=sharing