This is a recap of distance measuring methods on smartphones moved over from GPS Accuracy thread.
There is some confusion about what a smartphone can do to measure longer distances. That is changing and improving rapidly. Enthused with Solocator, I got an i12 Pro Max. Max accuracy is 9' to 13' instead of 16'. It is much faster at getting to max accuracy than a plain i6. And it does have the newly added LiDAR capability which may do great things in the future. Right now, that seems to be good for improving photos and measuring interior dimensions.
AirTags were just released on April 30, 2021. This uses the U1 chip in 11's and 12's. They may be able to do precise long range measurements later. Right now you can keep track of your keys with them. The direct range I saw was about Bluetooth or 5 meters. It resolves to 1 foot or .1 feet depending on how close you are.
Right now: Apple's in-house app "Measure" does 100'+ distances with useful accuracy. I tried it on a steel taped 200' and got 200' 2". Then taped accurately under a vehicle so straight line of sight is not necessary. Then did a 1,000' cal course wandering all over a 4 lane road and got 997' xx" and, moving slowly in a straight line, 998' 5". I showed it to a friend. He already had it on an iPhone 6S and it did fine on 10'. Just now ran a tape 15' up flight of stairs and got 14' 11" so it does point to point and not "earth's surface". It does record readings and take pictures of measurements. It will be fine for homing in on "middle of nowhere" mile markers for better than +-16' GPS.