Distance Measuring Apps for Smartphones

I'm finding it difficult to understand your third paragraph. How does the Measure app work? Is it LIDAR? If so, how can it work without a clear line of sight to the object your are measuring the distance to. Or is it measuring a travel distance, which is suggested by your "wandering all over the road?" And how could any of this help locate middle of nowhere mile markers without GPS?

How do they do it?  I do not know.  What I read is it is "augmented reality or AR." This has been around in some form going back to at least 1992.

Wikipedia says: "AR can be defined as a system that incorporates three basic features: a combination of real and virtual worlds, real-time interaction, and accurate 3D registration of virtual and real objects. 

What all is involved? Wikipedia again: "Modern mobile computing devices like smartphones and tablet computers contain these elements, which often include a camera and microelectromechanical systems (MEMS) sensors such as an accelerometer, GPS, and solid state compass, making them suitable AR platforms."

The developer might be using any hardware in that phone.  The U1 chip and LIDAR have only been on iPhone versions 11 and 12. As developers integrate these, we will see better apps. The Apple AirTag using LIDAR just went on sale in April.  It will resolve down to 0.1' but only from about (Bluetooth range) 20' away. 

For mile marks, I would like to get closer than +- 16' when I have to use coordinates.  Trail authorities are notoriously against ANY markings.  A set of pictures can't go on a course map.  But once you get within that +- 16', you can use Measure and get close enough to something identifiable without carrying a 100' steel tape. 

This is a screenshot of cal course runs.  It would be nice to have someone with a 6S or later try this. 

It is exciting to realize I no longer have to carry a 100' steel tape out on a trail course to provide an excellent description of a point.

So the purpose is to measure the distance from a permanent landmark to the mile mark?

Yes.

Oscar, for courses in which mile points occur at difficult-to-locate-on-the-ground places, I usually lay down a small strip of colored duct tape at the spot. This tape is robust enough to stay in place for many months. It leaves no mark when it is removed.

I have had good luck finding ground-truth established points on GE after measuring. It is then possible to use the Ruler function in GE to measure from the timing point to additional fixed objects.

Tried the "LiDAR Measuring" app again after realizing it is point-on-a-surface to point-in-space. It uses light and has to reflect off something.

First tried it to find height of desk and cat scratch post inside.  It was withing shaky hands range of exactly what steel tape said or about .10 inches.

Ran a tape down the hall on floor and fixed on chest high point on wall and got  "High Accuracy" readings out 25'.  They matched tape as nearly as you can eyeball from phone at chest level to tape on floor. 

Ran it up the street from a telephone pole. Still said "High Accuracy" at 150'.  You can move much faster than with Measure which seems to be point-in-space to point-in-space.

Then, ran a steel tape out from garage door with a 10 lb steel block holding tape down.  Fixed point chest high and walked backward.  Got eyeball chest to ground accuracy at 70'.  Put phone in pocket for a few seconds and readings were a couple of feet off when I took it back out. 

With some careful set up, marked a 30" board within about a kerf width of what the tape said.

Used correctly, you could "gain confidence" you had the right point on raceday. But neither Measure or LiDAR Measuring will replace a steel tape anytime soon. 

Last night using "LiDAR Measuring", chalked an 'x' on a utility pole and started backing away while aiming toward point on pole.  Chalked street at 100' on three trips.  Indicated resolution was .01' with "High Accuracy".  Points were in a 2' range.  "Measure", with a couple of "Slow Down" notices came out in middle of those.  There may be some techniques that will do better.