This answer is based strictly upon my personal experience.
I am not trying to sell you anything.
There are consumer grade GPS receivers which will get you approximately 3 meters of repeatable accuracy if they are used properly, and there is little or no overhead canopy.
I have had amazing results using a Garmin GLO which receives single frequency GPS, and Glonass data, along with WAAS corrections. ( About $110.00 )
It connected via bluetooth, and worked with multiple GIS, and mapping software applications, as well as various versions of the Windows operating system.
I have also used it with several models of semi-rugged, and rugged tablets without issue.
I have read that it is compatible with Android, Linux, IOS, and just about everything else out there. I only have Windows experience with it.
There were the occasional flyers that were in the five meter range, but in most cases, I was locating previously surveyed locations (Data collected with Survey Grade, Dual Frequency, RTK corrected, GPS/Glonass receivers) within a sub two meter radius.
One of the things I have found is, the units tend to work better when you are moving slightly. Just standing still, the position tends to walk around creating about a four meter circle.
The Garmin I was using had a 10 HZ update rate which created a very smooth line if I was mapping access routes, or roads.
The mapping data I collected, overlaid extremely well on previously located features.
There are other consumer grade external receivers that are probably capable of collecting data with this type of accuracy, I am just not experienced with them.
I have yet to see an embedded, discreet GPS chip within a mobile phone that approaches the results I have gotten with the Garmin.
Selective Availability has been turned off for a long time now, but it can be turned on at the whim of the Department of Defense.
It would really serve no purpose to do so at this point in time, or in any foreseeable scenario with regards to national security. Everyone knows where everything is. (Sorry, bad generalization.)
With regards to locating drainage tiles, I have done a great deal of this using a tool called a Magnetomatic. It is basically a telescopic car antenna attached to a swivel that is mounted through a handle. When you cross something that is buried, the antenna will swing in the direction of the object or line that is buried.
It works on plastic, poly, tile, transite, and metal lines, as well as fiber optic, and twisted pair copper lines. At least it does for me, and most of the people I work with.
Some people are unable to get it to work for them. Most of them are left handed, and the others are people who are unable to wear a movement style watch. (I believe these people have reversed polarity, but I cannot prove it.)
The process is similar to witching (divining) for water.
You can also bend two pinflags at a 90 degree angle to make short handles for each, put one in each hand, with the long ends parallel to each other, and parallel to the ground, about waist high, keeping them a shoulder's width apart. They will cross when you encountered something buried.
If you are using the GPS to measure a distance from a known location to find other unknown locations, say starting at a pump in the corner of a field, wanting to find the grid of drain pipes every 100 feet, the Garmin will get you close, but you will still want to verify the locations somehow, rather than blindly trusting the locations provided by the Garmin.