I'm late to the party, but there a couple of emails in this long thread asking if you can get better-than-consumer-grade GPS accuracy, to hunt up corners/monuments yourself.
I happen to have looked into this.
"Better than consumer grade" GPS is very expensive. Tens of thousands of dollars. But there are things you can do to get more accuracy from consumer grade gear, like what is in smartphones these days.
Even with perfect visibility of 10-12 or more satellites, consumer GPS can't be depended on to deliver more than about 15' (5m) accuracy. This is due to atmospheric (and other similar) effects. However, while it can be off by that much, a lot of the time it is OK *in this situation* to 6-7' (2m) or so. This is the type of error that you need really expensive equipment and processes, which can include real-time or post-measurement correction of the position measured, to eliminate. For mere mortals like us, you just need to be aware of it.
The second source of error is poor satellite visibility. Valleys, rock faces, even tree canopy can block, or even worse bounce, signals. Various algorithms try to compensate, with varying degrees of success. This you can do something about:
1. It will be much easier to GPS locate markers that are out in the wide open. To locate others, it's better to use the distances and bearings from a plat/survey, with an easier to find marker as a starting point.
2. Keep your GPS receiver out in the open, not in your pocket, inside your side-by-side, etc. In my case, I found it well worth it to buy a Garmin Glo 2 GPS external receiver (<US$100) that connects to my phone via Bluetooth. The underlying GPS hardware is only marginally better than the phone, but you can keep the external receiver on your hat, or on a pole, or on your Mule roof and get *much* better accuracy than if you're relying on GPS in your phone, held at waist height so you can see the screen.
3. Be patient. You probably have experienced having your phone's GPS position drift around for a minute after you query where you are. It can continue to drift for quite a while, and there are various interference effects that may take 30 mins or more to "settle down". So if you really care, occupy the point whose GPS coordinates you want to be sure of and wait, leaving your (external) receiver just sitting there.
Regarding distances and bearings, on iOS a very useful app is Theodolite, an "augmented reality" app that shows bearings, etc over what you see in your viewfinder. Note that plats will have somewhere on them written exactly how bearings on the survey are described, in particular if they are grid or astronomic, or relative to what reference bearing. You can figure out precisely what this means in your instance, but it will probably be easier to calibrate a known bearing (i.e. measure the Theodolite bearing of a known line at a known bearing on the survey, to get a correction factor you then apply to bearings you're trying to locate.)
I've used all of this to locate nearly all of the important corners/monuments of my land, based on 1991 surveys. It took a while, but it gave me the confidence I know my boundaries.
