The GPS's used in survey work are far different than the ones you might buy at your local sporting goods dealer. The major difference is in the antenna used which will determine the number of sattelites tracked simultaneously. This is particularly important if you are working back in under the trees. The small Garmins and Magellans you can buy are good, but they sacrifice antenna signal to noise ratio to get that tiny antenna that will fit into a handheld unit. They might say they are a 12 channel receiver, but how often do you see them tracking that many birds simultaneously, even out in the open? The birds are up there, if you have the antenna to see them. The more satellites you can track at the same time, the quicker your position will settle. They also culumatively average the position data so the longer you stay in a position, the more refined it becomes. You can do this with your handheld GPS, but you will have to output the position data to a computer to average it over time. You have probably seen survey crews using GPS, it will either look like a stick with a white dome antenna on the top about 6" in diameter or the antenna will be on a tripod over a survey mark. I worked with a guy last fall from the USGS getting him to remote sites to set up a receiver to take long term averages over USGS markers. The antenna was about 14" across and cost several thousand dollars.
I am working on a similar survey problem to lay out lines for fences. I have 3 good corner markers but so far have not found the 4th. It should be easy as it is one along the road and the corner marks are based on the road center line. My northern property line is also a quarter section line. Since the corner in question is along this line, the fences across the road give me a good idea of where to look but I havn't found it yet. I need to try the metal detector though, that is a good idea. The other three corners are marked with a piece of 1" steel pipe with an ID tag and the reference numbers that are stated on my deed. I also can't see from corner to corner so have been working on a method using a laser plumbbob to give me someting to sight on. I just need a night with enough moisture in the air to allow me to see the laser beam pointing straight up into the sky.