Thanks for all the feedback.
What I'm doing is not real critical. As I say, the corner posts have all been set by a professional surveyor so those are basically carved in stone so-to-speak. At least from a legal perspective. The cosmos itself may be changing, but I don't think that will change people's property markers.
Just for further information, for whatever it's worth.
I own 10 acres of land. Fortunately it was surveyed many years ago and all the corner posts are well established. However the lines between the corner posts have not been well-marked. In some situations the corner posts can be seen from each other so a direct line-of-sight is possible to establish those boundaries perfectly. In other places a line-of-sight cannot be established because of hills and dense forest.
Why am I concerned with marking out these boundary lines?
Well, recently two people bought property next to me. In one case the follow actually came onto my land by a major distance (i.e. he was 1/3 of the distance into my property) and he was putting up posted signs on my property. I had to go out and show him where the corner posts are. Fortunately once I showed him the surveyor's stakes he was quick to recognize and respect the true boundary. And in that case a line-of-sight was possible between the corner posts. So now we have that entire line marked off very clearly.
The gentleman was very good about it and didn't give me any strife. But, as I say, originally he was 1/3 of the way into my property putting up posted signs thinking that was his property line. So had I not said anything right away, that could have become a property line dispute at some point in the future.
That was on the back property line from my house. He was originally 1/3 of the way into my back yard. So I'm glad I was able to nip that in the bud and clear that up in a timely manner.
Shortly after that my next-door neighbor to my right died. His place is now up for sale. So I'm marking that boundary really well now so when the new owners move in there won't be any confusion over where the property line is. That property line is also line-of-sight between corners so again, no major problems there.
However, most of my property is on the other side of the road. And that's where the property lines are murky save for the corner posts. I've decided to take preemptive steps to mark these lines clearly before any new neighbors move in. And so that's where I wanted to mark the boundary lines as close as possible to their correct location. I don't mind being a few feet off one way or the other. Even the deed states +/- a few feet. But I don't want to 30 feet off, like I was the first time I tried to mark it.
I finally got it down to where I'm only 5 feet off at the very end of the line. I can live with that.
I was thinking that if an Arduino electronic compass could be used to mark out a perfect line I'd build one.
But if the electronic compass is only good to +/- 2 degrees, then I'm already doing better than that with my hand-held compass. So not point in building an electronic compass that won't even do as good.
I also thought I'd post this here because sometimes electronic makers come up with ingenious ways to improve the accuracy of various sensors. I've seen this done with distance senors on robots where by using more sensors and configuring them in certain ways accuracy can be improve over what a singe sensor can provide. I was thinking that maybe someone figured out a way to squeak out better performance from a compass sensor too.
Maybe by using several of them and calibrating out their mutual errors with a known measurement? Makers are always coming up with ingenious ways of solving these sorts of problems.
Fortunately I've already solved my problem. Simply by using a tripod to hold the compass steady and taking great care in reading the dial accurately I was able to mark off the line with only a 5 foot error over 565 feet. That's close enough for me.