Spotting scope and a compass worked for me. Set up the scope, pointed at something located with the compass and have a buddy walk towards it with a marker of some kind, I used yellow newspaper bags, that can be tied to trees etc along the way. When you get as far as you can see, move the scope, take another bearing and do it again. Should be close enough.

I had a similar problem, but also with some steep hills thrown in the mix. I ran a string from corner to corner. If a tree put a bend in the string, I cut it down. Took a while, but eventually ended up with a straight line of fence.

It is one side of a 40 acre so 440 yards. It is hilly but the biggest problem is the small trees. I am doing some Timber Stand Improvement to thin them out. I am sure my neighbor wouldn't mind if I would help out his side and kill his black and honey locusts but I have killed enough in my life and don't want to deny someone else the opportunity. Thanks for all of the ideas and help. If anyone else has any comments or thoughts feel free to post and I will keep reading. Thanks again.

Check back here in 36 hours. I'll post some ideas. From me a licensed land surveyor that has marked many miles of property lines.

I would use a $30,000 GPS receiver and could mark the line within a foot or so almost as fast as I could walk the line. You could also do this: Walk the line & count your steps. After you get to the end. start back flagging a straight line to the other corner. How: at a corner hang a flag (I use survey ribbon, but any thing will work, strips of cloth, plastic bags, etc.). Walk toward the second corner and hang a second marker. Now, continue to walk and place a third marker in line with the first two. A machete is useful to cut small brush/trees to help you see "the line" that you are marking. With some care you can mark a straight line. Continue to mark a straight line until you pass the second corner. At the second corner pace the distance from the straight line to the corner. The marked line needs to be adjusted by the amount of the error at the second corner. Example: the error at the second corner is 100 yards. So your error is about 22.7 yards per 100 yards of marked line. 22.7 x 4 = 90.8 walk down the marked line 40 yards and place a marker 90.8 yards from the marked line. Walk down the marked line another 50 yards (now 350 yards from the start) and place a marker 22.7 x 3.50 = 79.4 yards from the marked line. Your new flags will have the property line marked every 50 yards when finished. Now you can cut the line open between the corrected flags,. With some trial & error you can get the line marked with this method. Here are the correction for my example: 440 yard line has 100 yard error. Each yard of marked line has 100/440 = 0.23 yards of error. The marked line needs to be corrected as follows: @ 50 yards correct 0.23*50 = 11.5 yards, @ 100 yards correct 23 yards, @ 150 yards correct 34.5 yards, @ 200 yards correct 46 yards. @ 250 Correct 57.5, @ 300 correct 69 (0.23 x 300 = 69). This method is not as hard as it sounds. Good luck.

I have an APP, "Huntstand" that will show your and your neighbors property lines, and names of adjoining and local landowners.
You can measure distance between points, show the acreage of an area, and mark waypoints. I think it's free.
You should be able to do what you need with it.

The GPS hunting apps aren't nearly accurate enough to mark a property line. Some of them are so inaccurate you can see it by just looking at the map on the phone.

Borrow a buddies garmin that has the ONX hunt map in it. They are accurate to within 6 ft. according to my experience. Check with the coon hunting guys, some one close will have one.

So are the two corners diagonal?

I helped an old man years ago. He waited until a calm day,had me load saws, axe's, paint and brush and a metal barrel. We went to the easy to get to corner. Unloaded the barrel,set it 10 feet on his side of the corner pin. He made a fire in the barrel while I gathered green grass and leaves. He took the paint and brush and went to the other corner. He told me to wait 15 mins and start adding the green stuff. What a cloud of smoke it made,straight up. I added more green as the smoke would die down. 30 after the first smoke I saw him thru the trees,he was painting trees on both sides as he walked. The distance was about 700 yards. That was the easy part,took me 2 weeks to cut a drivable path through there. I was young it was work and I got a 22 bolt action for my hard work. My times have changed.

I like the sound of this.

Dad and I did this thirty years ago to mark his back line. Pulled off the nearest section monument, the next property over.
Ten years later had it professionally run. Dad's mark was off by six inches.