running a straight line through trees

I think you could do a good job with a cheap handheld GPS. The accuracy of these things is not great in an absolute sense - i.e. accuracy of finding a spot on the earth is +- 10 ft. But their relative accuracy is very good. This means they can find the same spot again very accurately. Test this by checking location of one marker, walk over to the other marker and check it's location. Then walk back to first marker - accuracy should be within a few inches. Similarly the GPS will be accurate between two markers for which it found the locations itself.
You would hold the GPS over each marker and record the latitude and longitude coordinates. Then the simplest way to compute the intermediate points is too select, say the mid-way and quarter-way points. You use the average of the coordinates for the midway point and then the averages of the midway and end points to get the quarter-way points. Then just walk along the line until the GPS says you are on the quarter-way and midway points. Pound in stakes there. It will be within a view inches accuracy.
You can greatly simplify the math if you set the GPS to use decimal degrees instead of degrees/minutes/seconds.
GPS does need a clear view of the sky to see the satellites. It won't work when the leaves are thick but it should see through the trees when the leaves have fallen. So from now to spring is your window of opportunity.
Note that you must keep the GPS within clear view of the sky tyhe whole time. You may lose the relative accuracy if you put it in the car and drive to the next marker. Again, you can test this by driving between the markers - if it finds each marker the second time with the accuracy you need then it is fine, if not, get out and walk.
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