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
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.