Tim:
Some good comments here, the ones that seems to make the most sense to look into are pane sizing, making sure you are on the the correct pane to mark, and this is one that I believe no one has mentioned…
If you are trying to map a weed line as Joe S. mentioned check out the depth… using the depth of the outer weedline, if the weeds stop at say 12′, then that depth marker should keep you on a better track. Also to better do this you should be able to insert a digital depth reading into your SI pane, thus making the reading of the depth contour easier.
Good luck!!!
Mark
Mark, curious how you do this on a helix! Can you let me know? Thanks