Not a final solution for measuring altitude above ground level while balloon mapping in the field, but I have found these duct tape tabs at 100' intervals to be quite handy. To set it up, I measured 100' on the ground and used a sharpie to mark small tabs of duct tape that folded directly over the dracon tether kite line. I have done a few flights with the tabs and the tape has not caused any problems with the spooling or release of the line. If there are multiple people doing the field work one person can remain focused on the balloon, while someone else is watching the line/spool activity.