New, higher end GPS units have a "line of sight" feature that could assist you with this. The premis is that you can lock the compass on a heading, walk the line and it will display these tracks on a map.
I think you will need more than a GPS device to do what you thinking though, you also need the map to overlay your tracks or "line", as you call it, on to. Topo and off road (i.e. not roadways) maps generally are only included with the more expensive units that are 400+ dollars.
If its that close I bet your neighbor would not care either way, kick him/her a few pints and just be a good neighbor. That usually goes a long way in my experience.
MapleME
Brunswick, Maine
2x5.5 Lapierre Raised Flue, AUF
150 taps on gravity line
2011 John Deere Gator XUV, gonna be a great hauler
Wood Wolf Splitter
One VERY understanding wife