Good work on finding the corner!

National Geographic's maps used USGS maps in an excellent viewer called "TOPO!". They used to come on CD's, with a state like Colorado on several disks. They migrated to DVD's with entire states. I have several of these and they are invaluable to my work. TOPO! became AllTrails (http://alltrails.com/) which is a cloud based service. A guy I work with uses this service but I still use the DVD's.

Magellan units of that era also used the Vantage Point software. Did you get that with the unit? As I recall, it had some useful measuring tools.

Do you have the original plat map? If so, you should be able to take the bearing of that property edge, and use your GPS to stay on that bearing until you get to the tree. Lots of error in that method.

Try a search with your county name and GIS. Many counties now have excellent GIS viewers with air photos of greater detail than google maps. Some viewers link to original plats, but if not, you can always get the book and page number of your property and obtain that plat from your county at a small fee. Some have data for download but typically those are shape files for GIS programs. (If Google Maps has parcel info, they got it from your county's GIS site) Free viewers are often available. Sometimes, the data is available for Google Earth (.kmz, .kml).