I was trying to confirm some GPS coordinates before my last trip to Joshua Tree and found this list which is similar to this list (Appendix IV), but when I try to convert the coordinates to the usual lat/lon, they don't really match up.
It seems that Easting/Northing coordinates can be defined in different ways depending on map projection type or offset origin?
I tried converters like https://www.engineeringtoolbox.com/utm-latitude-longitude-d_1370.html or http://www.zonums.com/online/coords/cotrans.php?module=14 (zone 11?)
For example, Mustang Ranch is listed at 3768017, 575615 which converts to 34.05013, -116.18072:
but is actually closer to 34.05194, -116.18145:
Headstone Rock - South Face is listed at 3760548, 578384 which converts to 33.98257, -116.15139:
but the actual location is more like 33.98439, -116.15240:
Do these seem like conversion errors or measurement errors?