The ornamental design for a tie down anchor, as shown.
FIG. 1 is a front elevational view of a tie down anchor formed in
accordance with the present invention, the other front elevational view
being an identical, mirror-image view;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof, with elements not included in the
invention shown in phantom;
FIG. 4 is a side elevational view thereof, the other side elevational view
being an identical, mirror-image view; and,