James Black wrote:

Where can I find mitre angles for creating a circular border around
the base of trees? For example, what is the angle cut needed to
create a 7 foot circle, an 8 foot, or 9 foot circle?

I presume the lengths are diameters and you really want a circle and
not an octogon which would be very easy.

Does the inside have to be circular or just the outside? Are you a
master craftsman with a heavy duty router and a router bit the length
of the thickness of a railroad tie?

The real question here is: How many pieces of wood are you prepared
to cut?

Dick