The are 135 degrees in each interior angles of a regular octagon: (8-2)*180/8 = 135 degrees.
[(Number of sides - 2)*(180 degrees)/( number of sides)]
If you connect the center of the octagon to each of its vertex by a straight line, and then bisect the sides with a perpendicular, you will create 16 congruent triangles. The angles on the sides are 135/2 degrees in measure.
You can use SOH-CAH-TOA to determine the lengths ofa the sides needed to determine the area of each triangle.
The formula for each triangle is (1/2)*10*10/tan(135/2 degrees), multiplying this value by 16 yields 331.37 square yards, accurate to 2 decimal places.