Octagon Earthworks State Memorial is currently leased to a golf club.

Newark Octagon, Newark Earthworks, Newark, Ohio

One of the most significant traces of ancient civilization in Eastern North America is the
complex array of geometric works arrayed on the earth, the greatest concentration of which covered
several square miles at Newark, Ohio. The Newark Octagon, the most northwestern of the earthworks
at Newark is the largest of all geometric earthworks. The contiguous earthwork of Observatory Mound,
Observatory Circle, the parallel embankments, and the Octagon combine to extend 2,898 feet.

Ancient Monuments Placemarks   -   Newark Octagon   -   Newark Circle   kmz files with map overlays.

Above. One of the Newark Octagon embankments and an opening mound in the far left. Each of the eight
vertices has a mound. Below. A surviving small circle, adjacent southeast of the Octagon, is now a golf green.

Excerpts from my 2001 paper Possible Geodetic Properties and Relationships of
Some Monumental Earthworks in the Middle Ohio Valley, A Preliminary Inquiry.

The relationships between Newark Earthworks and Marietta Square and the orientations and relationships of the various monuments of Newark Earthworks offer the best evidence of knowledge and practice of geodesy. The Newark Octagon and Marietta Square ratio of arc distance to longitude difference, the azimuth of the Newark Octagon, and the bearings between Newark Circle and the Octagon Circle infer precise understanding of the mathematical ratio of latitude to longitude. The precise commensuration of the Newark to Marietta arc distance and their longitude difference with the SO module (earth’s angular orbital motion per day) infers a precise understanding of astronomic constants and the scale of the earth. The redundancy of expression of the same relationships makes it very difficult to discount the relationships as coincidental. ... The fact that the largest of the monuments express these geodetic relationships also argues against coincidence. Given the distances involved, creating the relationships between the Middle Ohio monuments could have been accomplished using direct surveying and mathematics, and thus does not necessarily infer an astronomical method of the determination of longitude. If the Newark to Fort Center relationships are intentional, use of an astronomical method for determination of longitude is also inferred.

I conclude that the builders of the Middle Ohio earthworks understood aspects of geodesy, including accurate astronomical knowledge, knowledge of the scale and shape of the earth, knowledge of their location on the earth, and how to place find and point position.