Reference for Eruvin 112:1

תנו רבנן המרבע את העיר עושה אותה כמין טבלא מרובעת וחוזר ומרבע את התחומין ועושה אותן כמין טבלא מרובעת

Our Rabbis taught: If [a circular] town is to be [circumscribed by a] square<span class="x" onmousemove="('comment',' In connection with the calculations of the permitted Sabbath limits around it.');">¹</span> [the sides must be] drawn in the shape of a square tablet. The Sabbath limits also are then drawn in the shape of a square tablet.<span class="x" onmousemove="('comment',' This is explained infra.');">²</span> When the measurements<span class="x" onmousemove="('comment',' In connection with the calculations of the permitted Sabbath limits around it.');">¹</span>

Jastrow