A meridian which crosses the calendar revision office in Nishisanjo, Kyoto is used as the prime meridian, the basis of longitude, and the lines that cross at right angles to each degree of latitude and longitude are drawn on the basis of actual measured values (Some theories insist that such a method is similar to Sanson projection of the West).
京都西三条改暦所を通る子午線を本初子午線として経度の基準とし、実測値を元に経度・緯度1度ごとに直交する度線を引いてある（西洋のサンソン図法に近いとされるが異説もある）。