1. Cut the building plan along the edge. You need to cut off the 1/4 circle at the entrance of the above picture, and just cut off the other edge lines.
2. Find a pin (or sewing needle) and insert it into the upright you think of with a little force, then "hang" the picture with a pin. If the four sides are balanced, the needle tip is a vertical rod; If it is not balanced, repeatedly find the balance point on the map.
The pole is actually the center of gravity of the drawing. Stupid way, some trouble, but accurate. Draw with cardboard.