What is the meaning of correction lines?

Provisions in the rectangular survey (government survey) system made to compensate for the curvature of the earth's surface. Every fourth township line (at 24-mile intervals) is used as a correction line on which the intervals between the north and south range lines are measured and corrected to a full six miles. Range lines are only parallel in theory. Due to the curvature of the earth, range lines gradually approach each other. If they are extended northward, they eventually meet at the North Pole. The fact that the earth is not flat, combined with the crude instruments used in early days, means that few townships are exactly six-mile squares or contain exactly 36 square miles.