The Missouri Compromise’s dividing line was an arbitrary surveyor’s measure.

Evidence

In what became known as the Missouri Compromise, Congress decided that slavery would be permitted in Missouri but prohibited in all other parts of the Louisiana Purchase north of the 36°30ʹ parallel, a surveyor’s line that arbitrarily delineated North from South. But soon a second disagreement erupted when lawmakers in Missouri proposed a constitution that flat-out barred “free Negroes and mulattoes” from the state. — Nikole Hannah-Jones et al., The 1619 Project, 698