These are Google Earth screen captures from major cities in Vietnam. Using naked eyes as well as detectron2 detection model, we can see that the northern provinces have more houses with red roof than the southern provinces.

Among northern provinces, Bac Giang, Hanoi and Nghe An has more houses with red roof than Viet Tri.

Why are there more houses with red roofs in Northern cities than Southern cities in Vietnam ?

It is because of the blood type of females within the household or because of the winter weather in Northern provinces or because of the “Mái Ngói” tradition.

After sampling red roof houses in Hanoi, it seems like housewifes living in the red roof houses are XO.

The houses were marked with red roof so satelites from US, Russia, China, Japan or Korea can detect and observe houses in Vietnam like a Go game.