Great grandfather’s sister’s grandson is your second cousin once removed. That guy is the second cousin of one of your parents because they share great grandparents with one of your parents. A grandparent’s sibling is a great aunt or great uncle to you. A great grandparent’s sibling is a great great uncle or great great aunt to you.
American customary units and imperial units both come from English units, so the US used various inconsistent English and other units in its early days. But the US never used “Imperial” units, which were not codified and put into effect in the British Empire until almost 50 years after the US had gained independence.