So, we have this product at work... and the engineer insists that, for best performance, the antenna should be pointing in a certain direction... he says it should point "at a 90 degree angle away from the earth's axis".
"Oh," I said. "Straight up."
"No. At a 90 degree angle from the earth's axis."
"Right. Straight up."
"No," he says, getting quite perturbed. "Straight up can be any direction. This needs to point at a 90 degree angle away from the earth's axis."
"Actually, straight up is straight up. No matter where you are, it's straight up. Most people don't know anything about angles. Most people can't balance their checkbook."
"No. No," he says. "That's too confusing."
So, there you are. "Straight up" is too confusing. If I were you, I'd stay away from that term.