You can't just change negative numbers into positive numbers. They're completely different things.
I'm sure you worked with the number line in school. Do you remember how the negatives are on one side of zero, and the positives are on the other side ?
You dress a lot different when it's 40 degrees below zero outside, compared to what you put on when it's 40 degrees above zero. They're completely different numbers.
In the same way, 1,400 feet below sea level is the lowest spot on Earth, at the Dead Sea in Israel, but 1,400 feet above sea level is like somewhere in Nebraska or northern Texas. They're completely different numbers.
